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THESTORYOFEARTH
THESTORYOF
EARTH
TheFirst4.5BillionYears,fromStardusttoLivingPlanet_________________________
ROBERTM.HAZEN
VIKING
VIKINGPublishedbythePenguinGroupPenguinGroup(USA)Inc.,375HudsonStreet,NewYork,NewYork10014,U.S.A.•PenguinGroup(Canada),90EglintonAvenueEast,Suite700,Toronto,Ontario,CanadaM4P2Y3(adivisionofPearsonPenguinCanadaInc.)•PenguinBooksLtd,80Strand,LondonWC2R0RL,England•PenguinIreland,25St.Stephen’sGreen,Dublin2,Ireland(adivisionofPenguinBooksLtd)•PenguinBooksAustraliaLtd,250CamberwellRoad,Camberwell,Victoria3124,Australia(adivisionofPearsonAustraliaGroupPtyLtd)•PenguinBooksIndiaPvtLtd,11CommunityCentre,PanchsheelPark,NewDelhi–110017,India•PenguinGroup(NZ),67ApolloDrive,Rosedale,Auckland0632,NewZealand(adivisionofPearsonNewZealandLtd)•PenguinBooks(SouthAfrica)(Pty)Ltd,24SturdeeAvenue,Rosebank,Johannesburg2196,SouthAfricaPenguinBooksLtd,RegisteredOffices:80Strand,LondonWC2R0RL,EnglandFirstpublishedin2012byVikingPenguin,amemberofPenguinGroup(USA)Inc.
10987654321
Copyright©RobertM.Hazen,2012Allrightsreserved
LIBRARYOFCONGRESSCATALOGING-IN-PUBLICATIONDATAHazen,RobertM.,1948–ThestoryofEarth:thefirst4.5billionyears,fromstardusttolivingplanet/RobertM.Hazen.p.cm.Includesindex.ISBN:978-1-101-58068-41.Earth.I.Title.QE501.H3252012550—dc232011043713
PrintedintheUnitedStatesofAmericaDesignedbyCarlaBolteTimeLinesbyJeffreyL.WardNopartofthisbookmaybereproduced,scanned,ordistributedinanyprintedorelectronicformwithoutpermission.Pleasedonotparticipateinorencouragepiracyofcopyrightedmaterialsinviolationofthe
author’srights.Purchaseonlyauthorizededitions.
ALWAYSLEARNING
PEARSON
ToGregory:Changewillcome;
mayyouhavethewisdomandcouragetoadapt
CONTENTS____________
Introduction1 BirthTheFormationofEarth
2 TheBigThwackTheFormationoftheMoon3 BlackEarthTheFirstBasaltCrust
4 BlueEarthTheFormationoftheOceans5 GrayEarthTheFirstGraniteCrust6 LivingEarthTheOriginsofLife
7 RedEarthPhotosynthesisandtheGreatOxidationEvent8 The“Boring”BillionTheMineralRevolution9 WhiteEarthTheSnowball-HothouseCycle
10 GreenEarthTheRiseoftheTerrestrialBiosphere11 TheFutureScenariosofaChangingPlanet
Epilogue
AcknowledgmentsIndex
Introduction
OneofthemostarrestingimagesofthetwentiethcenturyisaphotoofEarthrise,takenin1968byahumantravelerinorbitaroundtheMoon.Wehavelongknownhowpreciousandspecialourworldis:Earthistheonlyknownplanetwithoceansofwater,withanatmosphererichinoxygen,withlife.Nevertheless,manyofuswereunpreparedforthebreathtakinglystarkcontrastbetweentheutterlyhostilelunarlandscape,thelifelessblackvoidofspace,andourenticingmarbledwhite-on-bluehome.Fromthatdistantvantagepoint,Earthappearsalone,small,andvulnerablebutalsomorebeautifulbyfarthananyotherobjectintheheavens.Wearejustifiablycaptivatedbyourhomeworld.Morethantwocenturies
beforethebirthofChrist,thepolymathGreekphilosopherEratosthenesofCyreneconductedtheearliestdocumentedexperimentonEarth.HebasedhisingeniousmeasurementofEarth’scircumferenceonthesimpleobservationofshadows.IntheequatorialtownofSyene,Egypt,atnoononthesummersolstice,heobservedtheSundirectlyoverhead.Averticalpostcastnoshadow.Bycontrast,onthesamedayatthesametimeinthecoastalcityofAlexandria,some490milestothenorth,asimilarverticalpostcastaslightshadow,thusrevealingthattheSunwasnotquitedirectlyoverheadinthatlocation.EratosthenesemployedthegeometricaltheoremsofhisGreekpredecessorEuclidtoconcludethatEarthmustbeasphere,andhecalculatedthatitmusthaveacircumferenceofabout25,000miles—remarkablyclosetothemodernestablishedvalueof24,902milesaroundtheEquator.Throughoutthecenturies,thousandsofotherscholars,afewwithnames
reveredbutmostlosttohistory,haveprobedandponderedourplanetaryhome.TheyhaveaskedhowEarthwasformed,howitmovesthroughtheheavens,whatit’smadeof,andhowitworks.Andmostofall,thesemenandwomenofsciencehavewonderedhowourdynamicplanetevolved,howitbecamealivingworld.Today,becauseofourremarkableandcumulativeknowledge,andbecauseofthewondersofhumantechnology,weknowmoreaboutEarththanancientphilosopherscouldhavefathomed.Ofcoursewedon’tknoweverything,butourunderstandingisrichanddeep.AndwhileourknowledgeofEarthhasincreasedsincethedawnof
humankind,refinedoverthemillenniaintoafixedunderstanding,muchofthatprogresshasrevealedthatthestudyofEarthisthestudyofchange.ManylinesofobservationalevidencepointtothefluctuatingnatureofEarth
yearbyyear,epochbyepoch.RhythmicallylayeredorvarvedsedimentdepositsinsomeglaciallakesinScandinaviadisplaymorethanthirteenthousandyearsofalternatingcoarserandfinerparticles—theconsequencesofswiftererosionduringannualspringthaws.FrozenglacialdrillcoresfromAntarcticaandGreenlandrevealmorethaneighthundredthousandyearsofseasonaliceaccumulations.Anddepositsofpaper-thinsedimentlayersfromWyoming’sGreenRiverShalepreservemorethanamillionyearsofannualevents.Eachofthoselayeringsrestsatopvastlyolderrocks,whichthemselveshintatgrandcyclesofchange.Measurementsofgradualgeologicalprocessespointtoevenmoreimmense
spansofEarthhistory.TheformationofthemassiveHawaiianIslandsrequiredslowandsteadyvolcanicactivity,successivelavablanketspilingupovertensofmillionsofyears.TheAppalachiansandotherancientroundedmountainrangesaretheresultofhundredsofmillionsofyearsofgradualerosionpunctuatedbygreatlandslides.Thesometimesherky-jerkymotionsoftectonicplateshaveshiftedcontinents,elevatedmountains,andopenedoceansoverthecourseofgeologicalhistory.Earthhasalwaysbeenarestless,evolvingplanet.Fromcoretocrust,itis
incessantlymutable.Eventodaytheair,theoceans,andthelandarechanging,perhapsatapaceunequaledinourplanet’srecentpast.Wewouldbefoolishnottocareabouttheseunsettlingglobalchanges,andindeedformanyofus,itseemsimpossiblethatwewouldn’t—ourcuriosityandcareforourhomecomesasnaturallytousasitdidtoEratosthenes.ButwewouldbeequallyfoolishtoaddressthecurrentstateofEarthwithouttakingfulladvantageofwhatitalreadytellsusaboutitssurprisingstoriedpast,aboutitsunpredictabledynamicpresent,andaboutourselvesandourplaceinitsfuture.
Mostofmylifehasbeenspenttryingtounderstandourvibrant,complex,changeablehome.Asaboy,Icollectedrocksandminerals,crammingmyroomfulloffossilsandcrystals,sidebysidewithrandombugsandbones.MyentireprofessionalcareerhasfollowedthatEarth-centrictheme.Ibeganwithexperimentsatthesubmicroscopicscaleofatoms,studyingthemolecularstructureofrock-formingminerals,heatingandsqueezingtinymineralgrainstodocumentthepressure-cooker-likeeffectsofEarth’sdeepinterior.Withtime,myviewexpandedtothegranderspatialandtemporaltapestryof
geology.FromthedesertsofNorthAfricatotheicefieldsofGreenland,fromtheshoresofHawaiitothepeaksoftheRockies,fromtheGreatBarrierReefinAustraliatoancientfossilizedcoralreefsinadozennations,Earth’snaturallibrariesrevealamultibillion-yearstoryofcoevolutionsharedbyelements,minerals,rocks,andlife.Asmyresearchprogramshiftedtotheplausiblerolesofmineralsinlife’sancientgeochemicalorigins,IhavereveledinstudiesthatsuggestthatthecoevolutionoflifeandmineralsthroughEarthhistoryisevenmorestrikingthanpreviouslyimagined—thatnotonlydocertainrocksarisefromlife,asevidentinlimestonecavernsacrossthecontinent,butthatlifeitselfmayhavearisenfromrocks.OverfourbillionyearsofEarthhistory,theevolutionarystoriesofmineralsandlife—geologyandbiology—haveintertwinedinastonishingwaysthatareonlynowcomingintofocus.In2008theseideasculminatedinanunconventionalpaperon“MineralEvolution”—acontroversialnewargumentthatsomewelcomedasperhapsthefirstparadigmshiftinmineralogyintwocenturies,whileothersvieweditwarily,asapossiblyhereticalreframingofourscienceinthecontextofdeeptime.Theancientdisciplineofmineralogy,thoughabsolutelycentraltoeverything
weknowaboutEarthanditsstoriedpast,hasbeencuriouslystaticanddetachedfromtheconceptualvagariesoftime.Formorethantwohundredyears,measurementsofchemicalcomposition,density,hardness,opticalproperties,andcrystalstructurehavebeenthemeatandpotatoesofthemineralogist’slivelihood.Visitanynaturalhistorymuseum,andyou’llseewhatImean:gorgeouscrystalspecimensarrayedincaseafterglass-frontedcase,withlabelsshowingname,chemicalformula,crystalsystem,andlocality.ThesemosttreasuredfragmentsofEartharerichinhistoriccontext,butyouwilllikelysearchinvainforanyclueastotheirbirthagesorsubsequentgeologicaltransformations.Theoldwayallbutdivorcesmineralsfromtheircompellinglifestories.Thattraditionalviewhastochange.ThemoreweexamineEarth’srichrock
record,themoreweseehowthenaturalworld,bothlivingandnonliving,hastransformeditselfagainandagain.Ourgrowingunderstandingofthetwinplanetaryrealitiesoftimeandchangehasmadeitpossibletoconjecturenotjusthowmineralsfirstcametobe,butwhen.Andtherecentdiscoveriesoforganismsinplaceslongconsideredinhospitable—insuperheatedvolcanicvents,acidicpools,Arcticice,andstratosphericdust—haveenlistedmineralogyasakeydisciplineinthequesttocomprehendtheoriginsandsurvivaloflife.IntheNovember2008issueofthefield’sflagshipjournal,AmericanMineralogist,mycolleaguesandIproposedanewwaytothinkaboutthemineralkingdomandits
incredibletransformationsthroughtheunexploreddimensionoftime.Weemphasizedthatmanybillionsofyearsago,therewerenomineralsanywhereinthecosmos.Nocrystallinecompoundscouldhaveformed,muchlesssurvived,inthesuperheatedmaelstromfollowingtheBigBang.Ittookahalf-millionyearsforeventhefirstatoms—hydrogen,helium,andabitoflithium—toemergefromthecauldronofcreation.Millionsmoreyearspassedwhilegravitycoaxedtheseprimordialgaseouselementsintothefirstnebulas,andthencollapsedthenebulasintothefirsthot,dense,incandescentstars.Onlywhenthosefirststarsexplodedintosupernovabrilliance,whenexpanding,coolingenvelopesofelement-richgascondensedthefirsttinycrystalsofdiamond,couldthecosmicmineralogicalsagahavebegun.AndsoIhavebecomeacompulsivereaderofthetestimonyoftherocks—the
compelling,ifsometimesfragmentaryandambiguous,storiestheyhavetotellofbirthanddeath,ofstasisandflux,oforiginsandevolution.ThisuntoldgrandandintertwinedtaleofEarth’slivingandnonlivingspheres—thecoevolutionoflifeandrocks—isutterlyamazing.Itmustbeshared,becauseweareEarth.Everythingthatgivesusshelterandsustenance,alltheobjectswepossess,indeedeveryatomandmoleculeofourflesh-boundshells,comesfromEarthandwillreturntoEarth.Toknowourhome,then,istoknowapartofourselves.Earth’sstorymustbeshared,too,becauseouroceansandatmospheresare
changingataraterarelymatchedinitslonghistory.Oceansarerising,whiletheyarebecomingwarmerandmoreacidic.Globalpatternsofrainfallarechanging,whiletheatmosphereisbecomingmoreturbulent.Polariceismelting,tundraisthawing,andhabitatsareshifting.Aswewillexploreinthepagesahead,Earth’sstoryisasagaofchange,butonthoserarepreviousoccasionswhenchangeoccurredwithsuchalarmingrapidity,lifeappearstohavepaidaterribletoll.Ifwearetoactthoughtfullyandintimeforourownsake,wemustbecomeintimatewithEarthandherstory.Foraswassublimelyevidentfromthatwondrouspicturesnappedfromalifelessworld239,000milesaway,wehavenootherhome.
InthetraditionofEratosthenesandthethousandsofcuriousmindsthathavefollowedhim,mypurposeinthisbookistoconveyEarth’slonghistoryofchange.AsimmediateandfamiliarasEarthmightseemtobe,itsvibrantstoryembracesasuccessionoftransformationaleventsalmostbeyondimagining.Totrulyknowyourplanetaryhomeandcomprehendtheaeonsthatshapedit,youmustfirstwrapyourmindaroundsevencoretruths.
1. Earthismadeofrecycledandrecyclingatoms.
2. Earthisimmenselyoldwhencomparedwithhumantimeframes.3. Earthisthree-dimensional,andmostoftheactionishiddenfromview.4. RocksaretherecordkeepersofEarthhistory.5. Earthsystems—rocks,oceans,atmosphere,andlife—arecomplexly
interconnected.6. Earthhistoryencompasseslongperiodsofstasispunctuatedbysometimes
sudden,irreversibleevents.7. LifehaschangedandcontinuestochangeEarth’ssurface.
TheseconceptsofEarth’sexistenceframetheintricatelylayeredstoriesofatoms,minerals,rocks,andlifeintheirvastepicsofspaceandtime;theywillreappearinthefollowingpages,ineveryphaseoftheuniverse’sfieryoriginandEarth’sprotractedevolution.ThecoevolutionofEarthandlife,thenewparadigmthatliesattheheartofthisbook,ispartofanirreversiblesequenceofevolutionarystagesreachingbacktotheBigBang.Eachstageintroducednewplanetaryprocessesandphenomenathatwouldultimatelyresculptourplanet’ssurfaceagainandagain,inexorablypavingthewayforthewondrousworldweinhabittoday.ThisisthestoryofEarth.
Chapter1
Birth
TheFormationofEarthSpanningthebillionsofyearsbeforeEarth’sformation
Inthebeginning,therewasnoEarth,oranySuntowarmit.OurSolarSystem,withitsglowingcentralstarandvariedplanetsandmoons,isarelativenewcomertothecosmos—amere4.567billionyearsold.Alothadtohappenbeforeourworldcouldemergefromthevoid.Thestagewassetforourplanet’sbirthmuch,muchearlier,attheoriginofall
things—theBigBang—about13.7billionyearsago,bythelatestestimates.Thatmomentofcreationremainsthemostelusive,incomprehensible,definingeventinthehistoryoftheuniverse.Itwasasingularity—atransformationfromnothingtosomethingthatremainsbeyondthepurviewofmodernscienceorthelogicofmathematics.Ifyouwouldsearchforsignsofacreatorgodinthecosmos,theBigBangistheplacetostart.Inthebeginning,allspaceandenergyandmattercameintoexistencefroman
unknowablevoid.Nothing.Thensomething.Theconceptisbeyondourabilitytocraftmetaphors.Ouruniversedidnotsuddenlyappearwheretherewasonlyvacuumbefore,forbeforetheBigBangtherewasnovolumeandnotime.Ourconceptofnothingimpliesemptiness—beforetheBigBangtherewasnothingtobeemptyin.Theninaninstant,therewasnotjustsomething,buteverythingthatwould
everbe,allatonce.Ouruniverseassumedavolumesmallerthananatom’snucleus.Thatultracompressedcosmosbeganaspurehomogeneousenergy,withnoparticlestospoiltheperfectuniformity.Swiftlytheuniverseexpanded,thoughnotintospaceoranythingelseoutsideit(thereisnooutsidetoouruniverse).Volumeitself,stillintheformofhotenergy,emergedandgrew.Asexistenceexpanded,itcooled.ThefirstsubatomicparticlesappearedafractionofasecondaftertheBigBang—electronsandquarks,theunseenessenceofallthesolids,liquids,andgasesofourworld,materializedfrompureenergy.Soonthereafter,stillwithinthefirstfractionofthefirstcosmicsecond,quarkscombinedinpairsandtripletstoformlargerparticles,includingtheprotonsandneutronsthatpopulateeveryatomicnucleus.Thingswerestillridiculouslyhot
andremainedsoforaboutahalf-millionyears,untiltheongoingexpansioneventuallycooledthecosmostoafewthousanddegrees—sufficientlycoldforelectronstolatchontonucleiandformthefirstatoms.Theoverwhelmingmajorityofthosefirstatomswerehydrogen—morethan90percentofallatoms—withafewpercentheliumandatraceoflithiumthrownin.Thatmixofelementsformedthefirststars.
FirstLight
Gravityisthegreatengineofcosmicclumping.Ahydrogenatomisalittlething,buttakeoneatomandmultiplyitbytentothesixtiethpower(that’satrillion-trillion-trillion-trillion-trillionhydrogenatoms)andtheyexertquiteanimpressivecollectivegravitationalforceononeanother.Gravitypullstheminwardtoacommoncenter,formingastar—agiantgasballwithepicpressuresatthecore.Asanimmensehydrogencloudcollapses,thestar-formingprocesstransformsthekineticenergyofmovingatomstothegravitationalpotentialenergyoftheirclusteredstate,whichtranslatesintoheatoncemore—thesameviolentprocessthatoccurswhenanasteroidimpactsEarth,butwithvastlymoreenergyrelease.Thecoreofthehydrogensphereeventuallyreachestemperaturesofmillionsofdegreesandpressuresofmillionsofatmospheres.Suchtemperaturesandpressurestriggeranewphenomenoncallednuclear
fusionreactions.Undertheseextremeconditions,thenucleioftwohydrogenatoms(eachwithoneproton)collidewithsuchforcethatneutronsaretransferredfromonenucleustoanother,makingsomehydrogenatomsmoremassivethanothers.Afterseveralsuchcollisions,aheliumnucleuswithtwoprotonsforms.Surprisingly,theresultingheliumatomisabout1percentlessmassivethantheoriginalhydrogenatomsfromwhichitformed.Thatlostmassconvertsdirectlytoheatenergy(justasitdoesinahydrogenbomb),whichpromotesevenmorenuclearfusionreactions.Thestar“ignites,”bathingitssurroundingswithradiantenergy,whilebecomingeverricherinheliumattheexpenseofhydrogen.Largestars,manyofthemmuchbiggerthanourSun,eventuallyusedupthe
prodigioussuppliesofhydrogenintheircores.Butextremeinteriorpressureandheatcontinuedtopromotenuclearfusion.Two-protonheliumatomsinastellarcorefusedtomakecarbon,thevitalelementoflifewithitssixprotons,evenasnewpulsesofnuclearenergytriggeredhydrogenfusioninasphericallayerofatomssurroundingthecore.Thencorecarbonfusedtomakeneon,neontomakeoxygen,thenmagnesium,silicon,sulfur,andonandon.Graduallythestar
developedanonionlikestructure,withlayeruponconcentriclayeroffusionreactions.Fasterandfasterthesereactionsoccurred,untiltheultimateiron-producingphaselastednomorethanaday.Bythispointinthefirststars’lifecycles,manymillionsofyearsaftertheBigBang,mostofthefirsttwenty-sixelementsintheperiodictablehadbeenbroughtintoexistencebynuclearfusionwithinmanyindividualstars.Ironisasfarasthisnuclearfusionprocesscango.Whenhydrogenfusesto
producehelium,whenheliumfusestoproducecarbon,andduringalltheotherfusionsteps,abundantnuclearenergyisreleased.Butironhasthelowestenergyofanyatomicnucleus.Aswhenablazingfiretransformseverybitoffueltoash,alltheenergyhasbeenusedup.Ironistheultimatenuclearash;nonuclearenergycanbeextractedbyfusingironwithanything.Sowhenthefirstmassivestarproduceditsinevitableironcore,thegamewasover,theresultscatastrophic.Untilthatpoint,thestarhadsustainedastableequilibrium,balancingitstwogreatinnerforces:gravitypullingmasstowardthecenter,nuclearreactionspushingmassoutwardfromthecenter.Whenthecorefilledwithiron,however,theoutwardpushjuststopped,andgravitytookoverinaninstantofunimaginableviolence.Theentirestarcollapsedinwardwithsuchswiftnessthatitreboundedoffitselfandexplodedinthefirstsupernova.Thestarwasrippedapart,blastingmostofitsmassoutward.
TheBirthofChemistry
Forthosereaderswhoseekdesigninthecosmos,supernovasarenearlyasgoodaplacetostartastheBigBang.Tobesure,theBigBangledinevitablytohydrogenatoms,andhydrogenatomsjustasinexorablyproducedthefirststars.Yetit’snotatallobvioushowstars,bythemselves,getyoutoourmodernlivingworld.Abigballofhydrogen,evenifitdoeshaveagrowingcore-boundcollectionofheavierelementsuptoiron,doesn’tseemtohelpmovethingsalonginaveryinterestingway.Butwhenthefirstbigstarsexploded,cosmicnoveltyensued.Thesefractured
bodiesseededspacewiththeelementstheyhadcreated.Carbon,oxygen,nitrogen,phosphorus,andsulfur—theelementsoflife—wereespeciallyabundant.Magnesium,silicon,iron,aluminum,andcalcium,whichdominatethecompositionsofmanycommonrocksandformalargefractionofthemassofEarth-likeplanets,alsoabounded.Butintheincomprehensiblyenergeticenvironmentoftheseexplodingstars,theseelementsfusedinnewandexoticwaystomakealltheperiodictable—elementswaybeyondnumbertwenty-six.
Soappearedthefirsttracesofmanyrarerelements:precioussilverandgold,utilitariancopperandzinc,toxicarsenicandmercury,radioactiveuraniumandplutonium.What’smore,alltheseelementswerehurledoutintospace,wheretheycouldfindoneanotherandclumptogetherinnewandinterestingwaysthroughchemicalreactions.Chemistryhappenswhenoneeverydayatombumpsintoanother.Everyatom
hasatinybutmassivecentralnucleuswithapositiveelectriccharge,surroundedbyacloudlikedistributionofoneormorenegativelychargedelectrons.Isolatedatomicnucleialmostneverinteract,exceptintheultimatepressure-cookerenvironmentsofstellarinteriors.Butelectronsfromoneatomareconstantlybumpingintotheelectronsofadjacentatoms.Chemicalreactionsoccurwhentwoormoreatomsmeetandtheirelectronsinteractandrearrange.Suchshufflingandsharingofelectronsoccursbecausecertaincombinationsofelectrons,notablycollectionsoftwoortenoreighteenelectrons,areparticularlystable.ThefirstchemicalreactionsfollowingtheBigBangproducedmolecules—
smallclustersofafewatomstightlyboundintoasingleunit.Evenbeforehydrogenatomsbeganfusingtogetherinstarstoformhelium,hydrogenmolecules(H2),eachwithtwohydrogenatomschemicallybondedtogether,formedinthevacuumofdeepspace.Eachhydrogenatomcarriesonlyoneelectron,whichisaratherunstablesituationinauniversewheretwoelectronsisamagicnumber.Sowhentwohydrogenatomsmeet,theypooltheirresourcestoformamoleculewiththatmagicnumberoftwosharedelectrons.GiventheabundanceofhydrogenfollowingtheBigBang,hydrogenmoleculessurelypredatedthefirststarsandhavebeenaperpetualfeatureofourcosmossinceatomsfirstappeared.Followingthefirstsupernova,asavarietyofotherelementsseededspace,lots
ofotherinterestingmoleculescouldform.Water(H2O),withtwohydrogenatomsbondedtoanoxygenatom,wasoneearlyexample.Chancesarethatnitrogen(N2),ammonia(NH3),methane(CH4),carbonmonoxide(CO),andcarbondioxide(CO2)moleculesalsoenrichedthespacearoundsupernovas.Allofthesemolecularspecieswouldcometoplaykeyrolesintheformationofplanetsandintheoriginsoflife.Thencametheminerals—microscopicsolidvolumesofchemicalperfection
andcrystallineorder.Thefirstmineralscouldhaveformedonlywherethedensitiesofmineral-formingelementswerehighenough,buttemperaturescoolenough,foratomstoarrangethemselvesinlittlecrystals.JustafewmillionyearsaftertheBigBang,theexpanding,coolingenvelopesofthefirstexploding
starsprovidedtheperfectsettingsforsuchreactions.Tinycrystallitesofpurecarbon—diamondandgraphite—wereprobablythefirstmineralsintheuniverse.Thosepioneeringcrystalswerelikeafinedust,theindividualgrainsindiscerniblysmallbutperhapslargeenoughtoaddabitofdiamondsparkletospace.Thesecrystallineformsofcarbonweresoonjoinedbyotherhigh-temperaturesolidsthatfeaturedthemorecommonelements,includingmagnesium,calcium,silicon,nitrogen,andoxygen.Somewerefamiliarmineralslikecorundum,thechemicalcompoundofaluminumandoxygenthatissovaluedinitsrichlycoloredvarieties,rubyandsapphire.Tinyamountsofthemagnesiumsilicateolivine,thesemipreciousbirthstoneofAugust,alsoappeared,joinedbymoissanite,asiliconcarbideoftensoldthesedaysasacheapsyntheticsubstitutefordiamond.Altogethertheinterplanetarydusthostedperhapsadozencommon“ur-minerals.”Andso,withtheexplosionofthefirststars,theuniversebegantogetmoreinteresting.Nothinghappensonlyonceinouruniverse(exceptperhapsfortheBigBang).
Scattereddebrisofoldexplodedstarswereconstantlysubjectedtotheorganizingforceofgravity.Thusremnantsoftheformerstellargenerationsinexorablyseedednewpopulationsofstarsbyformingnewnebulas,eachavastinterstellarcloudofgasanddustrepresentingthewreckageofmanypriorstars.Eachnewnebulawasmoreironrich,andalittlepoorerinhydrogen,thantheonebefore.For13.7billionyears,thiscyclehascontinued,asoldstarsproducenewstarsandslowlyalterthecompositionofthecosmos.Countlessbillionsofstarshaveemergedincountlessbillionsofgalaxies.
CosmicClues
Onceuponatime,fivebillionyearsago,ourfuturerealestateinthegalacticsuburbslayhalfwayoutfromtheMilkyWay’scenter,attheuninhabitededgeofastar-studdedspiralarm.Littlewastobefoundinthatunassumingneighborhood,apartfromagreatnebulaofgasandicyduststretchinglight-yearsacrossthedarkvoid.Ninepartsintenofthatcloudwerehydrogenatoms;ninepartsintenofwhatremainedwereheliumatoms.Iceanddust,richinsmallorganicmoleculesandmicroscopicmineralgrains,accountedfortheremaining1percent.Anebularcloudinspacecanlastmanymillionsofyearsbeforeatrigger—a
shockwavefromanearbyexplodingstar,forexample—beginsitscollapseintoanewstarsystem.Almost4.6billionyearsago,suchatriggerinitiatedourSolarSystem.Eversoslowly,overthecourseofamillionyears,theswirlingmassof
presolargasanddustwasdrawninward.Likeatwirlingice-skater,thebigcloudrotatedfasterandfasterasgravitypulleditswispyarmstothecenter.Asitcollapsedandspunfaster,thecloudbecamedenserandflattenedintoadiskwithagrowingcentralbulge—thenascentSun.Largerandlargergrewthatgreedyhydrogen-richcentralball,whichultimatelyswallowed99.9percentofthecloud’smass.Asitgrew,internalpressuresandtemperaturesrosetothefusionpoint,ignitingtheSun.CluestowhathappenednextarepreservedintherecordofourSolarSystem
—itsplanetsandmoons,itscometsandasteroids,anditsabundantandvariedmeteorites.OnestrikingfeatureisthatalltheplanetsandmoonsorbittheSunonthesameplane,andinthesamedirection.What’smore,theSunandmostoftheplanetsrotateontheiraxesinmoreorlessthatsameplaneanddirection.Nothinginthelawsofmotionrequiresthiscommonalityofspin;planetsandmoonscouldorbitandrotateanywhichway—northtosouth,easttowest,toptobottom,bottomtotop—andstillobeythelawofgravity.Ifplanetsandmoonswerecapturedfromdistant,randomsources,onemightexpectsuchahodgepodge.Theobservedorbitalnear-uniformityinourSolarSystem,bycontrast,suggeststhatplanetsandmoonsallcoalescedfromthesameflatrotatingdiskofdustandgasatmoreorlessthesametime.Allofthesegrandobjectspreservethesamesenseofrotation—thesharedangularmomentumoftheentireSolarSystem—fromthetimeoftheoriginalswirlingcloud.AsecondcluetotheSolarSystem’soriginsisfoundinthedistinctive
distributionofitseightmajorplanets.ThefourplanetsclosesttotheSun—Mercury,Venus,Earth,andMars—arerelativelysmallrockyworldscomposedmostlyofsilicon,oxygen,magnesium,andiron.Denserocks,likeblackvolcanicbasalt,dominatetheirsurfaces.Bycontrast,theouterfourplanets—Jupiter,Saturn,Uranus,andNeptune—aregasgiantsmadeprimarilyofhydrogenandhelium.Theseimmensesphereshavenosolidsurfaces,justanever-thickeningatmospherethedeeperyougo.ThisdichotomyofworldssuggeststhatearlyinthehistoryoftheSolarSystem,withinafewthousandyearsoftheSun’sbirth,anintensesolarwindblewleftoverhydrogenandheliumfarouttothecolderrealms.SufficientlyfarfromtheradiantSun,thesevolatilegasescouldcool,condense,andgatherintospheresoftheirown.Bycontrast,thecoarser,mineral-richgrainsofdustthatremainedclosertothehotcentralstarquicklyclumpedtogethertoformtherockyinnerplanets.DetailsoftheviolentprocessesthatformedEarthandtheotherinnerplanets
arebeautifullypreservedintheamazinglydiversevarietiesofmeteorites.It’sabitunsettlingtothinkthatourhomeisconstantlybeingpepperedbystones
fallingfromthesky.Infact,thescientificcommunitydidn’tpaythemmuchheeduntilabouttwohundredyearsago,thoughtherewascertainlynoshortageofcolorfulmeteoriteanecdotesinfolklore(includingseveraltalesinvolvingunfortunateFrenchpeasants).Evenwhenscholarsbegantodescribemeteoritefallsmoreformally,littleinthewayofreproduciblescientificevidencecouldbemusteredtodocumentthem,muchlessexplaintheirprovenance.TheAmericanstatesmanandnaturalistThomasJefferson,uponreadingthetechnicalreportfromYaleUniversityofanobservedmeteoriteimpactinWeston,Connecticut,quipped:“IfinditeasiertobelievethattwoYankeeprofessorswouldliethanthatstoneswouldfallfromheaven.”Twocenturiesandtensofthousandsofmeteoritefindslater,theirveracityis
nolongerinquestion.Asmeteoriteexpertscovermoreground,andasavidcollectorsviefortheraresttypes,museumandprivatecollectionsaroundtheworldhaveswelled.Foratime,theserepositorieswereskewedinfavorofdistinctiveironmeteorites,whoseblackcrusts,weirdlysculptedshapes,andunusuallyhighdensitymadethemstandoutagainsteverydayrocks.Butthe1969discoveryofthousandsofmeteoriteslyingonpristineAntarcticicefieldschangedthatperception.Meteoritesaretelltalecluestoourplanet’sorigins.Themostcommonand
ancientones,the4.566-billion-year-oldchondrites,datefromthetimejustbeforetheplanetsandmoonsoftheSolarSystemformed,whentheSun’snuclearreactorfirstturnedonandintenseradiantenergybroiledtheencirclingnebula.Theblastfurnaceeffectmeltedthedustydiskintoclotsofsmall,stickyrockdroplets,calledchondrules,aftertheancientGreekwordfor“grain.”RangingfromthesizeofBBstothatofsmallpeas,theseproductsoftheSun’srefiningfireweremeltedmultipletimes,inrepeatedpulsesofradiationthattransformedtheregionsclosesttotheSun.Clustersoftheseancientchondrules,cementedtogetherbyfiner-grainedpresolardustandmineralfragments,composetheprimitivechondritesthathavelandedontheEarthbythemillions.ChondritesprovideourbestviewofthebrieftimejustaftertheSunwasbornbutbeforetheplanetswereformed.Asecond,youngerclassofmeteorites,collectivelydubbedachondrites,dates
fromthetimewhentheearliestmaterialsoftheSolarSystemwerebeingreworked—melted,smashed,andotherwisetransformed.Thediversityofachondriticmeteoritesisastonishing—nuggetsofshinymetal,chunksofblackenedrock,someasfine-grainedasglass,otherswithlustrouscrystalsaninchacross.ImportantdiscoveriesofnewvarietiesarestillbeingmadeinsomeofEarth’smostremoteregions.
ThecontinentofAntarcticaholdsvastplainsofancientblueice—placeswhereitneversnowsandthefrozensurfacemayhaveremainedunchangedformanythousandsofyears.Rocksthathavefallenfromspacejustliethere,dark,out-of-placeobjectswaitingtoberetrieved.Internationaltreatiesbanningcommercialexploitationofthearea,coupledwithlimitedaccesstotheremoteicefields,ensurethattheseextraterrestrialresourceswillbepreservedforscientificstudy.Teamsofwarmlybundledscientistsinhelicoptersandonsnowmobilessystematicallyscourmileaftersquaremileoftheseforbiddingicedeserts.Theycarefullyrecordandpackageeachfind,makingsurethatnohand,nobreathcontaminatesitssurface.UponreturningtocivilizationaftereachAntarcticsummerseason,thesemeteoritehuntersdelivertheirtreasurestopubliccollections,mostnotablytheSmithsonianInstitutionstoragefacilitiesinsuburbanSuitland,Maryland,wheremanythousandsofspecimensarepreservedinultraclean,airtightstoragecabinetswithinbuildingsthesizeoffootballfields.Equallyrichinmeteorites,thoughfarlessconducivetoorganizedrecovery
andsterilecuration,areEarth’sgreatdesertsinAustralia,theAmericanSouthwest,theArabianPeninsula,andmostdramatically,NorthAfrica—thevastSaharaDesert.WordhasspreadamongSahara-crossingnomads—Tuaregs,Berbers,Fezzanis—thatmeteoritescanbevaluable.AsinglepreciouslunarmeteoritefoundsomewhereintheshiftingsandsofNorthAfricaearlyinthetwenty-firstcenturyisreputedtohavefetchedamilliondollarsinaprivatesale.It’seasyenoughforadesertridertogetdownoffhiscamelandcarryanoddstonetothenextvillage,wheresomeonefromanunofficialguildofmeteoritemiddlemen,networkedbysatellitephonesandskilledinhyperbole,willofferhimapittanceincash.Fromonedealertothenext,bagsofrocksarepassed,eachtimewithamarkup,untiltheyreachMarrakech,Rabat,orCairoandthencetraveltothebuyersoneBayandthebiginternationalrockandmineralshows.MorethanonceongeologytripstoremotepartsofMorocco,I’vebeen
offeredburlapbagsfilledwithtenortwentypoundsofrockspurportedtobemeteorites—“nomiddlemen,freshfromthedesert,justfoundlastweek.”Thesecash-only“deals”areoftenbrokeredindingy,windowlessbackroomsoftanmud-brickhouses,awayfromtheblazingdesertSun,whereit’salmostimpossibletoseewhat’sbeingoffered.Oncetheformalitiesofgreetingandthetraditionalcupsofmintteahavebeenshared,thesellerdumpsthecontentsonacarpet.Someoftherocksarejustrocks.Ballast.It’slikeatesttoseeifyouknowyourstuff.Afewwillbethecommonestsortofchondrite,thesizeofanoliveoranegg,somewithanicelymeltedfusioncrust,thefieryresultoffallingfastthroughthesky.Thestartingpriceisalwayswaytoohigh.Ifyousaythey’re
toocommon,asecond,smallerbagmayappear,perhapswithanironmeteoriteorsomethingevenmoreexotic.Irecallonedealworkedbyourguide,Abdullah,onadustysideroadafew
mileseastofSkoura.Theseller,adistantacquaintanceofquestionableintegrity,calledbysatelliteanddemandedsecrecy.“ItmightbeaMartian,”hetoldAbdullah.“Ninehundredgrams.Justtwentythousanddirhams.”About$2,400—ifitwasreal,ifitcouldbeaddedtothetwodozenorsoknownmeteoritesthatcamefromMars,it’dbeabargain.Theyarrangedthetimeandplace.Twonondescriptcarspulledupbesideeachother;threeofusgotoutandstoodinatightcircle.Therockinquestionwaslovinglyslippedfromavelvetpouch.Butitlookedlikeanordinaryrock(asdoallMartianmeteorites).Thepricedroppedtofifteenthousanddirhams.Thentwelvethousand.Buttherewasnowaytobesure,sowepassed.LaterAbdullahconfidedtomethathehadbeentempted,buttherearealwaysmoremeteorites.It’sbestnottobetoogreedywithonebigscore;noonetellsthetruth,andalldealsarefinal.AsinAntarctica,theequatorialdesertsrevealthenaturaldistributionofall
kindsofmeteorites,providingunrivaledcluestothecharacteroftheearlySolarSystemandthustheoriginsofourownplanet.Sadly,unlikemeteoritesfromAntarctica,mostofthesespecimenswillnevermakeittomuseumcollectionsforatleasttworeasons.Firstandforemost,thegrowingcommunityofamateurcollectors(fueledbyafewwealthyaficionadosandthereadilyavailableSaharanfinds)isintenselycompetitive.Anythingraresellsquicklyandforalotofmoney.Someofthosespecimenswillsurelywindupasdonationstomuseums,butmostarepoorlyhandled,andmuchofthescientificvalueinapristinefindissoonlostbycontaminationfromunprotectedhands,multipurposeclothbags,andtheubiquitouscameldung.Equallytroublingisthelackofanyusefuldocumentation,astowhenorwhereinthedesertthemeteoriteswerefound.Allthedealerswillsayis“Morocco,”whichisusuallyafalsehood,asmostofthesandySaharaliestotheeast,inAlgeriaandLibya—countriesfromwhichitisnowillegaltoimportspecimens.Sowithoutrigorousdocumentation,mostmuseumssimplywillnotaccept“Moroccan”or“NorthAfrican”meteorites.Inthosehostile,aridterrainsoftheSahara,ortheicefieldsofAntarctica,any
rockstandsoutasaforeignobjectfallenfromthesky.SuchanunadulteratedsamplingofthemeteoritepopulationgivesscientiststheirbestviewoftheearlieststagesoftheSolarSysteminwhichEarthformed.Chondritesrepresentalmostnineoutofeverytenfinds;therestarethediverseachondrites,belongingtothefew-million-yearserawhenouryoungSolarSystemwasaturbulentnebula,inwhichchondritesclumpedtogetherintolargerandlargerbodies:first
thesizeofyourfist,thenthesizeofyourcar,thenasmallcity—billionsofobjectsafewmilesorsoindiameterallvyingforspaceinthesamenarrowringaroundtheyoungSun.Largerandlargertheygrew:tothesizeofRhodeIsland,thenOhio,Texas,
Alaska.Asthousandsofsuchplanetesimalsunderwentthischaoticprocessofaccretion,theydiversifiedinnewways.Astheygrewtofiftymilesormoreindiameter,twocoequalsourcesofheatcompounded.Thegravitationalpotentialenergyofmanysmallobjectssmashingtogetherwasmatchedinintensitybythenuclearenergyoffast-decayingradioactiveelementslikehafniumandplutonium.Themineralsmakinguptheseplanetesimalswerethustransformedbyheat,whiletheirinteriorsmeltedoutright,differentiatingintoanegglikearrangementofdistinctivemineralzones:adensemetal-richcore(analogoustotheegg’syolk),themagnesiumsilicatemantle(theeggwhite),andthethinbrittlecrust(theshell).Thelargestplanetesimalswerealteredbyinternalheating,byreactionswithwater,andbytheintenseshockoffrequentcollisionsinthecrowdedsolarsuburbs.Perhapsthreehundreddifferentmineralspeciesaroseasaconsequenceofsuchdynamicplanet-formingprocesses.Thosethreehundredmineralsaretherawmaterialsfromwhicheveryrockyplanetmustform,andallofthemarestillfoundtodayinthediversesuitesofmeteoritesthatfalltoEarth.Fromtimetotime,whentwobigplanetesimalssmashedtogetherwith
sufficientforce,theywereblastedtosmithereens.(ThisviolentprocesscontinuestothisdayintheAsteroidBeltbeyondMars,thankstothegravitationaldisruptionsfromthegiantplanetJupiter.)Consequently,mostofthediverseachondritemeteoriteswefindtodayrepresentdifferentpartsofdestroyedminiplanets.Analyzingachondritesisthusabitlikeamessyanatomylessonfromanexplodedcadaver.Ittakestime,patience,andalotofbitsandpiecestogetaclearpictureoftheoriginalbody.Thedensemetalliccoresofplanetesimals,whichwoundupasadistinctive
classofironmeteorites,aretheeasiesttointerpret.Thoughoncethoughttobethemostcommontypeofmeteorites,theunbiasedAntarcticsamplingrevealsthatironsrepresentamodest5percentofallfalls.Planetesimalcoresmusthavebeencorrespondinglysmall.Thecontrastingsilicate-richmantlesofplanetesimalsarerepresentedinahost
ofexoticmeteoritetypes:howardites,eucrites,diogenites,ureilites,acapulcoites,lodranites,andmore—eachofdistinctivecomposition,texture,andmineralogy,andmostnamedforthelocalitywheretheearliestknownexamplewasrecovered.Someofthesemeteoritesarecloseanaloguestorocktypesfoundon
Earthtoday.Eucritesrepresentarathertypicalkindofbasalt—therocktypethatspewsoutoftheMid-AtlanticRidgeandblanketstheoceanfloor.Diogenites,composedprimarilyofmagnesiumsilicateminerals,appeartobetheresultofcrystalsettlinginalargeundergroundmagmareservoir.Asthemagmacooled,crystalsmoredensethantheirsurroundinghotliquidgrewandthensanktothebottomtoformaconcentratedmass,justastheydoinmagmachambersdeepinsideEarthtoday.Occasionally,duringaparticularlydestructivecollision,ameteoritehappened
tosnatchapieceofaplanetesimal’score-mantleboundary,wherechunksofsilicatemineralsandiron-richmetalscoexisted.Theresultisabeautifulpallasite—astunningmixtureofshinymetalandgoldencrystalsofolivine.Thinpolishedslabsofpallasite,withlightreflectingoffthemetalandpassingthroughtheolivinelikestainedglass,areamongthemostprizedspecimensintheworldofmeteoritecollecting.Asgravityclumpedtheearlychondritestogether—andascrushingpressure,
scaldingtemperature,corrosivewater,andviolentimpactsreworkedthegrowingplanetesimals—moreandmorenewmineralsemerged.Alltogether,morethan250differentmineralshavebeenfoundinallthevarietiesofmeteorites—atwenty-foldincreaseoverthedozenpresolarur-minerals.Thesevariedsolids,whichincludethefirstfine-grainedclays,sheetlikemica,andsemipreciouszircon,becamethebuildingblocksofEarthandotherplanets.Biggerandbiggertheplanetesimalsgrew,asthelargestswallowedthesmaller.Eventuallyafewdozenbigballsofrock,eachthesizeofasmallplanet,actedasgiantvacuumcleaners,sweepingswathsoftheSolarSystemcleanofmostofitsdustandgasastheycoalescedandsettledintonear-circularorbitalpaths.Whereanobjectultimatelywoundupdependedinlargemeasureonitsmass.
AssemblingtheSolarSystem
TheSun,whichhasthelion’sshareoftheSolarSystem’smass,dominateseverything.Oursisnotaparticularlymassivestarsystem,andsotheSunisamodestsortofstar—agoodthingforanearbylivingplanet.Paradoxically,themoremassiveastar,theshorteritslifetime.Thegreaterinteriortemperaturesandpressuresofbigstarspushnuclearfusionreactionsfasterandfaster.SoastartentimesthemassofourSunmightlastlessthanatenthaslong—severalhundredmillionyearsatmost,barelyenoughtimeforanorbitingplanettogetlifestartedbeforethestarexplodesinakillersupernova.Conversely,areddwarfstaratenththeSun’smasswilllastmorethantentimeslonger—onehundred
billionyearsormore—thoughtheenergyoutputofsuchaweakstarmightnotproveaslife-sustainingasourradiantyellowbenefactor.Ourmiddling-sizeSunhasstruckahappymedium:nottoolargeandshort-
lived,nottoosmallandcool.Anditsprojectednineortenbillionyearsofreliablehydrogenburningmeanthere’sbeenplentyoftimeforlifetogetgoing,andthere’splentymoreforittocontinuetoevolve.True,inanotherfourorfivebillionyears,theSunwillrunoutofhydrogeninitscoreandwillhavetoswitchtoheliumburning.Intheprocess,itwillswelltoamuchlessbenignredgiantstarmorethanonehundredtimesitspresentdiameter,engulfingpoorlittleMercury,firstscaldingandthenswallowingVenus,andmakingthingsprettyunpleasantonEarthaswell.Nevertheless,evenafter4.5billionyears,westillhaveplentyoftimebeforetheSunentersitscrotchetyoldageandlifeonEarthbecomesproblematic.OurSolarSystemhasanotherimportantbenefitforalivingplanet.Unlike
mostothers,oursisasingle-starsystem.Astronomersusingpowerfultelescopeshavefoundthatabouttwoofeverythreestarsweseeinthenightskyareactuallybinaries—systemsinwhichtwostarsorbiteachotherinadanceaboutacommongravitationalcenter.Asthosestarsformed,hydrogenaccumulatedattwoseparateplacestoformbiggasballs.Ifournebulahadbeenabitswirlier,withmoreangularmomentumand
consequentlymoremassoutintheregionofJupiter,ourSolarSystemwouldlikelyhavebeenadouble-starsystemaswell.TheSunwouldhavebeensmaller,andJupiter,insteadofbeingabighydrogen-richplanet,wouldhavegrowntobecomeasmallhydrogen-richstar.Perhapslifewouldhavethrivedbetwixtsuchapolarity.Perhapsanextrastarwouldhaveprovidedanextrasourceoflife-sustainingenergy.Butthegravitationaldynamicsoftwostarscanbetricky,andEarthmighthaveendedupaworldhostiletolifewithaneccentricorbit,wobblyspin,andwildclimaticswingsastwostronggravitationalattractorspulleditthiswayandthat.Asitis,thegasgiantplanetsareratherwellbehaved,withmodestsizeand
near-circularorbitsaroundtheSun.Jupiter,thelargestofthecast,weighsinatjustunderathousandthoftheSun’smass.That’slargeenoughtoexertsignificantcontrolonitsplanetaryneighbors;thankstoJupiter’sdisruptivegravitationalfield,theplanetesimalsthatmakeuptheAsteroidBeltnevercoalescedintoasingleplanet.ButJupiterisnotnearlylargeenoughtotriggernuclearfusionreactionsinitsowncore—thedefiningdifferencebetweenstarsandplanets.ThemoredistantringedplanetSaturnandtheevenmoreremoteandfrigidUranusandNeptunearesmallerstill.
Nevertheless,allthesegasgiantplanetswerelargeenoughtocapturetheirowngravitationallybounddisksofdebris,likelittlesolarsystemswithintheSolarSystem.Consequently,allfourouterplanetshavesuitesoffascinatingmoons,includingsomerelativelysmallasteroidsthatwereattractedandthenheldinorbitbythegiants’gravity.Othermoons,somealmostaslargeasthefourinnerplanetsandwithdynamicgeologicalprocessesoftheirown,formedmoreorlessinplacefromleftoverdustandgas—thedebrisofplanetaryconstruction.Infact,themostactiveobjectintheSolarSystemisJupiter’smoonIo,whichissoclosetothebigplanetthatitcompletesafullorbitonceeveryforty-onehours.Epictidalforcesconstantlystressthemoon’s2,260-milediameterandpowerahalf-dozenvolcanoeswithsulfurplumesthatextendmorethanahundredmilesfromthesurface,unlikeanythingelseintheSolarSystem.EquallyintriguingareEuropaandGanymede,bigmoonsroughlythesizeofMercuryandcomposedofnear-equalproportionsofwaterandrock.BothofthesebigmoonsarekeptwarminsidebyJupiter’sincessanttidalforces.Consequently,theybothhavedeep,encircling,ice-coveredoceans—placestargetedbyNASA,initsongoingsearchforlifeonotherworlds.Saturn,thenextplanetoutfromtheSun,isendowedwithalmosttwodozen
moons,nottomentionagloriousringsystemdominatedbysmallbitsofbrilliantlyreflectivewaterice.MostofSaturn’smoonsarerelativelysmall,somecapturedasteroidsandothersformedfromSaturn’sgassyleftovers;butitslargestmoon,Titan,isbiggerthanMercuryandenshroudedinathickorangeatmosphere.ThankstotheEuropeanSpaceAgency’sHuygenslander,whichtoucheddownonJanuary14,2005,wehaveclose-upviewsofTitan’sdynamicsurface.Abranchingnetworkofriversandstreamsfeedsfrigidlakesofliquidhydrocarbons;thedense,colorful,andturbulentatmosphereislacedwithorganicmolecules.Titanisyetanotherworldworthexploringforsignsoflife.Themostdistantgasgiantplanets,UranusandNeptune,arenolesswell
endowedwithinterestingmoons.Mostshowsignsofwaterice,organicmolecules,andongoingdynamicactivity.Neptune’sbigmoonTritonevenhasanitrogen-richatmosphere.AndbothUranusandNeptunehavetheirowncomplexringsystems,thoughapparentlytheyarecomposedofautomobile-sizechunksofdarkcarbon-richmaterial,quiteunliketheluminousparticlesthatconstitutetheicyringsofSaturn.
RockyWorlds
Closertohome,gravityalsoheldsway.Withmostofthehydrogenandhelium
blownoutwardtotherealmofthegasgiantsafterignitionoftheSun,theinnerSolarSystemhadmuchlessmasstoplaywith,andmostofthatconsistedofhardrocks—thestuffofchondriteandachondritemeteorites.Mercury,thesmallestanddriestrockyplanet,formedclosesttotheSun.Ahostilescorchedworld,thisinnermostplanetappearstobedeadandbattered:itsbillionsofyearsofintenselycrateredsurfaceispreservedunderanairlesssky.IfyouareeveraskedtonameobjectsintheSolarSystemwhereyou’dbetagainstlife,Mercuryshouldtopyourlist.Venus,thenextplanetout,isEarth’stwininsizebutradicallydifferentin
habitability,thanksinlargemeasuretoitsorbit,almostthirtymillionmilesclosertotheSun.Itmayhavehadamodeststoreofwaterearlyinitshistory,andevenashallowocean,butsubjectedtotheSun’sheatandsolarwind,mostVenusianwaterappearstohaveboiledoff,preventingthatworldfrombeingwet.Carbondioxide,thedominantgasinthethickVenusianatmosphere,sealedintheSun’sradiantenergyandcreatedarunawaygreenhouseeffect.TodayVenus’saveragesurfacetemperaturesexceed900degreesFahrenheit—hotenoughtomeltlead.Mars,onestopoutfromEarth,isalotsmaller—onlyatenthitsmass—butit
isinmanyrespectsthemostEarth-like.Likealltherockyplanets,Marshasametalcoreandasilicatemantle.LikeEarth,ithasanatmosphereandalotofwater.Itsrelativelyweakgravitycannoteasilyholdspeedinggasmoleculesintheupperatmosphere,sobillionsofyearshaveeatenawayatbothairandwater,andyetMarsstillholdswarm,wetundergroundreservoirswherelifemightmaintainatenuousrefuge.Nowondermostplanetarymissionshavetargetedtheredplanet.Earthitself,the“thirdrockfromtheSun,”issmackinthemiddleofthe
habitable“Goldilocks”zone.It’scloseenoughtotheSun,andhotenough,tohaverelinquishedsignificantamountsofhydrogenandheliumtotheouterrealmsoftheSolarSystem,butit’sfarenoughfromtheSun,andcoolenough,tohaveheldontomostofitswaterinliquidform.LiketheotherplanetsinourSolarSystem,itformedabout4.5billionyearsago,essentiallyfromcollidingchondritesandtheirsubsequentgravitationalclumpingsintolargerandlargerplanetesimals,overaspanofafewmillionyears.
DeepTime
LayeredintoalltheevidenceforhowtheSun,theEarth,andtherestofourSolarSystemwerebornistheconceptofimmensetimespans—4.5billionyears
andcounting.Americanslovetoquotethedatesoffamouseventsinhumanhistory.Wecelebrategreataccomplishmentsanddiscoveries,liketheWrightbrothers’firstflightonDecember17,1903,andthefirstmannedMoonlandingonJuly20,1969.WerecountdaysofnationallossandtragedylikeDecember7,1941,andSeptember11,2001.Andwerememberbirthdays:July4,1776,and,ofcourse,February12,1809(thecoincidentbirthdaysofCharlesDarwinandAbrahamLincoln).Wetrustthevalidityofthesehistoricmomentsbecauseanunbrokenwrittenandoralrecordlinksustothatnot-so-distantpast.Geologistsalsolovetoquotehistorictimemarkers:about12,500yearsago,
whenthelastgreatglaciationendedandhumansbegantosettleNorthAmerica;65millionyearsago,whenthedinosaursandmanyothercreaturesbecameextinct;theCambrianboundary,at530millionyearsago,whendiverseanimalswithhardshellssuddenlyappeared;andmorethan4.5billionyearsago,whenEarthbecameaplanetinorbitabouttheSun.Buthowcanwebesurethoseageestimatesarecorrect?There’snowrittenrecordofEarth’sancientchronologypastafewthousandyears,oranyinformingoraltradition.Fourandahalfbillionisanumberalmostbeyondreckoning.Thecurrent
GuinnessworldrecordforlongevityisheldbyaFrenchwomanwholivedtocelebrateher122ndbirthday—sohumansfallfarshortoflivingevenfor4.5billionseconds(about144years).Allofrecordedhumanhistoryismuchlessthan4.5billionminutes.AndyetgeologistsclaimthatEarthhasbeenaroundformorethan4.5billionyears.There’snoeasywaytocomprehendthisdeeptime,butIsometimestryby
takinglongwalks.JustsouthofAnnapolis,Maryland,twentymilesofstately,undulating,fossil-packedcliffsflankthewesternshoreofChesapeakeBay.Walkingalongthenarrowstripofsandbetweenlandandwater,onecanfindanabundanceofextinctclamsandspiraledsnails,corals,andsanddollars.Onceinawhileonaveryluckyday,asix-inchserratedsharktoothorsix-footstreamlinedwhaleskullturnsup.Theseprizedrelicstellofatimefifteenmillionyearsago,whentheregionwaswarmerandmoretropical,liketoday’sMaui,andmajesticwhalescametocalveandmonstersharkssixtyfeetlongfeastedontheweak.Thefossilspopulatethreehundredverticalfeetofsediments,laiddownoverthreemillionyearsofEarthhistory.Thelayersofsandandmarldipeversogentlytothesouth,sowalkingalongthebeachislikeastrollthroughtime.Eachstridetothenorthexposesslightlyolderlayers.TogetasenseofthescaleofEarthhistory,imaginewalkingbackintime,a
hundredyearsperstep—everypaceequaltomorethanthreehumangenerations.Amiletakesyou175,000yearsintothepast.ThetwentymilesofChesapeake
cliffs,ahardday’swalktobesure,correspondtomorethan3millionyears.ButtomakeevenasmalldentinEarthhistory,youwouldhavetokeepwalkingatthatrateformanyweeks.Twentydaysofeffortattwentymilesadayandahundredyearsperstepwouldtakeyouback70millionyears,tojustbeforethemassdeathofthedinosaurs.Fivemonthsoftwenty-milewalkswouldcorrespondtomorethan530millionyears,thetimeoftheCambrian“explosion”—thenear-simultaneousemergenceofmyriadhard-shelledanimals.Butatahundredyearsperfootstep,you’dhavetowalkforalmostthreeyearstoreachthedawnoflife,andalmostfouryearstoarriveatEarth’sbeginnings.Howcanwebesure?Earthscientistshavedevelopednumerouslinesof
evidencethatpointtoanincrediblyoldEarth—totherealityofdeeptime.Thesimplestevidenceliesingeologicalphenomenathatproduceannuallayeringsofmaterial;countthelayers,counttheyears.Themostdramaticsuchgeologicalcalendarsarevarvedeposits,thinalternatinglightanddarklayersthatrepresentcoarser-grainedspringsedimentsandfinerwintersediments,respectively.OnemeticulouslydocumentedsequencefromglaciallakesinSwedenrecords13,527yearsoflayering,withanewlight-darklayerdepositedeveryyear.ThefinelylaminatedGreenRiverShale,whichisexposedinscenicsteep-walledcanyonsinWyoming,featurescontinuousverticalsectionswithmorethanamillionannuallayers.Similarly,icedrillcoresthousandsoffeetdeepfromAntarcticaandGreenlandrevealmorethaneighthundredthousandyearsofaccumulation,yearbyyear,snowlayerbysnowlayer.Alloftheselayeringsrestatopvastlyolderrocks.MeasurementsofslowergeologicalprocessesstretchthetimescaleofEarth
historyevenfurther.ThemassiveHawaiianIslandsrequiredslowandsteadyvolcanicactivity,assuccessivelavablanketspiledup—atleasttensofmillionsofyears,basedonmodernratesoferuption.TheAppalachiansandotherancientroundedmountainrangeswereformedbyhundredsofmillionsofyearsofgradualerosion,andthebarelydetectablemotionsoftectonicplatesthathaveshiftedcontinentsandopenedoceansoperateincyclesofhundredsofmillionsofyearsaswell.Physicsandastronomyprovideevidencefordeeptimethatisnoless
persuasive.Thepredictabledecayratesofradioactiveisotopesofcarbon,uranium,potassium,rubidium,andotherelementsareexceptionallyaccurateclocksfordatingrock-formingeventsthatextendbackbillionsofyearstotheformationoftheSolarSystem.Ifyouhaveacollectionofonemillionatomsofaradioactiveisotope,halfofthemwilldecayoveraspanoftimecalledthehalf-life.Leavebehindamillionatomsofuranium-238,forexample,andcomeback
whenitshalf-lifeof4.468billionyearshaspassed,andyou’llfindonlyaboutahalf-millionatomsofuranium-238remaining.Therestoftheuraniumwillhavedecayedtoahalf-millionatomsofotherelements,ultimatelytostableatomsoflead-206.Waitanother4.468billionyearsandonlyaboutaquarter-millionatomsofuraniumwillremain.Theagedeterminationsoftheoldestprimitivechondrites—4.566billionyearsold—areobtainedbythisradiometricdatingmethod.ButwhatofthemanybillionsofyearsbeforetheSolarSystem?
Astrophysicists’measurementsofdistantgalaxiesinmotionpointtoauniversethatismucholderthan4.5billionyears.Allgalaxiesarespeedingawayfromus.Dopplershiftdata—theso-calledredshift—revealthatthemoredistantgalaxiesareretreatingevenfaster.Playthatcosmictapebackward,andeverythingconvergesdowntoapointabout13.7billionyearsago.That’stheBigBang.Thelightfromsomeofthesemostdistantobjectshasbeentravelingthroughspaceformorethan13billionyears.Thedataonthispointareunassailable.AnyclaimthatEarth’sageisten
thousandyearsorlessdefiestheoverwhelmingandunambiguousobservationalevidencefromeverybranchofscience.Theonlyalternativeisthatthecosmoswascreatedtenthousandyearsagotolookvastlyolder—aconclusionfirstexpoundedbyAmericannaturalistPhilipGossein1857,inhisconvolutedtreatiseOmphalos(namedaftertheGreekwordfor“navel,”becausemotherlessAdamwascreatedwithanavel,soastolookasifhewerebornbywoman).GossecatalogedhundredsofpagesofevidenceforanextremelyancientEarthandthenproceededtodescribehowGodcreatedeverythingtenthousandyearsagotolookmucholder.SomemayfindcomfortinthisCreationistloopholeofcreatedantiquity,
knownasprechronism.Toastrophysicists’observationsofstarsandgalaxiesthatarebillionsoflight-yearsaway,prechronistsretortthattheuniversewascreatedwithlightfromthosestarsandgalaxiesthatwasalreadyonitswaytoEarth.Rockswithancientratiosofradioactiveanddaughterisotopes,theyargue,werecreatedwithjusttherightmixturesofuranium,lead,potassium,andargontomakethemappearmucholderthantheyreallyare.Ifyouareofthisprechronistpersuasion,Isuggestyouskipaheadtochapter11,“TheFuture.”Otherwise,allowyourimaginationtoskipbackseveralbillionyearsintothepast,whenourplanetwasborn.ThebirthofEarth4.5billionyearsagowasadramathathasbeenrepeated
countlesstrillionsoftimesthroughoutthehistoryoftheuniverse.Everystarandplanetarisesinthetenuousnear-vacuumofspacefromgasanddust—individual
particlesofmattertoosmalltoseewiththeunaidedeye,yetsovastintotalextentthatwecanobserveimmensestar-formingcloudshalfwayacrossthegalaxy.Billionsofyearsago,gravityservedtomidwifetheSolarSystem’sbirth—theSunemergedasthesolitarygiantinalitterofplanetaryrunts.NuclearreactionsinflamedtheSun’ssurface,bathingitsplanetaryneighborsinlightandwarmth.Andsoourhometookitsfirsthaltingstepstobecomingalivingworld.Asalienassuchepiceventsmightseem,weallexperience,everydayofour
lives,thesamecosmicphenomenathatledtoEarth’sformation.TheverysameelementsandatomsthatforgedEarthalsomakeupourbodiesandourdwellings.Thesameuniversalforceofgravitythatassembledthestarsandplanetsfromdustandgas,andthatforgedtheelementsintostars,alsoholdsusfasttoourplanetaryhome.Whenitcomestotheuniversallawsofphysicsandchemistry,thereisnothingnewundertheSun.Thelessonsofrocks,stars,andlifeareequallyclear.TounderstandEarth,you
mustdivorceyourselffromtheinconsequentialtemporalorspatialscaleofhumanlife.Weliveonasingletinyworldinacosmosofahundredbilliongalaxies,eachwithahundredbillionstars.Similarly,welivedaybydayinacosmosagedhundredsofbillionsofdays.Ifyouseekmeaningandpurposeinthecosmos,youwillnotfinditinanyprivilegedmomentorplacetiedtohumanexistence.Thescalesofspaceandtimearetooinconceivablylarge.Butacosmosboundbynaturallawsthatleadinevitably,inexorablytoauniversethatpromisesthepossibilityofknowingitself,asscientificstudyinherentlysuggests,isacosmosthataboundswithmeaning.
Chapter2
TheBigThwack
TheFormationoftheMoonEarth’sAge:0toabout50millionyears
Acentralprincipleofthisbookisthatplanetarysystemsevolve—theychangethroughtime.What’smore,eachnewevolutionarystagedependsonthepriorsequenceofstages.Changesareoftengradual,takingmillionsorevenbillionsofyearstotransformaplanet’senvironment,butsuddenviolentandirreversibleeventscanforeveralteraworldinminutes.SoitwaswithEarth.Fromcountlessscatteredbitsandpieces,Earthformedrelativelyquickly,in
nomorethanamillionyearsbysomeestimates.Towardtheendofthisprocessafewdozenplanetesimals,eachseveralhundredmilesindiameter,sharedspacewiththeproto-Earth.Inaspanofaboutahundredthousandyears,asourplanetapproacheditspresentsize,thefinalstagesofthisprocessoccurredinepisodesofunfathomableviolence.Onceeveryfewthousandyears,aminiplanetsmackedintotheproto-Earthandwasswallowedwhole.Duringthatturbulenttime,Earthwasahot,blackenedsphere,punctuatedwith
glowingredcracks,toweringvolcanicmagmafountains,andincessantmeteorimpacts.Eachofthegiantimpactorssmashedintothesphere,blastingvaporizedrocksintoorbitanddisruptingtheentiresurfaceintoamolten,red-hotrockyslush.Spaceiscold,however.Followingeachgreatimpact,Earth’sairlesssurfacequicklycooledandblackenedagain.
StrangeMoon
ThisstoryofEarth’soriginsseemstobeneatandtidyexceptforonestrikingdetail:theMoon.It’swaytoobigtoignoreand,formuchofthepasttwocenturies,hasprovenexceedinglydifficulttoexplain.Smallmoonsareeasytounderstand.PhobosandDeimos,thetwoirregularcity-sizerocksorbitingMars,appeartobecapturedasteroids.ThedozensofmuchlargermoonsorbitingJupiter,Saturn,Uranus,andNeptuneareteenybycomparisontotheirhosts—muchlessthanathousandthofthemassoftheplanetstheycircle.Thelargest
moons,formedfromunclaimedremnantsoftheoriginalplanet-formingdustandgas,orbitthesegasgiantslikeplanetsinminiaturesolarsystems.Earth’sMoon,bycontrast,isrelativelyhugecomparedwiththeplanetitorbits:ithasmorethanaquarterofEarth’sdiameterandaboutone-eightiethofitsmass.Wheredidsuchananomalycomefrom?Thehistoricalsciences,especiallyEarthandplanetarysciences,dependon
creativestorytelling(albeitstoriesthatmoreorlessconformtothefacts).Ifmorethanonestoryseemstofittheobservations,thengeologistsadoptacautiousstanceknownas“multipleworkinghypotheses”—astrategyfamiliartoanyonewhoenjoysdetectivenovels.PriortothehistoricApolloMoonlandingsbeginningin1969,whenpristine
MoonrockswererecoveredandcarefulgeophysicalmeasurementsoftheMoon’sinteriorcouldbegin,threeprimesuspectsstoodoutinTheCaseoftheMassiveMoon.Thefirstwidelyacceptedscientifichypothesiswasthefissiontheory,proposedin1878byGeorgeHowardDarwin(whoisfarlessfamousthanhisnaturalistfather,Charles).InGeorgeDarwin’sscenario,theprimordialmoltenEarthwasspinningonitsaxissorapidlythatitstretchedandelongateduntilitflungoffaglobofmagmafromthesurfaceintoorbit(withalittlehelpfromtheSun’sgravitationalpull).TheMoon,inthismodel,isanEarthbudbrokenfree.Inoneimaginativevariantofthisdramatictale,thePacificOceanbasinremainsasatelltalemark—MotherEarth’sbirthingscar.Asecondcompetingidea,thecapturetheory,viewedtheMoonasaseparately
formed,smallerplanetesimaloccupyingmoreorlessthesamezipcodeasEarth,intheemergingSolarSystem.Atsomepoint,thetwobodiespassedcloseenoughtoeachotherthatlargerEarthcapturedsmallerMoon,swingingitintoaloopingorbitthathasgraduallysettleddown.ThatgreedygravitationalmechanismseemedtoworkwellenoughforthesmallerrockymoonsofMars,sowhynotforEarth?Thethirdhypothesis,theco-accretiontheory,positedthattheMoonformed
moreorlessinitspresentlocationfromalargecloudofleftoverdebristhatremainedinorbitaroundtheEarth.ThisplausibleideamimicswhatweknowabouttheSunanditsplanets,aswellasthegasgiantplanetsandtheirmoons.It’sacommontheme,seenoverandoveragainintheSolarSystem:smallerobjectsaccretefromcloudsofdust,gas,androcksaroundlargerobjects.Threecompetinghypotheses;whichoneiscorrect?Inquiringmindshadto
awaitdatafromtheMoonrocks—morethan840poundsofsamplesfromsixApollolandingsites.
TouchdownontheMoon
TheApolloMoonmissionstransformedplanetaryscienceinmanyways.Sure,theywereanunrivaledshowcaseforAmericantechnologicalprowessandbravado.Undoubtedly,theyprovidedatremendousboosttothemilitary-industrialcomplex.Andtheyinspiredcountlessinnovations,fromminicomputerstopolymerstoTang,providinganeconomicdriverthatmaywellhavepaidforthe$20billionworthofmissionsmanytimesover.It’snotsurprisingthatnationalprideandtheraceforthe“highground,”notlunarscience,weretheprimaryincentivesforthosecostlyanddangerousearlyMoonmissions.Evenso,itwouldbehardtooverstatetheimpactoftheApollomissionsand
theirtreasuretroveofMoonrocksonmygenerationofEarthscientists.Forallofhumanhistory,theMoonwastantalizinglyclose,justunderaquarter-millionmilesaway.Onaclearsummer’sevening,asthereddenedfullMoonrises,youfeelasifyoucouldjustreachoutandtouchit.Butwehadnosamples—nothingtotellusforsureofwhattheMoonwasmade,norwhen,norwhere.Withthefirstbatchoflunarsamples,wecould,forthefirsttimeinhumanhistory,literallytouchtheMoon(ascananyvisitortotheSmithsoniantoday).Myliteralfirstbreathoflunarsamplescameinthewinterof1969–70,during
mysenioryearatMIT,lessthanhalfayearafterApollo11’shistoricmission.Thestagehadbeensetafewmonthsearlier,onJuly24,1969,whenthefirsthumanstowalkontheMoonreturnedtoEarth.Inthoseearlydaysoflunarexploration,concernsofcontaminationbyalienmicrobesdictatedstrictquarantinepoliciesforastronautsandtheirsamples.SoassoonastheirmodulesplasheddowninthePacificnearHawaii,assoonasNeilArmstrong,BuzzAldrin,andMikeCollinswereretrievedbytheUSSHornet,theyandtheforty-fivepricelesspoundsofrocksandsoiltheyhadbroughthomefromspacewerehermeticallysealedinNASA’sMobileQuarantineFacility.FromHawaii,theywereallshippedtoHouston,tothenewLunarReceivingLaboratory,wherethespaceexplorersandtheirprecioussampleswereconfinedforalmostthreeweeks,incasesomethingreallynastyhadaccompaniedthembacktoEarth.Apollomissionsfollowedfastupononeanotherinthenextthreeyears.The
Apollo12lunarmoduleIntrepid,withastronautsCharlesConrad,Jr.,andAlanBean,toucheddownonNovember19,1969,andaweeklaterreturnedwithaboutseventypoundsofMoonrocksandsoil,whichwerewhiskedintotheHoustonquarantinefacilities.Bygoodfortune,mythesisadviser,thebrilliantandebullientDavidWones,wasamemberoftheApollo12LunarSamplePreliminaryInvestigationTeam.Thatsmallbandofscientistshadtheglorious
adventureofscrutinizingthesecondpreciouslunarsamplehaulwithastate-of-the-artarsenalofanalyticalmachines.Dave’sexpertisewasigneouspetrology—thestudyoftheoriginofrocksthatformfrommagma.AlloftheApollo11and12Moonrockswereigneousinorigin,sohewasingeologistheaven.Insomeways,itwashardduty,beinglockedupforthebetterpartofamonth
withafewotherintensescientists,underpressuretogatherunassailabledataonsomeofthemostcostlyandsignificantrocksamplesevercollected.Butitwasalsoincrediblyexcitingtobeamongthefirsthumanstohandlerocksandsoilfromanotherworld—thespacematterthatwouldonceandforalltellustheoriginoftheMoon.MyfirstglimpseoftheMoonupclosecameonDave’sreturntoMIT.Irecall
theelevatordooropeningonthetwelfthflooroftheGreenBuilding.TherewasDave,ofmodeststatureandbespectacled,flankedbytwobeefy,uniformed,gun-totingfederalagents.TheywereguardingtheMoonsamples,whichatthatpointwouldhavebeenworthmillionsonthecollectors’market.Everymilligramhadtobeaccountedfor.Davelookedtiredandnervous;hehadbeenawayforalongtime,hewasunderconstantscrutiny,andhestillhadajobtodo.Whenthesubjectoflunarsamplescomesup,mostpeopleimmediatelythink
ofMoonrocks,perhapssomethingchunkythatyoucanholdinyourhand.ButasignificantportionoftheApollomaterialwaslunarsoil,orregolith.Thefinest-grainedfractionofregolithispulverizedrockinfragmentssosmallthatyoucan’tresolvetheminamicroscope—theconsequenceofabatteryofcosmicinsults,frommightyasteroidstotheincessantsolarwind.Thisultrapowderhasstrangeproperties,mostnotablythatitstickstoeverythingittouches,likeXeroxtoner.Dave’staskwastotransfersomeofthispowderfromavialaboutthesizeofaCbatteryintothreeorfoursmallervials,aboutthesizeofAAAbatteries,fordistributiontonearbylaboratories.Itsoundseasy.Dumpthepowderfromthebigvialontoathree-inch-square
pieceofglassy-surfacedpowderpaper.Gentlyscoopsmallamountsintothesmallervials.Davehadperformedsimilaroperationshundredsoftimes,anditshouldn’thavetakenmorethanaminute.Butthestakeswerehigherhere.Humorlessguardswerestandingtoeitherside;asmallcohortofstudentswashovering,too.SoDave’shandshookabitashetippedthebigvial.Thestickypowderclungtotheglasssidesanddidn’twanttocomeout.Hetappeditwithhisindexfinger.Nothing.Tappedagain.ThenallofasuddenalltheMoondust—reallyonlyalittlepilethesizeofa
Hersheykiss,butitseemedlikealotunderthecircumstances—cameshlumpingoutallatonce,andthenpoof!Dustflewup,coatedDave’sfingers,andspilled
overtheedgeofthepowderpaperontothetable.Weallmusthavebreathedinsomeofthefinestairborneparticles.Noonesaidaword.Itwasn’tarealdisaster,asalmostnothingwaslost,andthepowderdid
eventuallygettransferred,andthefedsdideventuallyleavetohandoffthealiquotstootherlabs.Inretrospect,weallthoughtitwasprettyfunny.Andinacoupleofdays,abovethelabbenchwherethetransferhadbeencompleted,weneatlyframedthatthree-inchsquareofpowderpaperwithanear-perfectimprintofDaveWones’sleftindexfingerinMoondust.
FourmoreApolloMoonlandingsfollowed,culminatinginDecember1972withApollo17andthereturnofmorethan240poundsofsamplesfromtheTaurus-LittrowValley,aregionofsuspectedlunarvolcanism.Thatwasthelastmission;noonehasgonebackinfourdecades.Nevertheless,theMoonrocks,meticulouslycuratedinsterilevaultsattheLunarSampleBuildingatNASA’sJohnsonSpaceCenterinHouston(withasecurebackupcollectionatBrooksAirForceBaseinSanAntonio,Texas),continuetoprovideanamazingwealthofopportunitiesforresearchers.AfewyearsafterthelastApollomission,thosesamplesprovidedmemyfirst
realjob,asapostdoctoralfellowwiththeGeophysicalLaboratoryoftheCarnegieInstitution.Mytaskwastoexaminepilesof“fines”fromApollo12,Apollo17,andLUNA20(oneofthreeunmannedSovietmissionsthathadreturnedaboutathirdofapoundoflunarsamples).Interspersedthroughoutthefinedustoflunarsoilarelotsofsilt-andsand-sizegrains,andmyexactingjobwastoscanthousandsofthesegrains,bitbybit.Ispenthoursatamicroscope,peeringatgorgeouslittlegreenandredcrystalsandtinygoldenspheresofcolorfulglass—theremainsofviolentlyblastedrocksthathadbeensubjectedtobillionsofyearsofmeteoritebombardment.OnceI’disolatedafewdozenpromisingspecks,Isubjectedeachunusual
graintothreekindsofanalysis.Thefirstwassingle-crystalX-raydiffraction,totellwhatkindofcrystalIwasdealingwith.Mostofmystudiesfocusedonthecommonmineralsolivine,pyroxene,andspinel.IfIfoundagoodcrystal,I’dcarefullyorientthegrainandmeasureitsopticalabsorptionspectrum(thewayitsoakedupdifferentwavelengthsoflight).Greenolivinecrystals,forexample,typicallyabsorbredwavelengths;redspinelcrystals,bycontrast,absorbmoreinthegreenwavelengths.Ialsomeasuredspectraofanyunusualglassbeads,keepinganeyeoutfortelltalebumpsandwigglesintheabsorptionspectrumthatpointedtorarerelements—chromiumandtitanium,forexample.Discoveryofasmallpeakat625nanometers,aslightabsorptionofred-orangewavelengthscharacteristicoftheelementchromiumasitoccursontheMoonbutquite
differentfromchromiumonEarth,wasamemorable“eureka”moment.Finally,aftertheX-rayandopticalworkwasdone,Iusedafancyanalytical
machinecalledanelectronmicroprobetodeterminetheexactratiosofelementsinmysamples.TimeandtimeagainIconfirmedwhatothershadfound:theMoon’ssurfaceminerals,whilesimilarinmajorelementstothoseonEarth,areratherdifferentindetail.Theyhavemoretitanium;thechromiumisdifferent,too.TheseandothercluesfromtheApollorocksplacedsevereconstraintsonthe
varioustheoriesofhowtheMooncametobe.Foronething,itturnedoutthattheMoondiffersdramaticallyfromEarth,inthatitsdensityismuchlower;itdoesn’thaveabig,denseironmetalcore.Earth’scoreholdsalmostathirdofitsmass,buttheMoon’stinycoreislessthan3percentofitsmass.Second,Moonrockscontainalmostnotracesofthemostvolatileelements—thosethattendtovaporizethemomentthingsgetwarm.Thenitrogen,carbon,sulfur,andhydrogensocommonatEarth’ssurfacearemissingfromMoondust.ThisdeficiencymeansthatunlikeEarth,whichiscoveredinliquidwaterandwhosesoilscontainabundantwater-richmineralssuchasclaysandmicas,nowater-bearingmineralsofanykindhavebeenbroughtbackfromtheApollomissions.SomethingmusthaveblastedorbakedtheMoontoremovethosevolatiles,fortheMoon’ssurfaceisnowanunforgivinglydryplace.ThethirdkeyfindingoftheApollomissionsisbasedontheelementoxygen,
ormorespecificallythedistributionofitsisotopes.Eachchemicalelementisdefinedbythenumberofpositivelychargedprotonsinitsnucleus.Thenumberisunique—oxygenisjustanothernamefor“atomwitheightprotons.”Atomicnucleialsoholdasecondkindofparticle,theelectricallyneutralneutron.Morethan99.7percentofoxygenatomsintheuniversehaveeightneutrons(eightprotonspluseightneutronsyieldsanisotopecalledoxygen-16),whiletherarerisotopeswithnineortenneutrons(oxygen-17andoxygen-18,respectively)arepresentatafractionofapercent.Oxygen-16,oxygen-17,andoxygen-18arevirtuallyidenticalintheir
chemicalbehavior—youcouldbreatheanymixtureandwouldn’tnoticeadifference—buttheydohavedifferentmasses.Oxygen-18isheavierthanoxygen-16.Consequently,anytimeanoxygen-containingcompoundchangesstatefromasolidtoaliquid,orfromaliquidtoagas,thelessmassiveoxygen-16canmakethemovemoreeasily.IntheturbulentnascentSolarSystem,suchchangesofstatewerecommonplace,andtheyledtoshiftingamountsofoxygenisotopes.Itturnedoutthattheratioofoxygen-16tooxygen-18variesfromplanettoplanetandisverysensitivetotheplanet’sdistancefromtheSunwhen
itformed.TheApollorocksrevealedthattheMoon’soxygenisotoperatioisvirtuallyidenticaltoEarth’s.Inotherwords,EarthandtheMoonmusthaveformedataboutthesamedistancefromtheSun.SowheredidthesediscoveriesleavethethreecompetingMoon-forming
hypotheses?Theco-accretiontheorywasintroublefromthestart.IftheMoonformedfromEarthleftovers,thenitshouldhaveasimilaraveragecomposition.True,theMoonandEarthdomatchupintermsofoxygenisotopes,buttheco-accretiontheorycan’texplainthelargedifferencesinironandvolatiles.TheMoon’sbulkcompositionisjusttoodifferentforittohaveformedfromthesamestuffasEarth.Thecompositionaldisparitiesalsoposedinsurmountableproblemsforthe
capturetheory.TheoreticalmodelsofplanetarymotionsuggestthatanycapturedplanetesimalmusthaveformedinthesolarnebulaatmoreorlessthesamedistancefromtheSunasEarth,andsoitshouldhavemoreorlessthesameaveragecomposition.TheMoondoesn’t.Ofcourse,aMoon-sizeobjectmighthaveformedinsomeotherdistrictofthesolarnebulaandsubsequentlyadoptedanEarth-crossingorbit,butcomputermodelsoforbitaldynamicsrequirethatsuchaMoonwouldhavehadahighvelocityrelativetoEarth,makingsuchacapturescenarioallbutimpossible.ThatleavesGeorgeHowardDarwin’sfissiontheory.Itcansuccessfully
explainthesimilaroxygenisotopecompositions(EarthandMoonareonesystem)andtheirondifference(Earth’scorehadalreadyformed;theMoon-formingblobwasachunkofEarth’salreadydifferentiated,iron-poormantle).ItbeautifullyaccommodatesthefactthatonesideoftheMoonalwaysfacesEarth:Earth’srotationandtheMoon’sorbitfollowthesamespinningmotionaroundtheEarth’saxis—thesamesenseofturning.Butabigproblemremains:wherearethemissingvolatiles,nowabsentfromtheMoon?Thelawsofphysicsalsogetinthewayofthefissiontheory.Ataboutthetime
oftheApollomissions,computermodelingofplanetformationhadprogressedtoapointthattheoristscouldstudythedynamicsofarapidlyspinningEarth-sizeballofmagmawithconfidence.Simplyput,fissioncan’twork.Earth’sgravityismuchtoostrongtoallowabigblobofmoltenrocktobetossedoutwardintoorbit.Infact,amoltenEarthwouldhavehadtobespinningonitsaxisatanincrediblerate,aboutonceeveryhour,forittoflingoffaMoon-sizeglob.TheEarth-Moonsystemsimplydoesnothaveenoughangularmomentumforthattohappen.Bottomline:noneofthethreeprevailingtheoriesofMoonformationfitsthe
dataaftertheApollomissions.Theremustbeanotherexplanation.
TheTestimonyoftheMoonRocks
Planetaryscientistsarenothingifnotgoodstorytellers.ObservationsfromApollodisprovedallthreeoftheirpre-1969hypothesesabouttheMoon’sformation,butitdidn’ttakethemlongtocomeupwithanewideafromtheindisputablefacts.NewcompositionalcluesfromApolloprovidedonekey:theMoonmoreorlesslookslikeEarth.Ithasthesameoxygenisotopecompositionandmostofthesamemajorelements,thoughithaswaytoolittleironorvolatiles.Thatcompositionaldatahadtobeintegratedwiththeorbitalclueswe’dknownforthousandsofyears:theMoonorbitsEarthinthesameplaneandinthesamedirectionastheotherplanetsaroundtheSun.Earthdoeshaveaniggling23-degreetiltinitsrotationaxis(that’swhatcausestheseasons).AndonesideoftheMoonalwaysfacesus.Earliermodelsoflunarformationtendedtoignoreorbitalcluesbeyondthe
Earth-Moonsystem,includingsomestrikingexceptionstothegeneralpatternsofourSolarSystem.Foronething,Venusrotatesonitsaxisbackwardcomparedwithalltheotherplanets.Thatmaynotseemsignificant,butVenusisalmostaslargeasEarth—andit’srotatingthewrongway!EvenstrangerismassiveUranus,thethirdlargestplanet,whoserotationaxisissideways,soitseemstokindofrollalongitsorbitaroundtheSun.Themoonsofotherplanets,too,haveoddities.Neptune’slargestmoon,Triton,whichiscomparableinsizetoEarth’sMoon,orbitsatasteepangletotheplanet’srotationandintheoppositedirectionoftherestoftheSolarSystem.Thecultureofsciencehasanoddaspect—onethatmaybeoff-puttingtothose
notinthegame.Ontheonehand,wecomeupwithneattheoriesthatpackagelotsofoddfacts.SothefactthatalltheplanetsandmoonsorbittheSuninthesamedirection,inthesameplane,pointstoacommonoriginfromasingleswirlingnebula.Butthenwefindexceptionstotherule,andthoseexceptionsaresetasideascuriousanomalies.Venusrotatesthewrongway?Tritonorbitsthewrongway?Noproblem.Theiraberrationsareincidentaltothelargerscheme.Thesamekindofsituationcomplicatesmanypublicdebates,likethatover
globalwarming.Manyscientistspredictthatalteredatmosphericconditionswillraisetheaverageglobaltemperaturebyseveraldegrees.Butsuchchangescanalsocauseextremeweather,whichmaymeanworsesnowstormsinthesouthernUnitedStates.GlobalwarmingmayalteroceancurrentsliketheGulfStreamandultimatelyturnnorthernEuropeintoamuchcolderSiberian-typeicebox.Anomalieslikethisfueltheglobalwarmingnaysayers:scientistssaytheworldisgettinghotter,butyou’vejustsufferedthroughthebiggestsnowstorminyour
region’shistory.Howshouldyourespond?Ajudiciousresponseisthatnatureisamazing—rich,varied,complex,andintricatelyinterconnected,withamessy,longhistory.Anomalies,whetherinplanetaryorbitsorNorthAmericanweather,arenotjustinconvenientdetailstobrushaside:theyaretheveryessenceofunderstandingwhatreallyhappened—howthingsreallywork.Wedevelopgrandandgeneralmodelsofhownatureworks,andthenweusetheodddetailstorefinetheoriginalimperfectmodel(oriftheexceptionsoverwhelmtherule,weregrouparoundanewmodel).That’swhygoodscientistsrevelinanomalies.Ifweunderstoodeverything,ifwecouldpredicteverything,there’dbenopointingettingupinthemorningandheadingtothelab.InthecaseoftheoriginofEarth’sMoon,thoseexceptionstothesystematic
trends—thosenigglingorbitalanomalies—ledtotheconceptofthe“BigSplash”or“BigThwack”model,whicharoseinthemid-1970s.Theoriginalseriesofrelatedbutpoorlyconstrainedhypothesescoalescedintoconventionalwisdomatapivotal1984conferenceinHawaii,whereplanetaryformationexpertsgatheredtoweighalltheiroptions.Insuchaheadyenvironment,Ockham’srazor—thedemandthatthesimplestsolutiontoaproblemconsistentwiththefactsislikelytobecorrect—prevails.TheBigThwackfitthebill.Tounderstandthisradicalidea,thinkbackmorethan4.5billionyears,tothe
timewhentheplanetshadjustformedfromallthosesmallercompetingplanetesimals.AsEarthgrewclosetoitspresentdiameterofeightthousandmiles,itswallowedupalmostalltheremainingproximatebodiesinasuccessionofhugeimpacts.Thosepenultimatecollisionswithobjectsmanyhundredsofmilesacrosswouldhavebeenspectacular,buttheyhadlittleeffectonEarth,themuchmoremassiveproto-planet.Butnotallimpactsareequal.InEarthhistory,onesingleevent—oneday
morememorablethananyother—standsout.About4.5billionyearsago,whentheSolarSystemwasabout50millionyearsold,theblackproto-Earthandaslightlysmallerplanet-sizecompetitorwerejockeyingforthesamenarrowbandofSolarSystemrealestate.Thesmallerwould-beplanet(dubbedTheia,aftertheTitangoddesswhogavebirthtotheMoon)wasworthyofplanetarystatus—perhapstwotothreetimesthesizeofMars(orroughlyathirdofEarth’smass).Aruleofastrophysicsisthatnotwoplanetscansharethesameorbit.Eventuallytheywillcollide,andthelargerplanetalwayswins.SoitwaswithEarthandTheia.Increasinglyvividcomputersimulationsprovidetheprincipalmethodby
whichscientistsattempttounderstandwhatmighthavehappened.Abigcollisionisgovernedbythelawsofphysics,soonecanrunthousandsof
simulationswithallsortsofinitialconditionstoseeifaMoonresults.Theanswerisintimatelytiedtothestartingparameters:themassandcompositionoftheproto-Earth,themassandcompositionofTheia,theirrelativevelocities,andtheangleandaccuracyoftheblow.Mostcombinationssimplydonotwork;noMoonforms.ButafewmodelsaresurprisinglysuccessfulandproduceanEarth-Moonsystemratherliketheoneweseetoday.Inoneoft-describedversion,theimpactoccursasasolidsideswipe—big
TheiasmashesthebiggerEarthslightlyoffcenter.Seenfromspace,theeventplaysoutinslowmotion.Atthemomentofcontact,thetwoworldsseematfirsttogentlykiss.Thenoverthenextfourorfiveminutes,Theiaissmooshed,likeaballofsoftdoughhittingthefloor,withoutmucheffectonEarth.TenminuteslaterTheiaisprettymuchsquashed,whileEarthbeginstodeformoutofroundness.Halfanhourintothecollision,Theiaissimplyobliterated,whiletheinjuredEarthisnolongerasymmetricalsphere.Superhotrockhasbeenvaporized,blastingoutinluminousstreamsfromthegapingwoundandobscuringthedisruptedworlds.Anotherwidelycitedscenario,firstproposedinthe1970sandrefinedoverthe
nexttwodecades,wasdevelopedbytheoristAlastairCameronoftheHarvard-SmithsonianCenterforAstrophysics.Inhisintriguingscheme,Theiawasroughly40percentthemassoftheproto-Earth.Again,anoff-centerimpactoccurred,butinthisversion,TheiamoreorlessbumpedagainstEarthandbouncedoffasanelongatedblob,thenwaspulledbackinforthecoupdegrâce—asecondthwack,inwhichTheiadisappearedforever.Ineithercase,thecatastropheannihilatedTheia,whichsimplyvaporizedinto
animmenseincandescentcloud,tensofthousandsofdegreeshot,surroundingEarth.Theiahaddoneitsshareofdamageaswell.AsignificantchunkofEarth’scrustandmantlealsovaporizedandblastedoutwardtomixwithTheia’sscatteredremnants.Somematerialescapedtodeepspace,butmostofthesavagedremainswereretainedinorbitbyEarth’sunyieldinggravitationalgrip.Fromthisroilingcloud,densemetalfromthecoresofbothworldscommingledandcooledbackintoliquid,sinkingtoformanew,largercoreforEarth.Mantlematerialsalsomixedandvaporized,formingahellishlyhotglobe-encirclingcloudofvaporizedrock.Foraviolenttimeofdaysorweeks,Earthexperiencedanincessantrainoforange-hotsilicatedroplets,whichmergedwithashoreless,red-glowingmagmaocean.UltimatelyEarthseizedmuchofwhathadbeenTheiaandthusemergedamoremassiveplanet.NotallofTheiawascaptured,however.Higherupinspace,Earthbecame
encircledbyavastaccumulationofrockycollisionaldebris,mostlyanintimate
mixtureofthetwoplanetarymantles.Coolingrockydropletsstucktogether,withbiggerchunkssweepingupthesmaller.Inasortofinstantreplayofthegravitationalclumpingthatoriginallyformedtheplanets,theMooncoalescedrapidlyandmayhaveachievedmoreorlessitspresentsizeinafewyears.ThephysicsofplanetformationdictatewheretheMooncouldhaveformed.
Everymassiveobjecthasaninvisiblesurroundingsphere,calledtheRochelimit,insideofwhichgravitationalforcesaretoogreatforasatellitetoform.That’swhySaturnhasimmenseringsbutnomoonswithinaboutfiftythousandmilesofitssurface.Saturn’sgravitypreventsthoseicyparticlesfromcoalescingtoformamoon.Calculatedfromthecenterofarotatingobject,Earth’sRochelimitisabout
11,000miles,orroughly7,000milesupfromthesurface.Accordingly,modelsofMoonformationlocatethenewsatelliteatasafedistanceofabout15,000milesup,whereitcouldgrowinanorderlyfashionbysweepingupmostofthescatteredbitsandpiecesfromtheBigThwack.Andso,perhaps4.5billionyearsagobymostestimates,theMoonwasborn.Earthfounditselfwithacompanion,formedinlargepartfrompiecesofitself.ScientistsquicklyembracedtheBigThwacktheorybecauseitexplainsallthe
majorcluesbetterthananyothermodel.TheMoonlacksanironcorebecausemostofTheia’sironwoundupinsideEarth.TheMoonlacksvolatilesbecauseTheia’svolatileswereblastedawayduringtheimpact.OnesideoftheMoonalwaysfacesEarthbecausetheangularmomentumofEarthandTheiawerecoupledintoonespinningsystem.TheBigThwackalsohelpstoexplainEarth’sanomalousaxialtiltofabout23
degrees—afactornotwellhandledbyanyofthepreviousscenarios.TheimpactofTheialiterallytippedEarthontoitsside.Indeed,therealizationthatagiantimpactformedtheMoonhasledtospeculationaboutotherplanetaryanomaliesinourSolarSystem.PerhapslateBigThwackeventsofonekindoranotherarecommon,evennecessary.PerhapsthatexplainswhyVenusrotatesthewrongwayonitsaxisandwhyitlostsomuchofitswater.PerhapsalategiantimpactcausedUranustorotateonitsside.
ADifferentSky
TheMoon’sformationwasapivotalmomentinEarthhistory,withfar-reachingconsequencesthatareutterlyamazingandonlyjustnowcomingintofocus.TheMoonof4.5billionyearsagowasnottheromantic,silverydiskweseetoday.Longagoitwasafarmorelooming,dominant,andunimaginablydestructive
influenceonEarth’snear-surfaceenvironment.Itallboilsdowntooneamazingfact:theMoonformedonly15,000miles
fromEarth’ssurface—notmuchfartherthanaplaneflightfromWashington,D.C.,toMelbourne,Australia.Today,bycontrast,theMoonis239,000milesaway.Atfirstblush,itseemsutterlyimplausiblethatagiantMooncouldjustdriftawayfromEarthlikethat,butmeasurementsdon’tlie.Apolloastronautsleftshinymirrorsonthelunarsurface.LaserbeamsfromEarthbounceoffthemirrorsandreturntoEarthtoprovidedistancemeasurementsaccuratetowithinatinyfractionofaninch.Everyyearsincetheearly1970s,theMoonhasmovedfartheraway:3.82centimetersperyear,aboutoneandahalfinchesperyearonaverage.Thatdoesn’tsoundlikemuch,butitaddsupovertime—toamilefartherawayeveryfortythousandyearsatpresentrates.Playthetapebackward,anditpointstoaradicallydifferentsituation4.5billionyearsago.Foronething,theMoonlookedtotallydifferent.At15,000milesdistant,the
2,160-mile-diameterMoonwouldhaveappearedgigantic,likenothingwe’veeverseen.Itspannedalmost8degreesofarcinthesky—roughlysixteentimestheapparentdiameteroftheSun—andblockedmorethan250timesasmuchofthefirmamentastheMoondoestoday.Andthat’snotall.TheearlyMoonwasaviolentbodyofintensevolcanism,
quiteunlikethestaticsilvery-grayobjectweseenow.Itssurfacewouldhaveappearedblack,withglowingredmagma-filledcracksandvolcanicbasinseasilyvisiblefromEarth.TheprimordialfullMoonwasequallydramatic,thesurfacereflectinghundredsoftimesmoresunlightthaninourmodernera.Youcouldeasilyreadabookunderitsbrilliantillumination,butastronomicalobservationswouldhavebeenfutile.NostarsorplanetswouldhavebeenvisibleagainsttheyoungMoon’sintenseglare.Addingtothedramawashowfastthingsmovedthen.Inspace,thereisno
friction,sospinningobjectsjustkeepspinningforbillionsofyears.ThetotalamountofspinningenergyoftheEarth-Moonsystem—itsangularmomentum—ismeasuredbyacombinationoftwofamiliarcircularmotions.FirstisEarth’srotationaboutitsaxis;thefasterEarthrotates,themoreangularmomentumithas.TheMoon’sangularmomentum,bycontrast,dependsprimarilyonhowfarawayandhowfastitorbitsaroundEarth.Itsownrotationisnotasignificantpartoftheequation.ThetotalangularmomentumofEarth’srotationplustheMoon’sorbithasn’t
changedsignificantlyoverthelastseveralbillionyears,buttherelativeimportanceofthosetwomotionshaschangedalot.TodayalmostalltheangularmomentumoftheEarth-MoonsystemistiedupintheorbitingMoon,withits
239,000-miledistanceandtwenty-nine-dayorbitalperiod.ThemoremassivecentralEarth,withitsleisurelytwenty-four-hourday,hasonlyatinyfractionoftheMoon’sangularmomentum.(Bythesametoken,thedistantgasgiantplanetscarryalmostalltheSolarSystem’sangularmomentum,eventhoughthecentralSunhas99.9percentofthemass.)But4.5billionyearsago,thingswereverydifferent.WiththeMoononly
15,000milesaway,everythingwasturningridiculouslyfast,liketheice-skaterwho’spulledinherarmstospeedupherspin.Foronething,Earthrotatedonitsaxisonceeveryfivehours.Itstilltookafullyear(about8,766hours)togoaroundtheSun;thattimehasn’tchangedmuchinthehistoryoftheSolarSystem.Butthereweremorethan1,750shortdaysperyear,andtheSunroseeveryfivehours!Suchanestimateseemsbizarreanduntestable,butatleastacoupleofdirect
measurementsconfirmthisideaofshorterancientdays.Coralreefsareonecompellingformofevidence.Somespeciesofcoraldisplayexceedinglyfine-scalegrowthlinesthatrecordbothsubtledailyandmoreobviousannualcycles.Asexpected,moderncoralsshowabout365dailylinesforeveryyearofgrowth.ButancientfossilcoralsfromtheDevonianPeriod,aboutfourhundredmillionyearsago,displaymorethanfourhundreddailylinesperyear,pointingtoafasterrotationrate.Dayswereonlyabouttwenty-twohourslongbackthen,whentheMoonwasperhapstenthousandmilesclosertoEarth.Asecond,complementarymeasurementrestsontheeuphonicphenomenonof
tidalrhythmites,whicharefinelylayeredsedimentsthatrevealthedaily,lunar,andyearlycyclesofthetides.Exactingmicroscopicstudiesoftidalrhythmitesfromnine-hundred-million-year-oldrocksatBigCottonwoodCanyon,Utah,pointtoaworldwhenEarthdayswereonly18.9hourslong,whentheremayhavebeen464days—464sunrisesandsunsets—everyyear.ThecalculatedEarth-Moondistanceof218,000milesatthattimeimpliesarecessionrateverysimilartothatofmoderntimes:3.91centimetersperyear,slightlymorethanoneandahalfinchesannually.
LoonyWorld
NodirectevidenceyetdocumentsEarth’stidalcyclesmorethanabillionyearsago,butwecanbeconfidentthat4.5billionyearsagothingswerealotwilder.NotonlydidEarthhavefive-hourdays,butthenearbyMoonwasmuch,muchfasterinitscloseorbit,aswell.TheMoontookonlyeighty-fourhours—threeandahalfmoderndays—togoaroundEarth.WithEarthspinningsofastandthe
Moonorbitingsofast,thefamiliarcycleofnewMoon,waxingMoon,fullMoon,andwaningMoonplayedoutinfreneticfast-forward:everyfewfive-hourdayssawanewlunarphase.Lotsofconsequencesfollowfromthistruth,somelessbenignthanothers.
Withsuchabiglunarobstructionintheskyandsuchrapidorbitalmotions,eclipseswouldhavebeenfrequentevents.Atotalsolareclipsewouldhaveoccurredeveryeighty-fourhoursatvirtuallyeverynewMoon,whentheMoonwaspositionedbetweenEarthandtheSun.Forsomefewminutes,sunlightwouldhavebeencompletelyblocked,whilethestarsandplanetssuddenlypoppedoutagainstablacksky,andtheMoon’sfieryvolcanoesandmagmaoceansstoodoutstarklyredagainsttheblacklunardisk.Totallunareclipsesoccurredregularlyaswell,almosteveryforty-twohourslater,likeclockwork.DuringeveryfullMoon,whenEarthliesrightbetweentheSunandtheMoon,Earth’sbigshadowwouldhavecompletelyobscuredthegiantfaceofthebrightshiningMoon.Onceagainthestarsandplanetswouldhavesuddenlypoppedoutagainstablacksky,astheMoon’svolcanoesputontheirruddyshow.MonstertideswereafarmoreviolentconsequenceoftheMoon’sinitial
proximity.HadbothEarthandtheMoonbeenperfectlyrigidsolidbodies,theywouldappeartodaymuchastheydid4.5billionyearsago:15,000milesapartwithrapidrotationalandorbitalmotionsandfrequenteclipses.ButEarthandtheMoonarenotrigid.Theirrockscanflexandbend;especiallywhenmolten,theyswellandrecedewiththetides.TheyoungMoon,atadistanceof15,000miles,exertedtremendoustidalforcesonEarth’srocks,evenasEarthexertedanequalandoppositegravitationalforceonthelargelymoltenlunarlandscape.It’sdifficulttoimaginetheimmensemagmatidesthatresulted.EveryfewhoursEarth’slargelymoltenrockysurfacemayhavebulgedamileormoreoutwardtowardtheMoon,generatingtremendousinternalfriction,addingmoreheatandthuskeepingthesurfacemoltenfarlongerthanonanisolatedplanet.AndEarth’sgravityreturnedthefavor,bulgingtheEarth-facingsideoftheMoonoutward,deformingoursatelliteoutofperfectroundness.TheseepictidaldisruptionslieattheheartofwhytheMoonkeepsmoving
awayfromEarth.Howdoesa2,160-mile-wideobjectdriftfromamere15,000milesto239,000milesaway?Theanswerisfoundintheconservationofangularmomentum—theconstantsumofEarth’srotationalenergyplustheMoon’sorbitalenergy.ThelawsofphysicsdictatethatwhateverangularmomentumtheEarth-Moonsystemhadatitsorigin,itmuststillpossessinlargemeasuretoday.Fourandahalfbillionyearsago,agreattidalbulgesweptaroundplanetEarth
everyfewhours.ButbecauseEarth’ssurfacerevolvedarounditsaxisfaster
(everyfivehours)thantheMoonorbitedaroundthatsameaxis(everyeighty-fourhours),thetidalbulgewithitsextramasswasalwaysinthelead,constantlypullingontheMoonwiththeforceofgravity,makingitgofasterandfasterwitheveryorbit.Theimmutablelawsofplanetarymotion,firstproposedaboutfourhundredyearsagobytheGermanmathematicianJohannesKepler,statethatthefasterasatelliteorbits,thefartherawayithastobefromitscentralplanet.ButiftheMoonorbitsEarthfasterandfaster,andthusdriftsfartherandfartheraway,itmustalsogainangularmomentum.AtthesametimethatEarth’stidalbulgepulledtheMoonahead,thetidally
deformedMoonpulledbackonEarth’smassivebulgewithequalandoppositegravitationalforce,thusmakingEarthrotatemoreslowlyonitsaxiswitheveryrotation.That’swhereconservationofangularmomentumcomesin.ThefastertheMoonorbited,thefartherithadtobefromEarthandthemoreangularmomentumitpickedup.Tocompensate,EarthhadtorotateevermoreslowlyonitsaxistoconservethetotalangularmomentumoftheEarth-Moonsystem—again,thinkofthefigureskater,stretchingoutherarmsoncemoreandslowingherspin.Overthespanof4.5billionyears,Earth’srotationhasslowedfromonceeveryfivehourstoonceeverytwenty-fourhours,whiletheMoonhasmovedfartherawayandpickedupalotofangularmomentumintheprocess.Noteveryplanet-moonsystemhastofollowthisstoryline.Iftheplanetrotates
onitsaxismoreslowlythanitsmoonorbits,thenaninexorablebrakingeffectensues.Tidalbulgesontheplanetwilltrailbehind;themoonwillslowdownwitheachorbitandfalleverclosertoitsdoom.Eventuallythemoonwillspiralintotheplanetandbeswallowedup,inyetanothervariationontheBigThwacktheme.Perhapsthat’swhyVenus,withitswrong-wayretrograderotation,doesn’thaveamoon.Perhapssuchacataclysmicdemiseofaonce-orbitingmoonexplainswhyVenuslostitswaterandisnowahostile,scalding,lifelessworld.
EarlyinthehistoryoftheEarth-Moonsystem,theseexchangesofangularmomentumfromtheslowingEarthtotheacceleratingMoonwerevastlygreaterthantoday.InthefirstcenturiesaftertheMoon’sformation,bothbodiesweregirdledbyturbulentmagmaoceansthatcouldflexanddeform.ThegiantmagmatidesonEarth,andsimilarmagmabulgingontheMoon,probablycausedtheMoontorecedebytensorhundredsoffeetperyear,evenasEarth’srotationsteadilysloweddownfromitsinitialfreneticpace.Buttheseenormouslandtidescouldnothavelastedforlong.AstheEarth-Moondistanceincreased,thetidalforcesdecreasedevenmore:adoublingofthedistancecuttheforceofgravitybyafactoroffour.Atriplingofthedistanceandgravitationalforces
werebutaninthoftheirformerstrength.Repeatedtidalstressingdelayedbutcouldnotstopthesolidificationof
worlds.WithinafewmillionyearsoftheBigThwack,thesurfacesofbothEarthandtheMoonwerepavedwithhardblackrock.Landtides—thedeformationofsolidrock—werestillnottrivialinthoseearlydays,buttheywerenothinglikethemightydailyswellingsofthemagmaseathatprecededthem.
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TheMoonremainsaluminousreminderthatthecosmosisaplaceofintertwinedcreationanddestruction.Eventodaywearenotimmunetocatastrophiccosmicinsults:killerasteroidsandcometsstillcrossEarth’sorbitfromtimetotime.Millionsofyearsagoonebigrockkilledthedinosaurs;millionsofyearsfromnow,otherbigrockswillinevitablyfindtheirmark.Ifhumansurvivalisourgreatestcollectiveimperativeasaspecies,thenwewoulddowelltokeepwatchingtheskies,forournearestcosmicneighboroffersmutetestimony:whilechangeisusuallygradualandbenign,therecanbereallybaddays.
Chapter3
BlackEarth
TheFirstBasaltCrustEarth’sAge:50to100millionyears
Earthhassufferedmorethanafewtransformativeeventsinitslonghistory.TheBigThwackwassurelythemostdisruptiveand,intheconsequentformationoftheMoon,hadperhapsthemostfar-reachingeffects.Butsuchanoutcome—alargesolitaryMoonorbitingaplanetfullofvolatiles—isbynomeansaninevitableoutcomeofthelawsofchemistryandphysics.HaddetailsofthatancientinteractionbetweenEarthandTheiaunfoldedwithonlyslightvariations,theMoon-formingepisodecouldhaveturnedoutverydifferent.Hadtheimpactbeenbetteraimed,head-onanddeadcenter,muchmoreofTheia’smasswouldhavewoundupaspartofEarth.Inalllikelihood,wewouldnothaveasatellite,asTheiaandEarthwouldhavemergedintoonelargermoonlessworld.OrhadTheiajustmissedEarth,itsorbitmighthavebeensoalteredastobeflunginwardtowardVenusoroutwardtowardMars,perhapstoleaveEarth’sneighborhoodforever.Andhadtheimpactbeenmoreglancing,thedistributionofscattereddebrismighthaveproducedmultiple,albeitmuchsmaller,moonstograceEarth’snightsky.Chanceplaysasignificantrolethroughoutourdynamiccosmicneighborhood.
OurSolarSystem’shistoryisalitanyofthwacksandnear-misses.Theasteroidthathelpedtokilloffthedinosaursmightjustaswellhavebeenoffthemark,savingTyrannosaurusanditsdescendantstoevolvefortensofmillionsmoreyears.Perhapsbig-brainedbirdswouldhavebecomeintelligent,flyingtoolmakers.PerhapstheruntymammalsofthatextendedMesozoicErawouldhaveneveramountedtomuch.Withonlyalittletweakhereorthere,Earthwouldhavetakenadifferentpath.Butsomeaspectsofthecosmosareinevitable,deterministic.Theproduction
ofhugenumbersofprotonsandelectrons,andofcorrespondingamountsofhydrogenandhelium,washardwiredintoouruniversefromtheinstantoftheBigBang.Theformationofstarswasaninescapableconsequenceoftheproductionofhugeamountsofhydrogenandhelium.Thesynthesisofalltheotherelementsbynuclearfusionreactionsandbysupernovaswasequally
preordainedbytheformationofhydrogen-richstars.Andtheaccretionofallsortsofinterestingplanets—Earth-like,Mars-like,Jupiter-like,anddozensmoretypesthatareonlynowbeingdiscoveredorbitingdistantstars—followedwithcertaintyfromthesynthesisofallthosechemicalelements.Soitwasthatpost-TheiaEarthsettledintoaturbulenttimeofcoolingand
self-organization.Whatwasthatnascentworldlike?GeologistshavedubbedEarth’sfirstfivehundredmillionyearstheHadeanEon,inrecognitionofthehellishconditionsthatmusthaveprevailed.InformedspeculationpaintsaspectacularpictureofEarth’sHadeanEon:sulfurousvolcanicexhalations,riversofglowinglava,andasteadybombardmentofasteroidsandcometsincessantlydisruptedEarth’ssurface.WeareneverthelessseverelychallengedtoknowEarth’sfirstfewhundredmillionyearsinanydetail,forwearecompletelylackingintangibleevidence.OfEarth’sorigin,wehavetherichrecordoftheSolarSystem—theSunand
themyriadobjectsboundtoitbygravity.Tensofthousandsofmeteoritesprovidethemostintimateinteriorglimpsesoftheageofplanetesimals.DetailsoftheMoon’soriginarefoundineveryMoonrockandsoil.ButnothingatallsurvivesfromEarth’searliestdays,atleastnothingknownonEarthitself.Notafragmentofrocknoragrainofmineral.Remarkably,suchevidencemightstillexistintheformofmeteoritesthat
wereejectedfromEarth’searliestsurfaceduringgiantimpactsbillionsofyearsagoandsubsequentlylandedbackonEarthoronthenearbyMoon.Suchspecimensmustexist,perhapsinabundance,somevirtuallyunalteredinallthattime.Indeed,thequestforEarth’searliestrelicshasbeencitedasoneofmanyscientificrationalesforgoingbacktotheMoon.ExactinggeologicalsurveysofthelunarsurfacemightbeluckyenoughtofinderrantHadeanstonesandthusrevealtruthsaboutEarth’sinaccessiblepast.ButasniceasitwouldbetoholdapieceofEarth’sfirsthardenedsurface,we
aren’tcompletelystymied.ForwhileEarthhaschangedoverandoveragain,thelawsofchemistryandphysicshavenot.Fourandahalfbillionyearsago,thoselawsofchemistryandphysicsprevailed,astheyalwaysdo,butwithoutanymorereallybigthwacksorotherplanet-sizecomplications.
ElementalInevitability
TheearlyevolutionofEarthwasaconsequenceoftwointertwinedchemicalrealities:cosmochemistry(makingelements)andpetrochemistry(makingrocks).Firstcamecosmochemistryandthestellarproductionofalltheheavier
elements:everythingintheperiodictablebeyondhydrogenandhelium,whichareelementsoneandtwointhefirstrow.Inouruniverse,severalofthosechemicalelementsweredestinedtobecomedominant:oxygen,silicon,aluminum,magnesium,calcium,andironfaroutweighallotherheavyelements,particularlyintherockyterrestrialplanets.Thosesixelementsmakeup98percentofEarth’smass,astheydothemassofEarth’sMoonandofMercury,Venus,andMars.Eachofthese“bigsix”elementshasadistinctivechemicalstorytotell.Each,
initsownway,helpedmakeEarththewayitwouldinevitablybeaftertheBigThwack.Thekeyischemicalbonding.Recallthatatomsattachtoeachotherwhentheirfuzzyelectroncloudsinteractandshuffletoformmorestablearrangements—notablyatomswiththemagicnumbersoftwoortenoreighteenelectrons.Forthiskindofexchangetowork,someatomshavetogiveawayelectrons,whileothershavetoacceptthem.OxygenisEarth’smasterelectronacceptor.Everyoxygenatomhaseight
positivelychargedprotonsinitsnucleus,whichareelectricallybalancedbyeightnegativelychargedelectrons.Butoxygenisalwayslookingfortwoextraelectronstomakethemagicnumberoftenelectrons.Thatincessantacquisitiveurgemakesoxygenoneofthemostreactive,corrosivegasesinnature.It’sreallyrathernastystuff.Mostofusthinkofoxygen,firstandforemost,asanessentialpartofthe
atmosphere(the21percentorsothatkeepsusalive).ButthathappyatmosphericdevelopmentisarelativelyrecentchangeinEarthhistory.ForEarth’sfirsttwobillionyearsatleast,theatmospherewasutterlydevoidofoxygen.EventodayalmostallofEarth’soxygen—99.9999percentofit—islockedintorocksandminerals.Whenyouhikeupamajestic,ruggedmountainorwalkonawindswept,rockyoutcrop,mostoftheatomsbeneathyourfeetareoxygen.Whenyoulieonasandybeach,almosttwointhreeoftheatomsthatsupportyourweightareoxygen.Foroxygentoplaythiscriticalchemicalroleaselectronacceptor,therealso
havetobelotsofatomsthatcangiveawayorsharetheirelectrons.Themostprolificelectrondonorissilicon,whichaccountsforalmostoneineveryfouratomsinEarth’scrustandmantle.Siliconhasfourteenpositivelychargedprotonsinitsnucleus,whicharenominallybalancedbyfourteennegativelychargedelectrons.Siliconcommonlygivesupfourelectronstoachievethemagicnumberoftenelectrons,becomingasiliconionwithapositiveelectricalcharge.InEarth’srockycrustandmantle,thosefourrelinquishedelectronsarealmostalwaysgobbledupbytwooxygenatoms,whichbecomenegatively
chargedions.Consequently,strongsilicon-oxygenbondsarefoundinalmosteveryrock,mostnotablyinquartz,orSiO2—aone-to-twomixtureofsiliconandoxygenatoms.Tough,translucentquartzgrainslastalongtime.Theyaccumulatebytheuntoldtrillionsalongshorelinesandarebyfarthecommonestmineralofbeachsand.You’vealsoprobablyseenthebeautiful,sharplyfaceted,transparentcrystallinespecimensofquartzsoldinNewAgestoresas“powercrystals.”Whenyouholdsuchatreasureinyourhand,two-thirdsofwhatyou’reholdingisoxygen.Crystalswithsilicon-oxygenbonds,collectivelycalledsilicates,areEarth’s
mostcommonminerals,withmorethanthirteenhundreddifferentknownspecies(andmoreaddedalmosteverymonth).Theyarerichlyvariedintheiratomicstructuresandpropertiesbecauseoftheversatilityofthesilicon-oxygenconnection,whetherintheheartyweather-resistantstructureofquartzandfeldspar,ortheclusterarrangementsofgemmygreenolivineandredgarnet(thesemipreciousbirthstonesofAugustandJanuary,respectively),ortheneedlelikechain-silicatehabitsofsomenotoriousformsofasbestos,orthethinflatsheetsofmineralslikemica,onceusedasacheapsubstituteforwindowglass.Thoughlessabundantthansilicon,theelementscalcium,magnesium,and
aluminumallplaykeystructuralrolesinthemostcommonsilicaterocksthroughoutEarth’scrustandmantle.Aspositiveions,liketheirmorebountifulsiliconcousin,theyoccasionallybondwithoxygenalone,formingthecalciumoxidethatwerecognizeasthelawnchemicallime,therarecompoundmagnesiumoxide,and(whentraceamountsoftherarerelementschromiumortitaniumareincorporatedintoaluminumoxide)theprizedgemstonesrubyorsapphire.Itisthesixthofthebigsixelements,iron,thatisbyfarthemostversatile.
Eachoftheotherfive—oxygen,silicon,aluminum,magnesium,andcalcium—assumesonedominantchemicalpersonality.Oxygenalmostalwaysactsasanacceptoroftwoelectrons,siliconalmostalwaysactsasadonoroffourelectrons,aluminumasadonorofthreeelectrons,andmagnesiumandcalciumasdonorsoftwo.Butiron,elementtwenty-six,playsthreequitedistinctchemicalroles.Iron’sversatilityisunderscoredbyEarth’slayeredstructure.Aboutoneinten
atomsinEarth’soxygen-dominatedcrustandmantleisiron,whereasEarth’smetalliccoreismorethan90percentiron.Thissharpcontraststemsfromthefactthatthiselement’stwenty-sixelectronsareaprettyfarcryfromeighteen,thenearestmagicnumber,makingironadonorparexcellence.There’snowayironcangiveawayeightelectrons(nooneatomwillacceptthatmany)soithastomakedowithwhateveracceptorshappentobepresent.
Sometimesironactsjustlikemagnesiumandgivesuptwoelectronstobecomea+2ion.Ironinthisdivalentstatelendsadistinctivegreenishorbluishcolortomanymineralsandotherchemicals.Thecharacteristicgreencolorofthegemstoneperidot(aniron-bearingolivine)andthebluish-greencolorofoxygen-starvedbloodinyourveinsaretelltalesignsofdivalentiron.Inthisguise,ironbondstooxygeninaone-to-oneratio.Andbecausemagnesiumandironatomsaresimilarinsize,theseelementsoftensubstitutefreelyforeachotherincommonmineralsofEarth’scrustandmantle.SomeofEarth’smostabundantminerals,includingolivine,garnet,pyroxene,andmica,havevariantsthatdisplayprettymuchanymagnesium-to-ironratio,fromcolorlessversionswith100percentmagnesiumtodark-huedvarietieswith100percentdivalentiron.Ironisnotrestrictedtothe+2state,however.Inthepresenceoflotsof
electronacceptors,itreadilygivesupathirdelectrontobecomea+3ion.Thistrivalentformofironlendsacharacteristicbrick-redcolortoitshost.Redrust,redsoils,redbricks,andoxygen-richredbloodowetheirvividhuestotrivalentiron.Likealuminum,whichalsoadoptsthe+3state,trivalentironbondsinatwo-to-threeratiowithoxygentoformFe2O3—acommonmineralnamedhematite,foritsbloodredcolor.Justasmagnesiumoftenproxiesforiron’sdivalentform,aluminumcommonlyreplacesiron’strivalentvariant.Themineralsgarnet,amphibole,andmicadisplayeveryimaginablealuminum-to-ironratio,withtheiron-richvarietiespresentingasredratherthangreen.Sowiththeextremelyusefultrickofswitchingbackandforthfromthe+2to
the+3state(we’llcomebacktothisremarkableabilityinacouplebillionyears,whenlifefirstcomesonthescene),ironinitsdivalentandtrivalentguisesactsliketheothermembersofthebigsix.Butwait—ironhasonemorecriticalroletoplayinEarth:itcanrathereasilyformametal.Allthetypesofchemicalbondsintroducedsofarinvolveanexchangeof
electrons,resultinginions.Silicon,aluminum,magnesium,calcium,andirongivethelectrons;oxygentakeththemaway.Consequently,theselinkagesarecalledionicbonds.However,metalsadoptaverydifferentbondingstrategy.Inametal,eachatomgivesuponeormoreelectrons,tobecomepositivelycharged.Butthosedisenfranchisedelectronshangaroundinthemetalinakindofsticky,negativelychargedsea,whichholdsallthepositivelychargedatomstogetherlikeregimentedarraysoflittleBBsinmolasses.Ironmetalisavastcollectionofironatomsthatcollectivelysharesuchdelocalizedelectrons.Theconsequencesofthiscommunalbehaviorareprofound.Foronething,all
thosesharedelectronsarefreetomovearound,sometalsmakeexcellentconductorsofelectricity(electricitybeingnothingmorethanthecontrolledflow
ofelectrons).Bycontrast,inionicallybondedmaterialsmadeofoxygenplusmagnesiumorsilicon,everyelectronislockedintoplacesotightlythatelectricitycan’tpossiblyflow.Anotherconsequenceofmetallicbondingisthatsuchmaterialstendtobendratherthanbreak.Theelectronseasurroundingtheatomscanbefoldedandtwistedwithoutlosingitscollectivestrength,quiteunlikethebehaviorofmostbrittlerocksandminerals.Theperceptivereaderwillhavenotedthatironisnotaloneinperformingthis
metal-formingtrick.Aluminummetalcans,foil,andhouseholdwiringarecommonplace;magnesiummetalalloysareamainstayofhigh-techracingcarsandothertoys;andthesemimetalsiliconliesattheheartofeveryelectronicgadget(henceSiliconValley).Butmetallicaluminum,magnesium,andsiliconaremodernmarvelsofthechemicalindustry.Ittakeshugechunksofenergytoripthosestubbornelementsawayfromoxygen,andtheirmetallicstatesalmostneverforminnature.Ironislesscommittedtooxygenandmorefickleinitschemicalbonding
partners.Unlikesilicon,aluminum,magnesium,orcalcium,itisperfectlyhappytolinktootherelectronacceptors,notablysulfur—ironsulfideistheshinymineralpyrite,orfool’sgold.Unlikethoseotherelements,ironreadilyformsadensemetalthatsinkstothecenterofplanetsandformstheirmassivecores.
MoltenEarth
Thebigsixelements,eachofwhichisaninescapableconsequenceoftheevolutionofexplodingstarsandterrestrialplanets,arealsoresponsibleforEarth’smostabundantrocks.Theirdistinctivechemicalbehaviorssetourplanetonanirreversiblecourseoftransformationintotheworldweinhabittoday.Butbeforerockscouldform,Earthhadtocool.ImagineonceagaintheviolentyearsfollowingtheMoon-formingimpact.For
afewdaysorweeks,whatwouldbecomeEarthandwhatwouldbecometheMoonwerestillbeingsortedout.NeitherEarthnortheMooninthoseearlypost-Theiadayshadanysolidsurface.Thesecoalescingcompanionglobeswerebothboundedbyanencirclingmagmaocean,roilingandincandescentred,peltedbyanincandescentmoltensilicaterainattemperaturesofthousandsofdegrees.AstheairclearedofTheia’sremains,blast-furnace-likeheatradiatedfrom
Earthintothecoldvacuumofspace,inexorablycoolingtheplanet’soutershell.Evenso,cosmiceventsconspiredtokeepEarth’ssurfacemoltenawhilelonger.Foronething,bigasteroidskeptpoundingtheplanet.Eachcollisionaddedmore
thermalenergy,superheatingtheimpactarea,thwartinganyattemptstoformastablecrust.Intensegravity-inducedtidesfromthenearbyMoonalsohelpedtomaintainEarth’sliquidstate,asanequatorialbulgeofturbulentmagmasweptaroundtheplaneteveryfivehours,fragmentingtheorganizationofanythin,solidveneer.Earth’samplestoreofhighlyradioactiveelements—boththeshort-lived,heat-generatingisotopesofaluminumandtungstenandthelong-livedradioactiveisotopesofuranium,thorium,andpotassium—continuedtoaddevenmoreheat.Andayoungandgrowingatmosphere,fueledbythevolcanicreleaseofvaporsrichincarbondioxideandwater,mayhaveamplifiedtheseeffectsbyinducinga“supergreenhouse”state.Foranunknownlengthoftime,perhapsahundredyearsorahundred
thousandyears—ageologicalblink—themoltenstateprevailed.Butcoolingandhardeningwerepreordained.Thesecondlawofthermodynamicsdemandsthathotobjects,lackingsignificantnewenergyinputs,mustcool,andthehottertheobject,thefastertherateofcooling.Threefamiliarmechanismsfacilitatethistransferofheat.Firstthere’s
conduction.Whenahotterobjecttouchesacoolerobject,heatenergymustflowfromhottocool.Thisprocess,painfullyobviousifyourfeethaveeverbeenburnedfromwalkingonsunbakedpavementoryourhandhaseverblisteredfromtouchingastoveburner,resultsfromtheconstanttwitchingofatoms.Atomsinhotterobjectsexperiencemoreviolentmotions.Whenacoolerobject,withmoreslowlywigglingatoms,contactsahotterobject,withmorefreneticatoms,someofthatviolentmotionistransferredbyatom-to-atomcollisions.Ifthehotobjectyoutouchishotenough,itcandisruptthemoleculesinyourskin,killingcells,causingaburn.Conductionisafinewaytotransferheatlocally,fromoneobjecttoanadjacentobject,butitisapoorchoiceforheattransferonaplanetaryscale.Itjusttakestoolongtomoveheatfromonewigglingatomtothenext.Convection,bywhichcollectionsofhotatomsmovethermalenergy
wholesale,isabetterplanetarychoiceforcooling.Youexperienceconvectionwheneveryouboilwater.Pourwaterinapan,turnuptheheat,andwait.Theprocessisslowatfirst,asthehotpancontactsthecoldwaterandtransfersheatbyconduction—wigglebywiggle,metalatomsinthepanjostlemoleculesofwater.Butsoonanothermechanismtakesover.Heatedvolumesofwateronthebottombegintoexpandandrisethroughthecooler,denseroverlyingwater,transferringheatenmassetothesurface.Simultaneously,thecooler,densersurfacewaterssinktothehotbottom.Fasterandfastertheheatexchangeprogresses,withcolumnsofwaterrisingandsinking,untilyoureacharolling
boil.Throughtheconvectivecyclingofhotwaterupandcoolwaterdown,largevolumesofwaterspreadheatthroughouttheliquidinafastandefficientdance.OnthegrandscaleofEarth,convectionappearsoverandover—incooling
offshorebreezesonasummerday,ingrandoceancurrentsthatsweepfromtheEquatortotheArctic,intheturbulentlightning-lacedfrontsofthunderstorms,inboilinghotspringsandspurtinggeysers.AndsoitisinEarth’sinterior;cooler,densermagmasandrocksnearthesurfacesink,ashotter,less-densemagmasdeepdownrisetothesurface.ForallofEarth’shistory,convectionhasbeentheprimarydriverofplanetarycooling.Andthenthere’sradiation,thethirdmechanismofheattransfer.Anyhot
objectradiatesheattoitscoolersurroundingsintheformofinfraredradiationthattravels186,000milespersecondthroughavacuum.Thisfamiliarformofenergy,abundantlyevidentwheneveryourelaxandsoakuptheraysoftheshiningSun,issimilarinitsbehaviortowavesofvisiblelight(thoughheatradiationhasslightlylongerwavelengths).PerhapsthemostobviousinfraredenergysourceistheSun,whichbathesEarthininfraredradiationthattravelsacrossthevacuumofspaceinabout8.3minutes.Anelectricspaceheater,atoastyfireinyourfireplace,andanoldhotwaterradiatorareotherfamiliarexamples.Everywarmerobjectradiatesheattoitscoolersurroundings.Yourbodyisnoexception.That’swhyacrowdedauditoriumcangetsouncomfortablywarm—eachpersonradiatesheatlikeahundred-wattlightbulb—afactonemayeasilyverifybyputtingonapairofnight-visiongoggles,whichmakepeopleandotheranimalsthatemitinfraredradiationappeartoglowbrightlyinthedark.Therateofheattransfer,whetherbyconduction,convection,orradiation,
dependsonthedifferenceintemperaturebetweenthehotterandcolderobjects.Conductionisswifter,convectionmorevigorous,andradiationmuchmoreintenseifthetemperaturedifferencesarelarge.Earthisawarmplanet.OrbitingtheSunasitdoesinthecoldnessofspace,itisalwaysradiatingheatintothevoid.Butthered-hotpost-TheiaEarthblasteditsexcessheatenergyintospaceatarateunmatchedinmoderntimes.Itliterallyglowedintheblackvoidofspace.
FirstRocks
GivenEarth’sprodigiousheatlosstospace,theformationofarockycrustwasinevitable.Somewhere,probablynearoneofEarth’slesstidallystressedpoles,themoltensurfacecooledjustenoughforthefirstcrystalstoform.Butcooling
andcrystallizingwasfarfromasimpleevent.Manyeverydaysubstanceshaveawell-definedtemperatureatwhichacoolingliquidbecomessolid—thefamiliarfreezingpoint.Liquidwaterfreezesat32degrees,silverymercurymetalat–38degrees,andethanol(thecommonalcoholinbooze)at–179degreesFahrenheit.Butmagmaisdifferent.Itisacuriosityofmagmathatitdoesn’thaveonesinglefreezingpoint(althoughfreezingpointinthecontextofmagmaatmorethan2,500degreesFahrenheitseemssomethingofanoxymoron).Let’sbeginwiththeimmediatepost-Theiainferno4.5billionyearsago,atime
whenEarthanditsMoonsharedaradiantsilicatevaporatmosphereattemperaturesof10,000degreesFahrenheit.Thathellishrockgascooledrapidlyandeventuallycondensedintodropletsandrainedmagmaontothenewtwinworlds,asitinexorablycooledtobelow5,000degrees,then4,000degrees,then3,000degrees.That’swhenthefirstcrystalsstartedtoform.SuchstoriesofEarth’sfirstrocksarethescientificdomainoftheexperimental
petrologists,womenandmenwhodevisenovellabtechniquestobakeandsqueezerocksinordertomimicconditionsofEarth’sdeepinterior.Thequesttodiscoverrockoriginsfacestwotechnicalchallenges.First,youneedtocontrolincrediblyhightemperaturesofthousandsofdegrees,farhotterthananyovenorfurnaceinyourhome.Todoso,scientistscraftplatinumwireintometiculouslyspacedcoils,throughwhichtheydeliverhighelectricalcurrentstoachievetemperatureextremes.Evenmorechallenging,thesetemperaturesmustbeappliedwhilesamplesaresubjectedtocrushingpressurestensorhundredsofthousandsoftimesthatoftheatmosphere.Forthisexactingtask,researchersenlistmassivehydraulicramsandgiantviselikepresses.Formorethanacentury,theCarnegieInstitution’sGeophysicalLaboratory,
myscientifichome,hasbeenacenterfortheseheroicquestsforEarth’sdeeptruths.Forashortwhile,beforehisuntimelydeathbyhospital,IhadthechancetoworksidebysidewithHattenS.Yoder,Jr.,oneofthepioneersofexperimentalpetrologyandtheworld’sforemostexpertontheoriginsofbasalt.Imposing,dynamic,enthusiastic,andattentive,Yoderwasliterallyatoweringfigureinthefield.AsanavalofficerinWorldWarII,hewasintimatelyfamiliarwithgiganticmetalhardware.Inthe1950s,hejoinedtheGeophysicalLaboratoryandusednavalsurplusgunbarrelsandarmorplating,stillpaintedbattleshipgray,tobuildthehigh-pressurelabthatwouldframenotjusthishalf-centurycareerbutourcomprehensionofthegroundwestandon.ThecenterpieceofYoder’sdevicewasa“bomb”—amassivesteelcylindera
footindiameter,twentyincheslong,withaninch-diameterbore.Oneendofthebombwasconnectedtoaseriesofgaspumps,compressors,andintensifiersthat
couldgenerateastaggeringtwelvethousandatmospheresofgaspressure—thepressurefoundtwenty-fivemilesbeneathEarth’ssurface—withapent-upenergyequivalenttotheexplosivepowerofastickofdynamiteiftheapparatuseverfailedcatastrophically.Thebomb’sotheropenendacceptedafoot-longrocksampleassemblyandagianthexagonalnut,sixinchesindiameter.Wesealedthedevicebytighteningthenutwithathree-foot-long,twenty-poundwrench.ThebeautyofHatYoder’sapparatuswasthatwecouldloadpowderedrock
andmineralsamplesintolittlegoldtubes,packthetubesintoacylindricalheater,andsecuretheentireassemblyinsidethebomb’spressurechamber.Pumpupthepressure,turnontheelectricheater,andthebombdidallthework.Eachexperimentalrunhelduptosixsmallgoldtubes;eachrunlastedfromafewminutestoafewdays.HatYoder’sremarkableinventionwasideallysuitedtostudyhowrocksevolveinEarth’scrustanduppermantle.WhatHatYoderandhiscolleaguesfoundwasthatanincandescentmeltrich
inthebigsixelementswilltypicallybegintosolidifybyformingcrystalsofthemagnesiumsilicateolivineasitcoolsbelowabout2,700degreesFahrenheit.OnbothEarthandtheMoon,duringthatlong-agocoolingperiod,beautifultinygreencrystalsbegantogrowinthemagmaasmicroscopicseeds,whichexpandedtothesizeofBBs,peas,grapes.Butolivineistypicallydenserthantheliquidinwhichitgrows,sothosefirstcrystalsbegantosink,fasterandfasterasthecrystalsgrewlargerandlarger,accumulatinghugedeepmassesofnearlypurecrystals—formingastunninggreenrockcalleddunite.OnEarththisrockisrelativelyraretoday,appearingatthesurfaceonlyonthespecialoccasionswhenmountain-buildingactivitiesofupliftanderosionexposethedistinctivedense,deep-formedolivinecumulates.Thecontinuoussinkingofolivinecrystalsgraduallyalteredcoolingmagmas
insideEarthandtheMoon.Theremaininghotmeltschangedcomposition;astheybecameprogressivelydepletedinmagnesium,theybecamecorrespondinglymoreconcentratedincalciumandaluminum.OntheMoon,asthemagmaoceancontinuedtocool,asecondmineralstartedtoform.Anorthite,afeldsparmadeofcalciumaluminumsilicate,begantocrystallizealongsideolivine,formingpaleblocks.Unlikeolivine,anorthiteislessdensethanitshostliquid,soittendstofloat.OntheMoon,immensequantitiesofanorthitepoppedtothesurfaceofthemagmaoceantoformavastcrustoffloatingfeldsparmountainrangesrisingasmuchasfourmilesabovethemoltensurface.Thesewhitish-graymasses,whichstilldominate65percentoftheMoon’sreflectivesilveryface,arecalledtheLunarHighlands.Risingastheydiddirectlyfromthemagmaocean,theyaretheoldestknownformationsontheMoon.Apollosamplesrevealarangeof
ancientagesforthesedistinctiveanorthositesfromasyoungas3.9billionyearstoalmost4.5billionyears,shortlyaftertheBigThwack.OnEarth,withitswettercomposition,deepermagmaoceans,and
correspondinglygreaterinternaltemperaturesandpressures,asomewhatdifferentscenariounfolded.AsmallamountofanorthiteprobablycrystallizedearlyinEarthhistoryinsomenear-surface,low-pressureenvironments,butitwasaratherminormineral.Instead,magnesium-richpyroxene,thecommonestofthechainsilicateminerals,appearedinabundance,tocomminglewitholivineinathickcrystalslush.Earth’searliestrocksthuspredominantlyfeaturedolivineandpyroxeneinahard,greenish-blackrockcalledperidotite.VarietiesofperidotitebegantocrystallizethroughoutEarth’souterfiftymiles,probablycommencingmorethan4.5billionyearsagoandcontinuingformanyhundredsofmillionsofyears.Inspiteofitsearlyabundance,peridotite,too,isrelativelyrareatEarth’s
surfacetoday.Byonepersuasivescenario,raftsofperidotitehardenedandcooledtoformEarth’sfirsttransientrigidsurface.Butcoolingperidotite,likeitsdunitepredecessor,issignificantlydenserthanthehotmagmaoceaninwhichitformed.Theperidotitesurfacelayerthuscracked,buckled,andsankbackintothemantle,todisplacemoremagmathatcooledtoformmoreperidotite.Overaspanofhundredsofmillionsofyears,themantleitselfslowlysolidified,ridingakindofperidotiteconveyorbeltthatoperatedinEarth’souterfiftymiles.Theratioofdensesolidperidotitetomagmaincreased,untiltheuppermantlewasmostlysolidolivine-pyroxenerock.
CoreTruths
Deeperinthemantle,fiftytotwohundredmilesbeneaththecrust,coolingandcrystallizationmusthaveproceededinasimilarfashion,albeitmoreslowly.Detailsoftheprocessremainuncertain—thenextgenerationofhigh-pressure,high-temperatureapparatusmustbebroughttobeartosortoutthecomplexities—butseparationofcrystalsfrommeltsbysinkingandfloatingprobablyplayedasignificantrole,astheydidinthenearersurfaceenvironment.Muchofwhatweknowofthosehidden,deepdomainscomesfromthe
scienceofseismology,thestudyofsoundwavesspeedingthroughEarth’sdeepinterior.Earthisconstantlyringinglikeabell:crashingtides,rumblingtrucks,andearthquakesbothbigandsmallallconspiretoshakeEarthandpropagateseismicwaves.Andlikesoundwavesinasteep-walledcanyon,seismicwavesechowhentheybumpintoasurface.SeismicwavesrevealthatEarth’sinterioris
acomplexlylayeredplace.Atthemostbasicanatomicallevel,Earthistriplylayered—ithasathin,
lower-densitycrustatthesurface,athick,higher-densitymantleinthemiddle,andathicker,reallydensemetalliccoreinthecenter.Eachofthosethreedomainscontainsfurtherlayering.Themantle,forexample,isdividedintothreesublayers—uppermantle,transitionzone,andlowermantle.Theperidotite-dominateduppermantleextendsdownperhaps250miles,atwhichdepthpressureforcestheatomsinolivinetopackintoadensersilicatecrystalformcalledwadsleyite,thedominantmineralofthemantle’stransitionzone.Thelowermantle,150milesfartherdown,featuresanevendenserassemblageofmagnesiumsilicates.Thepressuresinthelowermantlearesohigh—hundredsofthousandsoftimesthesurfacepressure—thatsilicon-oxygenbondsadoptanevendenser,moreefficientpackingarrangementofatomscalledperovskite.Seismicstudiesdocumentthenatureandextentofeachofthese
mineralogicallydistinctmantlelayers,andbyandlargethetransitionsfromonetothenextareneatandtidy.Theexactdepthsofthetransitionsvaryslightlybytenortwentymilesfromplacetoplace—beneaththecontinentsversustheoceans,forexample—buteachboundaryappearstoberelativelysmoothandwellbehaved.Bycontrast,seismologyprovidestantalizingevidencetosuggestthatthecore-mantleboundaryisanespeciallycomplicatedzone,ratherdifferentfromthecleanmantle-mantletransitions.Toafirstapproximation,thecore-mantleboundaryproducestheexpectedstrongecho.Indeed,thedensitycontrastbetweensilicatemantleandmetalcoreissoextremeastocreateaphysicalboundaryassharpasthatbetweenairandwater—producingthestrongestreflectedseismicsignalfromEarth’sdeepinterior.Morethanacenturyago,thatdividewasoneofthefirsthiddenfeaturesofEarth’sdeepinteriorthatseismologistsdiscovered.Aperfectlysmoothandregularboundarywouldproduceasharp,focused
seismicreflection—anechoingresponsethatcouldberecordedasadistinctivespikeonaseismometer.Butseismicsignalsreflectingoffthecore-mantleboundaryareoftenmessy,smearedout,brokenup.There’sextrastructuredownthere,likeirregularlumpsorpilesofdebris.Geophysicists,whoarenotalwaysknownforemployingthecatchiestterminology,callthislumpychaoticzonetheD´´(Ddoubleprime)layer.(Astrophysicists,whocoinedsuchimaginativetermsasbrowndwarf,redgiant,darkenergy,andblackhole,arerathermoresuccessfulatthescientificnamegame.)ThecomplexityofthisdeepD´´featureisinparttheresultofthesharp
densitycontrastbetweenthecore’shomogeneousironmetalandthemantle’s
variedoxygen-richminerals.Allmantlemineralsfloatonthedensecorelikecorksonwater,butthesediversemineralscandifferwidelyintheirdensities.Intheprimordialmagmaocean,somesilicatessank,andsomefloated.Asaresult,bigchunksoftheearliestcrystallizedsolidssankthroughthemantleallthewaydown,tofloatlikeraftsonthemetalcore.Someseismologistsenvisionthree-hundred-mile-high“mountains”withirregularpilesofdensemineralsrestingonthecore-mantleboundary,wheretheychaoticallydeflectseismicsignals.Surprisingly,theremayalsobebigcore-mantleboundarypuddlesandponds
ofunusuallydensesilicateliquid,perhapsrichintheelementsaluminumandcalciumaswellasaslewof“incompatibleelements”thatseemtobemissingfrominventoriesofEarth’souterlayers.Wehavenoeasywaytobesure,butseismologistspointtodeep,localized“ultra-lowvelocityzones,”intheD´´layerjustabovethecore-mantleboundary,whereseismicwavestravelabout10percentslowerthantheydoinadjacentrocks.Slowseismicwavesareoftenatelltalesignofliquid.Suchdeepliquidlakesandpondscouldalsoprovideaneatsolutiontothatnigglingmissing-elementproblem:juststickalltheincompatibleelementsintheinaccessibleD´´layer,wheretheyareforeversequesteredinthatenigmatic,heterogeneouszoneofmineralogicaljunk.Andwhatofthecoreitself?WhenEarthwasveryyoung,thedense,iron-rich
core,morethan2,000milesindiameter,hadfullyformedandwasprobablyentirelymolten(unliketoday,whentheinnercoreappearstobeagrowingballofsolidironcrystals750milesindiameter).Temperaturesatthatsharpcore-mantledividinglinemayhaveexceeded10,000degreesFahrenheit,whilepressuresexceededamilliontimesthatofourmodernatmosphere.Thehotcorewasfromtheoutset,andremainstothisday,adynamicplaceof
swirlingliquidmetalcurrents.OneimportantconsequenceofthesecurrentswastheearlygenerationofEarth’smagneticfield—themagnetosphere,whichislikeanimmenseelectromagnet.Magneticfieldsbendthepathsofelectricallychargedparticles,soEarth’smagnetosphereprovidesakindofinvisibledeflectorshieldtotheintensebombardmentofsolarwindandcosmicrays—abarrierthatisperhapsaprerequisitefortheoriginsandsurvivaloflife.Thecoreisalsoanimportantsourceofheatenergythathelpstodrive
convectioninthemantle.Eventodayplumesofhotmagmafromthecore-mantleboundaryrisealmosttwothousandmilestothesurfaceinvolcanichotspotssuchasHawaiiandYellowstone.Remarkably,thefixedlocationsoftheseplumesonthesurfacemaybedictatedbydeeptopography.Thethree-hundred-mile-tallmountainsoftheD´´layermayactasthermalblanketslyingonthehotcore,soit’spossiblethathotspotsoriginateatthedeepest,heat-releasingvalleys
betweenthoseepic,hiddenmountains.
Basalt
Atheart,themineralevolutionstoryrestsonapreordainedsuccessionofrocktypes,eachmineral-formingstagefollowinglogicallyfromthepreviousstage.Earth’sfirstperidotitecrustwasacriticalbutfleetingjuvenilephase,bornoftheprimordialmagmasea.Whenitultimatelycooledandhardened,itprovedtoodensetoremainanywherenearthesurfaceandthussankbackintoEarth’sdepths.Another,lessdenserockwasrequiredtogirdletheglobe.Thatrockwasbasalt.Blackbasaltdominatesthenear-surfacerocksofeveryterrestrialplanet.The
asteroid-scarredexteriorofMercuryismostlybasalt.Soarethescorched,mountainoussurfaceofVenusandtheweatheredredsurfaceofMars.TheMoon’sdarklysplotchedmares(seas),whichcontrastsovividlywiththepalergrayanorthositehighlands,arethehardenedremainsofimmenseblackbasalticlakes.AndonEarth,70percentofthesurface,includingallthefloorsofalltheoceans,isunderlainbybasaltcrust.Basaltcomesinavarietyofflavors,buttwoessentialsilicateminerals
dominatethemall.Onekeymineralisplagioclasefeldspar,byfarthemostimportantaluminum-bearingmineralinterrestrialplanetsandmoonsandEarth’scommonestcrustalmineral.MyMITprofessorDaveWonesonceadvisedmethatifIwasevershownamysteryrockandquizzedastoitsmineralogy,Ishouldjustsay“plagioclase,”andI’dberight90percentofthetime.Thesecondessentialmineralogicalingredientofbasaltispyroxene,thecommonchainsilicatealsofoundinperidotite.Pyroxeneisoneofahandfulofcommonmineralsthatcanincorporateallofthebigsixelements(andmanymorelesscommonelements,aswell).Tounderstandtheoriginsofplagioclaseandpyroxene,thetwoessential
mineralingredientsofbasalt,considerthestrangefreezingandmeltinghabitsofrocks.Fourandahalfbillionyearsago,asEarth’smagmaoceancooled,olivineformedfirst,thenalittlebitofanorthite,andfinallyalotofpyroxene.Theresultingmagnesiumsilicaterockwasperidotite,whichformedmuchoftheuppermantle.Asmassesofperidotiteformedandsank,theywerereheatedandpartiallyremelted.Oureverydayexperiencewithmeltingsuggeststhatthechangefromsolidto
liquidtakesplaceatonespecifictemperature.Watericemeltsat32degrees,mosthouseholdcandlewaxatabout130degrees,anddensemetalleadat621
degreesFahrenheit.Butrockmeltingisn’tsosimple;mostrocksdon’tmeltentirelyatonetemperature.Ifyouheatperidotitetoabout2,000degreesFahrenheit,thefirstmeltwillappear.(Itwillappearsooneriftheperidotiteisrichinvolatilewaterandcarbondioxide.)Thecompositionofthosefirstmicroscopicdropletsdiffersdramaticallyfromthatofthebulkofperidotiterock.Theinitialmelthasalotmorecalciumandaluminum,alittlemoreironandsilicon,andalotlessmagnesium.Thisinitialliquidisalsoalotlessdensethanitsperidotitehost.Consequently,evena5percentmeltingofperidotiteinthemantlegeneratesalotofmagmathatgathersalongmineralgrainboundaries,collectsinfissuresandpockets,andrisestowardthesurface—magmathatwilleventuallybecomebasalt.OverbillionsofyearsofEarthhistory,thepartialmeltingofperidotitehasgeneratedhundredsofmillionsofcubicmilesofbasaltmagma.Moltenbasaltcomestoplanetarysurfacesintwocomplementaryways.The
morespectacularisthroughvolcaniceruptionslikethoseinHawaiiandIceland,withfierymagmafountainsandriverlikeflows.Suchdramaticeruptionsareaconsequenceofwaterandothervolatiles,whichremainlockedinthesilicateliquidatthehighpressuresmorethanamiledown,butwhichtransformexplosivelytogasnearthesurface.Suchexplosivevolcanismcanejectashandtoxicgaseshighintothestratosphereandcanhurlcar-sizevolcanic“bombs”morethanamileoutwardtosmushthesurroundingcountryside.Layerbylayer,thesebasalteruptionsoflavaandashcanbuildmountains
severalmilestallandcoverthousandsofsquaremilesinblackrock.Thistypeofbasalticlavaflowandashisextremelyfine-grainedandrichinglass,theconsequenceofliquidcoolingsorapidlythatcrystalsdon’thavetimetoform.Theresultisafeaturelessblackcrustofhardenedlava.Otherdistinctiveolivinebasalts,whichoccuronlywhenperidotiteispartiallymeltedatrelativelyshallowdepthsoflessthantwentymiles,containafewlustrousolivinecrystalsthatformedundergroundinthefirststageofmeltsolidification.Thegreencrystalsdecoratetheotherwiseblandblackrock.Ittakesalotofexplosiveforceformagmatobreakthroughtothesurface,so
asignificantfractionofbasalticmagmanevermakesitaboveground.Rather,thesered-hotliquidsarestuckfarunderground,wheretheycoolmoreslowlyandforminch-long,lathe-shapedfeldsparandpyroxenecrystalsinrockscalleddiabaseorgabbro.Sometimesthemagmaisinjectedintonear-verticalcracksinsubsurfacerockstoformsmooth-faceddikes.Ifthehostrockissoftanderodesawaymillionsofyearslater,theresultcanbealong,straightdiabasewallthatcanlookuncannilylikeacrumblingarchaeologicalsite.Alternatively,ifthe
magmaisinjectedbetweenflat-lyinglayersofsedimentaryrocks,itcanformathickblanketlikesill.ThePalisadescliffsontheHudsonRiver,prominentjustupstreamfromNewYorkCityonthewesternshoreoftheHudsonRiver,aretheresultofoneofaseriesofbasalticsillsthatdipgentlytothewesttoformparallelhighlands(andsomeofthemostexpensiverealestate)innorthernNewJerseyandsouthernNewYork.Stillothertimestheliquidsitsandcoolsinirregularmagmachambersthatcanformmilesundergroundandthatstretchformilesacross.Butwhatevertheirultimategeometry,diabaseandgabbroarereallyexactlylikebasalt.Withtheinevitableformationofthebasalticcrust,Earthforthefirsttime
enjoyedasturdy,solidsurfacethatcouldfloat.Beforethecrust,whenmagmaandperidotitealonedefinedtheplanet’ssurface,notopographicfeaturecouldriseforlongtoasignificantheightabovethemeanelevation.Red-hotperidotitemushisnotnearlystrongenoughtosupportamountain.Buttoughbasalt,withitsrelativelylowdensity,isadifferentstory.Theaveragedensityofbasaltismorethan10percentlowerthanthatofperidotite.Thatmeansafloatingmassofbasalttenmilesthickwillprojectmorethanamileabovethemagmaocean.Rapidlyaccumulatingvolcanicconescouldriseevenhigher,perhapsmorethantwomilesabovethemean.Asaconsequence,Earth’sblemishedsurfacebegantodevelopsomerealcharacter.
HostileWorld
Seenfromspace—fromthesafedistanceoftheyoungMoon,forexample—Earth’sbasalticveneerappeareddeepblackwitharcuateredcracksandlocalbrightpointswhereimmensefountainingvolcanoesbrokethesurface.Jetsofdirtywhite,ash-ladensteamobscuredsomeofthemostvolatile-richvolcanicconesandadjacentportionsoftheglobe.Imagineyourselfbackmorethan4.4billionyearsintimetothenewlyminted
blacksurfaceofthatHadeanEarth.Youcouldnothavesurvivedlongintheharsh,alienlandscape.Meteorsincessantlybombardthesurface,crackingthethinbrittleblackcrust,showeringshatteredrockandgobsofmagmaacrosstheplains.Countlessvolcanicconesrise,growingsteadilytoheightsofmanythousandsoffeet,theirimmensemagmafountainspoweredbytheexplosivereleaseofsteamandothervolatilesthatwill,somefineday,coolenoughtobecometheoceansandatmosphere.Notraceoflife-supportingoxygenistobefound.OnthisunforgivingyoungEarth,yournostrilsareassaultedbyfoul-smellingsulfurouscompounds,yourskinscaldedbyventingsteam,youreyes
burnedbythenoxioushotgases.Yourexcruciatingdeathagonieswillbebriefonsuchahostileworld.TherecedingMooncontinuedtoplayamajorroleinshapingthecrust.Globe-
spanningtidesofrockandmagma,thoughlessextremethaninthefirstcenturiesfollowingTheia’sdemise,repeatedlycrackedandbuckledthesurface,openingfissuresthatoozedred-hotrockymushandthwartedtheformationofasolidsurface.TheMoon’suncomfortableproximityalsoperpetuatedEarth’sinsanelyrapidrotation—five-hourdayspersisted,accompaniedbymegastormsandultratornadoesfarmoreseverethananythinghypedontoday’sWeatherChannel.Butbeneaththatwretchedsurface,Earth’sinexorableevolutiontoaliving
worldhadbegun.Thewell-mixed,molteninteriorbegantoseparateintovolumesofdistinctivecompositions—matterthatwouldbecomethecontinentsanddeep-seacrust,theatmospheresandoceans,plantsandanimals.Heatingandcoolingandcrystallization,crystalseparationbysettlingandfloating,accumulationofperidotite,partialmelting—theseprocessesshapedEarththroughitsinfancy4.5billionyearsago,andtheypersistevenuntothepresentday.Earth’svastreservoirofinternalheat,thecentralthemeofthischapter,
continuestoplaythedominant,transformativeroleinshapingourplanetaryhome.Todaythemostobviousmanifestationsofthisdeep,hotrealmareintermittentvolcanoes,withtheirfierymagmafountainsandred-hotriversofmoltenrock.Eruptinggeysersandsulfuroushotspringsalsohintatahellishhiddensubsurfacerealm.ThroughoutEarth’s4.567-billion-yearhistory,asheatinexorablyworkeditswayoutwardfromtheincandescentcentertothefracturedcrustandthenceintothecoldnessofspace,thesurfacehasbornethebrunt.BuffetedbytheswirlingconvectionofthemantleandstressedbytheMoon’sincessantpull,thecrusthasbentandbuckled,crackedandtwisted.Continentshaveconstantlyshuttledacrosstheglobe,rippingapart,colliding,andscrapingpastoneanotherintheongoingheat-drivendanceofthetectonicplates.Everydayofourlives,Earth’sinnerheatreworkstherocksonwhichwelive,recyclesthewaterwedrink,andalterstheairwebreathe.Becauseofheat,Earthwasdestinedforabrieftimetobeablackworld,
glazedwithathinbasalticveneer.Butthatbriefjuvenilephasecouldnotlastlong.Anewvolcano-bornlayerofbrilliantbluewasabouttogirdtheglobe.
Chapter4
BlueEarth
TheFormationoftheOceansEarth’sAge:100to200millionyears
Earth’sinfancy,itsfirsthalf-billionyearsorso,isshroudedinmystery.Rocksandmineralsprovidetangibleevidenceformostofourplanet’sstoriedpast,butfewrocksormineralssurvivefromthatmostancientHadeantime.Asaconsequence,anynarrativeofEarth’sinitialcoolingandthesubsequentwateringofitsblacksurfacemustbebasedonspeculationsinformedbyexperiments,models,andcalculations.Evenso,someuncertaintieswillalwaysremain.That’snotabadthing.Whatmakeseachdayatthelabnewandexcitingisthe
richnessof“whatweknowwedon’tknow”andthepossibilityeachdaythatwewilldiscoversomesmallcluethatbringsusclosertotruth.Evenmoretantalizingistheprospectofdiscoveringaspectsofthenaturalworldthat“wedidn’tknowwedidn’tknow”—discoveriesthatincreasethebreadthofmystery.*Thesenewwaysofaskingquestions—“Howdidmineralsevolve?”forexample,ratherthansimply“Whataretheirchemicalandphysicalproperties?”—pavethewayforbreakthroughs.It’simportanttotakeaninventoryofwhatwedon’tknow.Alltheevidence
suggeststhattheMoonformedbyanepicimpact,yetwecan’tbesureexactlywhenthecollisionoccurred,norwhatwerethenuancesofTheia’sfinaltrajectory.Followingthatcolossalcollision,wecanimagineanincandescenttorrentialrainofsilicatesontoEarth’storturedmagmaocean,butthedurationandrateofcoolingofsuchasuperheatedworldarepoorlyconstrainedandwillremaintopicsofmuchdebatefordecadestocome.TheproximityandrecessionrateofthenewlyformedMoon,thoughcriticaltounderstandingthedynamicsandevolutionofearlyEarth,areequallyuncertain.Likewise,nooneknowswhentheoceansfirstformed,norexactlywhattheylookedlike.Butformtheydid,andthefollowingstoryisbasedonthebestevidenceavailableandsoisasgoodasitgetsforthetimebeing.TheblackEarthcouldnotremainblackforlong.Global-scalevolcanism
spewedhotnitrogen,carbondioxide,noxioussulfurcompounds,andwatervaporintothethickeningatmosphereatratesofbillionsoftonsperday.Those
volatileelementsandcompounds—theverysamemoleculesthatformedthevariedicesoftheformernebula,theverysameatomsthatyouarenowbreathingandthatmakeuptheintricatetissuesofyourbody—playedmanyrolesintherapidlyevolvingEarth.Whenhotwatermixedwithrockmagmas,itloweredtheirmeltingtemperaturesandturnedthemintoasuperheatedsoupthatrosetowardthesurface.Closetothesurface,thegasesdissolvedinthatmagmaticsouptransformedfromliquidtoviolentlyexpandinggasinmassivevolcanicexplosions,muchasashakensodawillblastitswayoutofaconfiningcan.Water-richfluidsalsodissolvedandconcentratedrareelements—beryllium,zirconium,silver,chlorine,boron,uranium,lithium,selenium,gold,andmanymore—thatwouldeventuallybecomethegreatorebodiesofEarth’sdiversifyingcrust.Atthechaoticsurface,roaringriversandcrashingwavesbecameprincipalagentsfortheerosionofrock,theformationofEarth’sfirstsandybeaches,andtheaccumulationofthickeningnear-shorewedgesofsediments.Inshort,waterbecamethechiefarchitectofEarth’ssolidsurface.Anyfocusonoceansandatmospherereflectsasomewhatanthropocentric
view,forthesefluidbodiesaretrivialcomponentsoftheplanetasawhole.Oceanstodayrepresentonlyabout0.02percentofEarth’stotalmass,whiletheatmosphereisnomorethanonepartpermillionofitsbulk.Nevertheless,oceansandatmospherehaveexerted,andcontinuetoexert,disproportionatelylargeinfluencesinmakingEarththeuniqueworldthatitis.Fourprincipalplayers—nitrogen,carbon,sulfur,andwater—taketheleading
rolesasEarth’smobile,gaseouscomponents.Alloftheseingredientsareproducedabundantlyinlargestars,allarewidelydispersedinsupernovaexplosions,andallbecameconcentratedinthemostprimitivecarbon-richchondritemeteoritesmorethan4.56billionyearsago.Inmanyrespects,theaveragecompositionofchondritemeteoritesclosely
matchesthatofEarthtoday.Thebigsixelementsdiscussedinchapter3(oxygen,silicon,aluminum,magnesium,calcium,andiron)areremarkablysimilarintheirproportions,asarenumerouslesscommonelements.ButevenacursorystudyofthosefascinatingancientobjectsrevealsthatmuchofEarth’soriginalinventoryofvolatilesismissingfromtoday’splanet.Themostprimitivechondritesaveragemorethan3percentcarbon,butalltheknowncarbonreservoirsonEarthnowadduptolessthan0.1percent.Similarly,thewatercontentofchondritesismuchgreaterthanEarth’smodernaverage—perhapsahundredtimesmore.Suchgrosscompositionaldisparitiespointtoachaoticandviolentpast.MostofEarth’svolatilesmusthavebeenlosttospaceordeeplyburied,farbeyondourabilitytosample.
ThekeytounderstandingEarth’searlytransformationfromablasted,inhospitableblackplanettoacooler,habitableblueplanetliesinthestoryofitsperipateticvolatiles.ButnovolatilessurvivedinanunalteredstatefromtheEarth’sfirsthalf-billionyears.Almostallthenitrogenandcarbon,allthesulfurandwater,havebeenrecycledcountlessthousandsoftimes,asthesameatomshavebeenusedoverandoveragain.Chondritemeteoritesprovideaquantitativestartingpointforourguesswork;thefewknownrockandmineralsamplesfromEarth’sfirstbillionyears,coupledwithdatafromtheMoonandotherobjectsintheSolarSystem,furtherdelineateourspeculations.AndaswasthecasewithunderstandingtheevolutionofmantleandcrustduringEarth’sfirsthundredmillionyears,aswellastheformationofstarslongbeforethat,thekeytoanyviablescenarioisknowledgeoftheimmutablecharacteristicsoftheelementsinquestion—inthisinstance,thephysicalandchemicalpropertiesofvolatilenitrogen,carbon,sulfur,andwater.Ofthesefouringredients,nitrogenistheeasiesttodealwith.It’sachemically
inertgasthatformsfewminerals,playsalmostnoroleinrockformation,andtendstoconcentrateintheatmosphere.OnlysincetheriseoflifehasthenitrogencyclehadmucheffectonEarth’souterlayers.CarbonandsulfurwouldalsocomeintomuchgreaterprominenceoneortwobillionyearsintoEarth’sexistence,whenlifeandanoxygen-richatmospheretransformedthesurfacerealm.Butthefourthingredient,water,hasbeencentraltoEarth’sstoryfromtheget-go.
Water:AShortCV
Water’svariedgeologicalrolesfollowfromtheuniquechemicalpropertiesofhydrogenoxide.Recallthathydrogeniselementnumberone,whileoxygeniselementeight;neitherelementhasamagicnumberoftwoortenelectrons.Eachelectron-acceptingoxygenatomseekstwomoreelectronstoreachthemagicnumberten,whileeachhydrogenatomwithoneelectrontosharewantsonemore.Themolecularresultisatwo-to-oneratioofhydrogentooxygen:H2O.TheatomsinthiscompactunitformaVshape:thecentrallargeroxygenatomisflankedbytwohydrogenbumps,notunlikeMickeyMouse’sears.Theoxygenatom,havingborrowedelectronsfromtwohydrogenatoms,assumesaslightnegativeelectricalcharge,whileeachhydrogenatomiscorrespondinglyslightlypositive.Theresultisapolarmolecule,withopposedpositiveandnegativeelectricallychargedparts(Mickey’searsandchin,respectively).Suchpolarityinwater’smoleculesaccountsformanyofitsdistinctive
properties.Polarwaterisasupersolvent,becauseitspositiveandnegativeendsexertstrongforcesthatcanpullapartothermolecules.Consequently,tablesalt,sugar,andmanyotheringredientsdissolverapidlyinwater.Mostrockstakeabitlongertodissolve,butovermillionsofyears,theoceanshavebecomerichinalmostallthechemicalelements.(Asaconsequence,everycubicmileofoceanwaterholdsaboutfourhundredpoundsofgold—morethan$10millionworthatthepreciousmetal’srecenthighvalue,ifonlywehadthetechnologytoextractit.)Thisunparalleledabilityofwatertodissolveandtransportotherchemicalsalsomakesitanidealmediumfortheoriginsandevolutionoflife.AlllifeonEarth,andperhapsalllifeinthecosmos,dependsonwater.Thepolarityofwatermoleculescausesthemtobondstronglytooneanother:
thepositivesideofonemoleculeattractsthenegativesidesofothermolecules.Consequently,iceisanunusuallystrongmolecularsolid(afactyou’llnotsoonforgetifyou’veeverfallenhardwhileice-skating).Unusuallystrongintermolecularbondingalsoresultsinwater’sunusuallyhighsurfacetension—afascinatingpropertythatallowssmallinsectsliterallytowalkonwater.Surfacetensionalsoleadstocapillaryaction,whichcauseswatertorisethroughthestemsofvascularplantsandallowstreestosoarhundredsoffeetabovetheland.Roundedraindrops,pulledtogetherbythestrongmutualattractionofwatermolecules,areyetanothermanifestationofsurfacetensionandavitallinkinmaintainingEarth’sunusuallyrapidwatercycle.Nonpolarvolatilemoleculeslikemethaneandcarbondioxidecan’tformsuchdroplets.Theywouldjustfloatintheatmosphereasanultrafine,pervasivemist,so“rain”wouldbeunknownonaplanetdominatedbythoseatmosphericgases.Strongbondsbetweenmoleculesresultinanotherofwater’smostcuriousand
importantproperties:liquidwaterisabout10percentdenserthansolidice.Inalmosteveryknownchemicalcompound,thesolidsinksinitsliquid—asituationthatisintuitivelylogicalbecausetheregular,repeatedpackingofmoleculesinsolidscontrastswiththeirrandomdistributioninliquids.Thinkaboutstoringshoeboxesinthebackroomofashoestore.Neatstacksandrowsofboxes(likeperfectlyalignedmoleculesinasolidcrystalstructure)takeupmuchlessvolumethanarandompile(likechaoticallytumblingmoleculesinaliquid).Butinwater,themoleculesactuallypackmoreefficientlyintheirrandomliquidstatethantheydoinorderlyicecrystals.Theimportantconsequenceisthatice,whetheritisacubeinyourdrink,a
layeronafrozenriverorstream,oragianticeberg,floats.Wereitnotforthisunusualcharacteristic,manybodiesofwaterwouldfreezesolid,bottomtotop,ratherthanformingathick,protectivesurfacelayeroficeeverywinter.Insucha
solidlyfrozenworld,aquaticlifewouldbeseverelychallenged,whilethevitalwatercyclewouldallbutcometoahalt.Curiously,thesamephenomenonisoneofseveralfactorsthatfacilitateice-skatingandskiing.Thehighpressureexertedbythebladeofyourskatepressingdownonsolidicehelpstoproduceathinlayerofdenserliquidwateroverwhichyoucanglide.Ifthetemperaturebecomestoocold,typicallybelowabout–100degreesFahrenheit,thelubricatingwaterlayerdoesn’tform,andice-skatingandskiingbecomemuchmoredifficult.Yetanotherdistinctivecharacteristicof“pure”waterisitslackofpurity.No
matterhowcarefullyfilteredordistilled,waterisnevermadeentirelyofH2Omolecules.Asmallfractionofthosethree-atomunitsinevitablysplitsapartintopositivelychargedhydrogenions(hydrons,orH+ions,whichareactuallyjustindividualpositivelychargedprotonswithoutanyelectronsattached),plusnegativelychargedhydroxylgroups(OH−ions).HydronsquicklylatchontowatermoleculestoproduceH3O+hydroniumions.Whatwecallpurewateratroomtemperaturecontainsequalnumbersofpositivehydroniumandnegativehydroxylgroups,ataconcentrationthattranslatestoapHof7(a“powerofhydrogen”of10−7molesofhydroniumgroupsperliter,inchemistryterms).AprimefocusofspeculationonEarth’searliestoceansistheirpHandsalt
content.Watereasilydissolveslotsofimpurities,someofwhicharepositivelychargedlikesodium(Na+)orcalcium(Ca2+)ions,andothersnegativelychargedlikechlorine(Cl−)orcarbonate(CO32−)ions.Aruleofthumbisthatthenetelectricalchargeofanybulkwatersolutionmustbezero:thetotalnumberofpositivechargesmustbebalancedbyanequalnumberofnegativecharges.Inpurewateratroomconditions,10−7molesofH3O+isperfectlybalancedby10−7molesofOH−.Inacids,however,excessH3O+hastobalancenegativeions(suchaschlorineinhydrochloricacid,HCl).Inbases,excessOH−hastobalancepositiveions(suchassodiuminthebasesodiumhydroxide,NaOH).ThestrengthofacidsandbasesisquantifiedbythepHscale.Smallervalues
ofpHpointtoacidicsolutionswithmoreH3O+thanOH−ions.AslightlyacidicsolutionwithpH6(typicalofuntreatedtapwaterinmanylocalities)hastentimesmorehydroniumionsthananeutralsolutionatpH7.Morestronglyacidicliquidsincludecoffee(pH5,withahundredtimesmoreH3O+),vinegar(pH3,withtenthousandtimesmoreH3O+),andlemonjuice(pH2,withonehundredthousandtimesmoreH3O+).Bycontrast,basesareliquidswithmoreOH−thanH3O+,andthuswithpHvaluesgreaterthan7.Commonbasesincludebakingsoda(pH8.5),milkofmagnesia(acommonantacidwithpH10),andhousehold
ammoniacleaners(pH12).Asweshallsee,thepHandsalinityofEarth’searliestoceansremainsahotlydebatedmatter.
Water,WaterEverywhere
Waterisoneofthemostabundantchemicalsinthecosmos.Themorewelook,themorewefind,anditspresenceonotherplanetsandmoonsandcometsprovidescluestoitsabundanceonEarth,aswellasthepossibledistributionofwater-dependentlifeintheuniverse.Telescopicobservationscanbetricky,sinceourwater-richatmospheretendstoobscureallbutthemostconcentrateddepositsofH2Oonmoredistantsources.Nevertheless,someobjectsindeepspacerevealanicysurfaceviatheirdistinctiveabsorptionofinfraredradiation.Thisspectroscopicfingerprintrevealsthatsomecometsandasteroids
incorporatesignificantamountsoffrozenwater.AstronomershavedocumentednumerousicyworldsinourSolarSystem,fromPlutoanditscompanionmoonCharon,toSaturn’sluminousicerings.Allofthegasgiantplanets,thoughprimarilymadeofhydrogenandhelium,holdsignificantstoresofwatervaporintheirdenseatmospheres.AndJupiter’slargemoonsEuropaandCallistoarenowthoughttofeatureaniceveneerafewmilesthickovermuchdeeperencirclingoceansofwater.Closertohome,theotherterrestrialplanetsatfirstglanceappeartoberather
dry.MercuryissoclosetotheSunthatanynear-surfacewaterhaslikelybeenbakedoff,makingthishottestofplanetsalsothemostdesiccated.Venus,thenextplanetout,mayinitiallyhavehadanEarth-likeshareofwater,buttodayitappearstobealmosttotallylackinginnear-surfaceH2O.Itsthick,superheatedcarbondioxideatmospherespeaksofarunawaygreenhouseeffectandancientlossofwhatevernear-surfacewatermighthavebeentherewhenitformed.Mars,withitswhitepolaricecapsthatexpandandrecedeinconcertwiththe
687-daycycleofMartianseasons,isanaltogetherdifferentstory.Astronomershavelongspeculatedthattheredplanetmightbeawet,livingworld.Inthe1870s,duringaparticularlycloseorbitalapproachofMarstoEarth,ItalianastronomerGiovanniSchiaparellidocumenteddark,linearfeaturesthatheinterpretedasnatural,possiblywater-bearingchannels,orcanaliinItalian.WhentheEnglishtranslationofhisoriginaldescriptionserroneouslycalledthemcanals,implyinghigh-techengineeredstructures,theideaofanintelligent,extinctMartianracetookonalifeofitsown.MostnotableoftheseMarslifeenthusiastswasHarvard-trainedastronomerPercivalLowell,whobecame
obsessedbySchiaparelli’sdiscoveriesinthe1890s.Heusedhisfamily’swealthtoconstructaprivateobservatoryinFlagstaff,Arizona,andtherehedevotedhimselftothestudyofMars.Employingastate-of-the-arttwenty-four-inchtelescopeundertheclearArizonaskies,heimaginedthathecouldresolveavastnetworkofcanalsstretchingfromthepresumablyice-coveredpolestothedesiccatedEquator.Inhisimmenselypopularbooks,Mars(1895),MarsandItsCanals(1905),andMarsastheAbodeofLife(1908),Lowelldescribesthelastdesperatetechnologicalmasterpieceofavanished,water-starvedrace.Lowell’scolorfulimaginingsfueledawaveofsciencefictionstoriesand
novels(includingH.G.Wells’s1898classic,TheWaroftheWorlds)butdidlittletoconvincethescientificcommunitythatMarsiswet,muchlessliving.Inspiteofmorethanacenturyofsubsequentstudiesemployinglargerandlargertelescopes,supplementedbyaflurryofsophisticatedMarsflybymissions(commencingwithMariner4in1965),orbiters(Mariner9wasthefirstin1971),andlanders(startingwithVikingin1976),definitiveevidenceforMartianwaterrepositoriesandtheirsignificantextentprovedelusive.Thepresenceofwatericeinthenorthernpolarregionswasfinally,unambiguouslydocumentedbytheVikingmissionswithspectralmeasurementsinthelate1970s,butonlysince2000,withanarsenalofinstrumentsonthelatestgenerationofsatellites,coupledwithscrapingtoolsonthePhoenixlanderandtheSpiritandOpportunityrovers,hasthetruevastextentofwaterandthenatureofitsrepositoriesonMarsbeenrevealed.TodaymostofMars’swateroccursassubsurfacepermafrostandperhapsas
groundwaterindeeperwarmzones—potentiallyhugerepositoriesthatremainhiddenfromthedryoutermostlayer.Hintsoftheextentofthissubsurfacewaterwereprovidedin2002bytheMarsOdysseyspacecraft,whichcarriedasophisticatedneutronspectrometer.WhencosmicraysbombardMars’ssurface,theycandislodgeneutronsfromhydrogen-rich(thatis,water-bearing)deposits.ThespectrometerwasdesignedtodetectsuchneutronsastheysprayoutfromawideswathoftheMartiansurface,fromtheEquatortohighlatitudes.However,theseintriguingresultsraisedasmanyquestionsastheyanswered,fortheexactformofwater—liquid,ice,ormineralbound—couldnotbedetermined.In2007NASA’sMarsReconnaissanceOrbiter,usingground-penetrating
radar,providedamuchhigher-resolutionpictureofthisburiedwater.Thesepioneeringmeasurementsdetectedglacier-sizeaccumulationsoficeinthemid-southernlatitudes.Morerecently,theEuropeanSpaceAgency’sMarsExpressOrbiteremployedasimilarradarsystemtodetectdeepiceacrossawiderswathoftheplanet.Someareasnearthesouthpolerevealice-richzonesmorethan
fifteenhundredfeetdeep.Indeed,Marsmayholdaquantityofsubsurfacefrozenwaterequivalenttoaglobe-spanningoceanhundredsoffeetdeep.SoEarth’soceansmayhaveoncehadaMartiancousin.Watercanalsoberevealedbythepresenceofdistinctiverocksandminerals.
NASA’sPhoenixlanderanditspluckyrovers,SpiritandOpportunity,foundabundantcomplementaryevidenceintheformofmineralsformedthroughwater-rockinteractions.Water-richclaymineralsturnouttobeacommonphenomenoninthenear-surfaceenvironmentofMars,andtheymayrepresentmuchofthehydrogen-richmaterialthattheneutronexperimentsobservedseveralyearsearlier.Evaporitemineralscharacteristicofdried-uplakesoroceansarealsocommon,asisopal—apoorlycrystallizedvarietyofquartzthattypicallyformsinwetsedimentsontheoceanfloor.Asplanetaryscientistsexaminetheredplanetwithneweyes,theyalsosee
moreandmoreevidencethatwateronceflowedfreelyacrossthesculptedMartiansurface.High-resolutionphotosrevealancientrivervalleysandgulliesstrewnwithboulders,teardrop-shapedislands,slumps,andbraidedstreamchannels.Theselandformscutthroughlayersofsedimentthatappeartohavebeenlaiddowninshallowlakesorseas.Indeed,thebeachliketerracesthatgirdlethenorthernhemisphereofMarsimplythatnorthern-latitudeoceansmayoncehavecoveredmorethanathirdoftheMartiansurface.Ifso,thencoolerMarsmayhavebeenablue,life-givingplanetmanymillionsofyearsbeforeEarth.Andthenthere’stheMoon—akeytounderstandingthehistoryofwateronits
largercompanion,Earth.TheMoonisbone-drybyconventionalwisdom(actuallydrierthanbone,whichretainsasignificantwatercomponentevenwhenbakedinthedesertsun).Multiplelinesofevidencepointtothisaridity:Earth-basedtelescopesrevealnocharacteristicinfraredabsorption;MoonrocksfromallsixApollolandingsitesheldnodetectabletracesofwater(atleastby1970analyticalstandards);andthefindingofunrustedironmetalafterfourbillionyearsonthelunarsurfacewouldseemtoprecludeevenatraceofcorrosivewater.It’safunnythingaboutconventionalwisdom,though.Eventuallysomeone
willchallengewhateveryoneelseknowstobetrue,andonceinawhilesomethingreallyinterestingwillbefound.In1994asingleflybyoftheClementinemissionproducedradarmeasurementsthatwereconsistentwithwaterice,thoughmanyplanetaryscientistswereunconvinced.FouryearslatertheLunarProspectoremployedneutronspectroscopytodetectasignificantconcentrationofhydrogenatoms,andhencepossiblywatericeorwater-
containingminerals,nearthepoles.Still,manyexpertspointedtoimplantedhydrogenionsfromtheSun’ssolarwindasamorelikelysourceofthesignal.TheninOctober2009NASAsmashedtheupperstageofanAtlasrocketintooneofthesecraters(theCabeuscrater,nearthesouthernlunarpole)andscrutinizedtheplumeofimpactdebrisforsignsofH2O.Sureenough,theflurryofdustincorporatedasmallbutsignificantamountofthelife-givingstuff—enoughtorenewinterestinlunarwateranditspossibleorigins.Threeback-to-backarticlesinSciencethatsameOctoberestablishedthatevidenceforwaterontheMoonisnowunambiguous.EnterErikHauriandhiscolleaguesattheCarnegieInstitution.Usinganion
microprobe—ahighlysensitiveinstrumentthathadn’tbeenavailabletothefirstgenerationofscientistswhostudiedtheApollosamples—Hauri’steamhasrevisitedthecolorfulglassbeadsofthesortIstudiedinmyfirstgeologyjob,pickingoutMoonspecks,backin1976.Otherscientistshadexaminedtheglassbeadsforsignsofwaterdecadesearlier,buttheirdetectioncapacitieswerenomatchfortheionmicroprobe’sabilitytoresolvemeasurementsatthescaleofamillionthofaninch.Hauriandhiscoworkerspolishedavarietyofglassbeadssothattheirroundcrosssectionswererevealedintheionprobe.Thebeads’outerrimsprovedtobeverydry,withonlyafewpartspermillionwater,butthecoresofthelargestbeadshaveasmuchasahundredpartspermillion.Overbillionsofyears,mostoftheglassbeads’originalwaterhasevaporatedtospace,morefromtheoutsidesthanfromthecores.However,basedonthesignificantamountofremainingwaterdeepinsidethebeads,HauriandhiscolleaguescalculatethattheoriginalwatercontentoftheMoon’smagmamayhavebeenashighas750partspermillion—alotofwater,comparabletomanyvolcanicrocksonEarth,andmorethanenoughtodrivesurfacevolcanismthatwouldhavedispersedmagmainexplosiveeruptionsbillionsofyearsago.IfthatmuchwaterpoweredvolcanoesintheMoon’spast,thenagreatdealof
watermuststillbelockedsomewhereintheMoon’sfrozeninterior.AndsincetheMoonformedprimarilybyTheia’swholesaleexcavationofEarth’sprimordialmantle,ourplanet’sdeepinteriorlikelyholdsprodigiousamountsofunseenwateraswell.
TheVisibleWaterCycle
HowevermuchwaterweendupfindingonMarsortheMoon(anditnowappearsthere’salot),EarthremainsourSolarSystem’soneandonlywaterworld.ThestoryofEarth’swater—howmuchthereis,whatformitassumes,
whereitresides,andhowitmoves—ismorethanalittlecomplicated.Untilasrecentlyasthe1990s,theoceanswerethoughttobebyfarthelargestwaterrepository,storingabout96percentofEarth’saccessibleinventory.Icecapsandglaciers,withabout3percenttoday(andperhapsnomorethan5or6percentevenaticeagepeaksofglacialadvance),comeinatadistantsecond.Groundwater(allthenear-subsurfaceH2O,bothinwell-definedaquifersandasmorewidelydispersedstores)accountsfor1percent,whileallthelakes,rivers,streams,ponds,andtheatmospherecombinedrepresentnomorethanaboutahundredthofapercentofEarth’snear-surfacewatersupply.Allthiswaterisinconstantmotion,shiftingfromonerepositorytoanotheron
ascaleofdaystomillionsofyears.Thedynamic,life-sustainingwatercyclerepresentsthemostobvioussourceofchangeonourperpetuallychangingplanet.Imaginethepossibleperambulationsofasinglewatermolecule—amoleculeformedfromanoxygenatomandtwohydrogenatomsthathaveexistedformanybillionsofyears.BeginwithourmoleculeinthemightyPacificOcean,wheremostofEarth’snear-surfacewatermoleculesspendmostoftheirtime.Agreatoceanriverofcoldwater,theCaliforniaCurrent,sweepsthatmoleculefromnearAlaskasouthwardalongtheCaliforniacoasttoBajaandtheEquator.Asthesurroundingwaterwarmsandrises,themoleculereachesneartheoceansurfaceandbeginsanepicclockwisejourneyaroundtheNorthPacific—firsttheNorthEquatorialCurrentthatflowswesttocurvepastJapan,thentheNorthPacificCurrentheadingeasttoNorthAmerica.AsourmoleculeonceagainmovesclosetoCalifornia,ithappenstorisetothesunlitoceansurfaceandevaporatesintotheatmosphere,wherecloudsarebeginningtoform.Prevailingwindssweepthethickeningmassofraincloudseastward,across
thedesertSouthwest,intotheelevatedterrainoftheRockyMountains.Asthecloudsriseintohigher,coolerelevations,therainbeginstofall.OurmoleculeeventuallydescendstoEarthaspartofaraindrop;itfollowsasinuouspathfromrivulet,tocreek,tostream,toaswollenriverthatoverflowsitsbank.Tothispointthewatermolecule’smovementshavebeenswift—ayearortwotocycletheentirePacificOcean,adayortwotojointhecloudsandfallasrain,aweekorsotoflowacrossthehillyterrain.Butasitsoaksdeeperintothegroundandmergeswithavast,hiddenaquifer,themoleculemayspendmanythousandsofyearscreepingthroughthesubsurfacerealm.Herehumanactionsalternature’sancientrhythm,forwater-hungryfarmers
pumpoutimmensequantitiesofdeepwatertosustainagricultureinthesemiaridSouthwest.Aquifers,minedoftheirwateratunsustainablerates,arethusdryingup.Ourmoleculesuccumbstothistrendandfindsitselfbackatthesurface,
splashingoveraTexascornfield,whereitquicklyevaporatesbackintothecloudlessskyandcontinuesitseastwardjourney.Thisstorycycleswithoutend.Somemoleculesaretemporarilybrokenapart
intohydroniumandhydroxylions,onlytorecombineintonewwatermoleculeswithnewatomiccompanions.OthermoleculesbecomefrozenintothickAntarcticice,wheretheywillremainlockedformillionsofyears.Stillotherwatermoleculesundergochemicalreactionstobecomepartofclaymineralsinthesoil.Lifehasbecomeanintegralpartofthewatercycleaswell.Plantstakeup
watermoleculesandcarbondioxideandcombinethem,intheSun-drivenprocessofphotosynthesis,tomanufactureroots,stems,leaves,andfruit.Andwhenthosenutrient-richplanttissuesareeatenbyanimalsandbrokendownbythemetabolicmiracleofrespiration,thewasteproducts,exhaledwitheverybreathwetake,arereassembledmoleculesofcarbondioxideandwater.
TheDeepWaterCycle
Inthemid-1980sEarthscientistsbegantothinkinearnestaboutwaterataglobalscale,forthenear-surfacewatercyclecan’tbethewholestory.Becauseweknowthatmagmasoriginatingtensorhundredsofmilesdownholdenoughwatertocauseexplosivevolcanism,wecanassumethatsilicatemineralscrystallizeddeepinsideourplanetmustsomehowtrapH2O.Theremustbeadeep,hiddenpartofthewatercyclethatcouldtellusmuchabouthowandwhenEarthbecametheocean-bathedplanetitistoday.Theexperimentalapproachtodeepwaterhasfocusedonthepossibilitythat
themostcommonofminerals—olivine,pyroxene,garnet,andtheirdenserdeep-Earthvariants—maybeabletoincorporateasmallamountofwateratmantleconditions.Thestudyofwaterin“nominallyanhydrous”minerals,whichbecameamajorfocusofhigh-pressuremineralogyinthe1990s,yieldedastonishingresults.Itturnsoutthatathighpressureandtemperature,it’srelativelyeasyforsomemineralstoincorporatelotsofhydrogenatoms,whicharethemineralogicalequivalentofwater(becausehydrogenatomscombinewithoxygenintheseminerals).Mineralsthatareinvariablydryinthecooler,low-pressureenvironmentsoftheshallowcrust—whereexplosivevolcanismreleasesanywater—canbecomeratherwetinthedeepmantle.Inprinciple,theexperimentalstrategyisprettystraightforward.Takeasample
ofolivineorpyroxene,addwater,heatwhilesqueezing,andseewherethewater
goes.Inpractice,it’snotsoeasy.InordertoreproduceEarth’sdeepermantleconditions,thesamplemustbepressurizedtohundredsofthousandsoftimestheatmosphericpressure(equivalenttomillionsofpoundspersquareinch)andsimultaneouslyheatedtotemperaturesashighas4,000degreesFahrenheit.Toaccomplishthisdauntingfeat,scientistsemploytwocomplementaryhigh-pressureapproaches.Somerelyonmassive,room-sizemetalpressesthatexerttonsofpressureona
tinysample—elaboratevariationsonHatYoder’spressurebombfromhalfacenturyago.Oneoft-usedexperimentalassemblyinvolvesfournestedstageslikeRussiandolls:eachstagesurroundsandhugsthenext,focusingimmensepressuresontoasmallerandsmallervolume.First,agiantpairofmetalplatessqueezesfromaboveandbelowwithawesomeforcethatcanreachthousandsoftons.Thosemassiveplatesexertaviselikegriponthecleversecondstage,whichconsistsofsixcurved,interlockingsteelanvils—threeaboveandthreebelow—whichinturnpressuniformlyfromallsidesagainstathirdstagewithacube-shapedclusterofeighttungstencarbideanvils.Thesampleofpowderedmineralpluswatermustbetightlyencasedinsidetheinnermostfourthstage,oftenwithagoldorplatinumlinersothatthereactantsdon’tsquirtoutthesides.Asifgeneratingpressureisn’thardenough,thesamplemustalsobebakedwithelectricalheatersburieddeepinsidethesampleholder,andthetemperaturehastobemeasuredcontinuouslywithadelicateloopofspecialwirecalledathermocouple.AnotherpopularexperimentalapproachtosimulatingEarth’sdeepinterioris
thediamondanvilcell,whichgeneratesextremepressuresbysqueezingtwodiamondswithflattipstogether.Firsttaketwotypicalhalf-caratbrilliant-cutgemdiamonds,justlikethestonesinoldweddingrings,andpolishdownthesharppointsonthebottomtoflat,circularsurfacesatwentiethofaninchacross—whatwillbecometheanvilfaces.Thenmountthediamondsinapreciselyalignedmetalvise,andbetweenthemplaceathinpieceofmetalwithatinyholepunchedout.Centertheholeovertheopposeddiamondanvils,loaditwithwaterandmineralpowder,andsqueeze.Amodestforceonthediamondscreatesatremendouspressurebecausetheanvilsaresosmallandthusconcentratetheforce.DiamondanvilcellshavesustainedrecordhighpressuresequaltothethreemillionatmospheresfoundinEarth’sinnercore.Thebeautyofthediamondanvilcellisthatyoucanseeyourpressurizedsamplebylookingthroughthetransparentgems.Abatteryofanalyticalspectroscopictechniquescanbebroughttobear,andit’seasytoheatthesampletomantleconditionswithahigh-poweredlaser,whichcanalsoshinethroughthetransparentdiamond
anvils.Ifeverythinggoeswell—ifthedesiredpressuresandtemperaturesarereached
andmaintained,ifthethermocoupledoesn’tbreak,ifthesampledoesn’tleak—thenthetrickyanalyticaltasksbegin.Somewater-bearingmineralslikeclaysandmicasareeasytorecognize,buthowdoyoumeasureafewpartspermillionofwaterinanotherwisedrysample?Theionprobeisoneoption;itshighsensitivityandspatialresolutionledtoErikHauri’sdiscoveryoftraceamountsofwaterinlunarvolcanicglass.Infraredspectroscopy,whichcanrevealcharacteristicbondsbetweenoxygenandhydrogen,isanotherusefultool.Newlyformedbondsbetweenhydrogenandoxygenalterthewaythatinfraredradiationinteractswithacrystal—changesthatcanrevealwaterenteringthemineralstructure.Nevertheless,cautiouscolleagues(andwaryrivalsloathtobescooped)willalwaysraisethepossibilitythatexperimentsareflawedoranalyticaltechniquestooinsensitive.Asinglefluidinclusion—aminutepocketofwatertoosmalltoseewithamicroscope—canyieldafalsesignalinsuchfinickymeasurements.Asinanynewscientificendeavor,ittookawhilefortheseexperimentsto
catchon,butthemorescientistslooked,themoremineralsemergedaslikelyhostsfordeepwater.Olivineandpyroxeneinthelowercrustarefairlydry,holdingnomorethanahundredthofapercentwater.Butraisethepressuretomantleconditionsof100,000atmospheresandthetemperatureto2,000degreesFahrenheit,andolivinetransformstowadsleyite,whichcanincorporateawhopping3percentwater.ThecorrespondingEarthlayer,themantle’stransitionzonefromabout255to410milesdeep,isoneofthewettestplacesintheplanetandmayholdninetimesallthewaterintheoceans.Mineralsofthelowermantlearelesswaterlogged,buttheymakeupforthatintheirhugevolume—halfofEarth’stotal—sothelowermantleisestimatedtoholdanothersixteentimesthewaterinEarth’soceans.Giventhelikelihoodofotherwater-richmineralsandthatEarth’sironcoreprobablyholdsalotofhydrogenaswell,thedeepinteriormaystoremorethaneightyoceans’worthofwater.
FirstOcean
Conservativeestimatesplaceproto-Earth’soriginalbudgetofvolatilesatmorethanahundredtimesmodernlevels.Indeed,oneoftheprincipalchallengesinmodelingthehistoryofEarth’svolatilesisfiguringouthowmuchwaslost—andhowitescaped.Ofsomethingswecanbesure.Fromdayone,volatileswerereleased
prodigiouslyfromthedeepinterior,asmegavolcanoespumpedhugequantitiesofsteamintoarapidlythickeningatmosphere.Inthefirstfewmillionyearsofproto-Earth’sexistence,thatfirstatmospheremayhavebeenmanytimesdenserthanthatofthemodernworld.Watermayhavepouredoutontothesurfaceinliquidform,coolingthefirstrocksandformingwide,shallowseaswithinafewtensofmillionsofyears.AndthentheBigThwackblasteditallaway.Almosteverymoleculethathad
workeditswaytothesurfacewaslosttospace,inwhatamountedtothepushingofagiantresetbutton.WehavenoreasonableestimatesastohowmuchofEarth’sstoreofnitrogen,water,andothervolatileswaslostinthatsingleevent,butitwasalot.Dozensofsmaller-scaleimpactsofhundred-mile-diameterrockscausedunimaginabledisruptionsforanotherfivehundredmillionyears,eachonevaporizingasignificantfractionoftheoceansandfurtherdiminishingthevolatileinventory.Nonetheless,withinafewmillionyearsfollowingtheBigThwack,water
vaporhadagainbecomeaprincipalcomponentoftheprimordialatmosphere,formingaglobaltempestofturbulentdarkclouds,howlingwinds,shatteringlightning,andunceasingtorrentialrain.Thesurfaceofthestorm-lashedbasaltcrustcooledandhardened,aslowlyingbasinsgraduallyfilled,slowlyformingtheoceans.Foratimetheencroachingseascreatedaglobalsauna,asthethinveneerofsurfacewaterpenetratedcracksandfissures,contactedthehotrocksbelow,andreturnedtothesurfaceasgargantuangeysersofroaringsteamandsuperheatedwater.Suchintensewater-rockinteractionsservedtohastencrustalcooling,makingwayfordeeperponds,thenlakes,thenoceans.Theexacttimingoftheglobalocean’sformationisunknown,buttantalizing
evidencehasemergedintheformofEarth’soldestcrystals.SomeofthemostancientrocksonEartharethree-billion-year-oldlayersofsedimentsinthearidsheep-ranchingterrainofWesternAustralia,knownastheJackHills.Thesand-sizemineralandrockfragmentsthatmakeupthosesedimentswouldhaveerodedfromvanishedrockformationsthatmusthavebeenmucholder.Atinyfractionofthosesandgrains,notmorethanoneinamillion,ismadeofthemineralzircon—zirconiumsilicate(ZrSiO4),oneofthetoughestmaterialsinnature.Individualzircongrains,typicallysmallerthantheperiodattheendofthis
sentence,firstformedasaminoraccessorymineralinigneousrocks.Imaginebasaltsolidifyingoutofameltthatholdsonlyatraceoftheelementzirconium.Mostofthechemicalelements,whetherrareorcommon,easilyenterthecrystalstructuresofpyroxene,olivine,andfeldspar.Butzirconiumhasnohomein
commonminerals.Rather,itseeksitsownkindandthusformsisolated,tinyzirconcrystals.Severalfactorsacttogethertomaketheseeasilyoverlookedzirconcrystalsa
uniquesourceofinsightabouttheearliestEarth.First,zirconscanlastalmostforever(atleastforallofEarthhistory).Asinglecrystalofzirconcanerodefromonerock(perhapstheigneoushostwhereitfirstcrystallized),thenbecomepartofasedimentarysandstonerock,andthenerodeagainandagainandagainforbillionsofyears.Thesameindividualzircongraincanberecycledthroughadozendifferentsedimentaryrockformations.Second,zirconcrystalstelltimebecausetheyreadilyincorporatetheelement
uranium,whichcanmakeup1percentormoreoftheiratoms.Radioactiveuranium,withahalf-lifeofabout4.5billionyears,isnature’sultimatestopwatch.Onceazirconcrystalforms,itsuraniumatomsarelockedin,andtheybegintodecayatasteadyrate;halfofthemdecayonaverageevery4.5billionyears,eachoneultimatelytransformingintoastableatomoflead.Theratioofdwindlingparentaluraniumatomstogrowingdaughterleadatomsprovidesanaccurateestimateofthezirconcrystal’sage.Finally,twoofeverythreeatomsinzirconareoxygen,whichprovidesclues
aboutthetemperatureofformation.RecallthatonelineofevidenceregardingtheMoon’sformationwasthedistinctiveratioofoxygen’sstableisotopes:EarthandtheMoonhaveidenticalratiosofoxygen-16tooxygen-18,whichimpliesthattheyformedatasimilardistancefromtheSun.Inasimilarlineofreasoning,theratioofoxygen-16tooxygen-18inazirconcrystalpointstothetemperatureatwhichitgrew:heavieroxygen-18-enrichedsamplespointtoacoolertemperatureofformation.Forigneousrocks,thistemperaturecanbeasensitiveindicatorofthewatercontentofthemagmainwhichzirconcrystalsgrew,becausewaterlowersthetemperatureatwhichcrystalsgrow.What’smore,isotoperatiosfromwaternearEarth’ssurfacetendtobeevenmoreenrichedinheavyoxygen,sozirconcrystalswithextremelyhighoxygen-18contentshavebeeninterpretedashavinginteractedwithsurfacewater.Inthisway,zirconcrystalsfromEarth’searliestrockscansurvivemany
cyclesoferosionanddeposition,whiletheypreservedetailsoftheage,temperature,andwatercontentoftheiroriginalenvironment.Allthatinformationgleanedfromcrystalsbarelylargeenoughtoseewithoutamicroscope!ThebottomlineisthatmanyindividualzirconcrystalsfromtheJackHillsof
Australiaaremorethan4billionyearsold,withoneaged,ancientsandgrainclockinginataremarkable4.4billionyears.Thatoldestzirconcrystal—indeed,
theoldestknownsurvivingsolidfragmentofEarth—hasasurprisinglyheavyoxygenisotopecomposition.Somescientistsconcludethat4.4billionyearsago,whenEarthwasonlyabout150millionyearsold,thesurfacewasrelativelycoolandwet:hencetherewereoceans.Otherexpertsaren’tsosure.Theypointoutthatzirconcrystalscanbe
incrediblycomplicated:that4.4-billion-year-oldgrain,aswellasvirtuallyallitsslightlyyoungercompanionsfromtheJackHills,hasanancientcrystalcore.Butdetailedmappingofeachindividualcrystalrevealsconcentriclayersofyoungerzircongrownaroundoldlayers.It’snotuncommonforasinglegraintodisplayabillion-yearrangeofagesfromcoretorim,withcorrespondinglycomplexvariationsinoxygenisotopes.Iftheoldercorewasalteredduringmorerecentpulsesofcrystalgrowth,thenthetruecharacterofEarth’sancientsurfacemightbeobscured.Whatevertheeventualoutcomeofthezirconstory,mostexpertsagreethatnot
muchmorethanonehundredmillionyearsaftertheBigThwack,Earthhadbecomeabrilliantbluewaterworldwithamile-deepencirclingocean.Fromspaceitwouldhaveappearedasanultramarinemarble,swirledwithwhitewispsofcloudstobesure,butpredominantlyabreathtakingblue.(Theocean’scolorarisesfromsimplephysics.Thesunlightthatbathesthesurfacecomprisesallthecolorsoftherainbow—reds,yellows,greens,andblues—butwaterabsorbstheredendofthespectrummoreeasily,sooureyesperceivethepredominanceofthescatteredbluewavelengthsoflight.)Andwhatoftheland?TodaythecontinentsoccupyalmostathirdofEarth’s
surface,butatthedawntimeofourplanet,duringthehellishHadeanEon,continentshadnotyetformed.Theprimordialblueoceanwasbrokenonlybyisolatedsteamingvolcanicislandsthatpokedabovethewaves.Theircone-shapedcontoursandnarrow,rubblyblackbeaches,randomlydottingtheglobefromthepolestotheEquator,weretheonlyfeaturestobreakthewaterymonotony.AswethinkbacktoEarth’searliest,globe-spanningocean,wewonderwhatit
waslike.Wasithot?Probablyatfirst,giventhestill-coolingglobalmagmaoceanbeneath.Wasitfreshorsalty?Saltisperhapsthemostdistinctivepropertyofmodernoceanwater,butitmightseemreasonabletoassumethatEarth’sfirstoceanstartedoutfresh,withfewdissolvedchemicals,andonlygraduallyachievedthesaltinesswefindtoday.Onthecontrary,recentevidencesuggeststhatthehotearlyoceanquicklybecamefarsaltierthantoday.Commontablesalt,sodiumchloride(NaCl),readilydissolvesinhotwater.TodayabouthalfofEarth’ssaltistiedupinlandlockedsaltdomesandotherevaporitedeposits
relatedtodried-upbodiesofsaltwater.Mostofthissaltissequesteredinthicklayersdeepunderground,butduringEarth’sfirsthalf-billionyears,therewerenocontinentsonwhichtoharborsalt.Consequently,thesalinityofthefirstoceanmayhavebeenasmuchastwicethatofthemodernworld.Moreover,otherelementsdissolvedinthewarmoceanwater—primarilyiron,magnesium,andcalciumthatdominatebasalt—wouldhavebeenpresentinhigherconcentrationsaswell.ScientistsalsowonderiftheHadeanoceanwasacidicorbasic.Themost
criticalsinglefactorcontrollingtheocean’spHandsalinityisatmosphericcarbondioxide.Bymostaccounts,theCO2contentoftheearlyatmospherewasthousandsoftimeshigherthantoday’svalueofabitlessthanfourhundredpartspermillion(thoughyearbyyearwearerapidlyapproachingandsoonwillsurpassthatlevel).AlotmoreCO2intheHadeanairmeantalotmoreCO2inthewateraswell,andthatmusthavehadsignificantconsequencesforbothpHandsalinity.Carbondioxidecombineswithrainwatertoformcarbonicacid,H2CO3.Intheocean,thiscarbonatepartlydissociatestohydrogenions,whichformhydroniumions(theH3O+ofacids)plusbicarbonate(orHCO3−).ThatnetadditionofH+turnstheoceansmoreacidic,perhapstoapHaslowas5.5.Suchacidicoceanconditionsinturnprobablyacceleratedtheweatheringofbasaltandotherrocks,addingevenmoresolutestoanalreadysaltyocean.
TheFaintSunParadox
Asifthedetailed,sometimesconflictingstoriesofEarth’sfirstoceanweren’tcontroversialenough,there’soneadditionalbigwrinkletocontendwith:accordingtoincreasinglysensitiveastronomicalobservationsandastrophysicalcalculations,starslikeourSunundergoaslow,inexorablebrighteningovertheirlifetimes.Bytheseestimates,theyouthfulSunof4.4billionyearsagowas25to30percentlessbrightthanitistoday.What’smore,theSunwouldhaveremaineduncomfortablyfaintforatleastanother1.5billionyears.Iftoday’sSunweresuddenlytodimbythatextremeamount,Earthwouldquicklyenteradevastatingiceboxphase;theoceanswouldfreezesolidfromthepolestotheEquator,andmostlifeonEarthwoulddie.Onlythehardiestorganisms,deep-subsurfacemicrobiallifeandanimalslivinginprotectedhydrothermalzonesassociatedwithvolcanoes,couldsurvivesuchacatastrophicclimatechange.GivensuchacoolerearlySun,Earthmustsurelyhavequicklyfrozenover.
Andyetgeologicalevidenceforabundantsurfacewateratleastasfarbackas
fourbillionyearsisunambiguous.Sedimentsfrombothshallow-anddeep-waterenvironmentsarecommon.Lifebeganandthrivedduringthatinterval.How,then,couldtheearlyoceanhavebeenliquid?CertainlysomeoftheheatdeficiencycausedbythemuchfainterSunwas
compensatedforbyamuchhotterEarth.Followingthecrustingoveroftheprimordialmagmaocean,therewasstillalotofhotmoltenrockandvolcanicactivitytowarmthesurface.Anoceanonsuchaplanetwouldhavebeencontinuouslyheatedfrombelow,astheblackcrustslowlythickenedandcooled.TheleadinghypothesistoexplainthefaintSunparadoxpointstoan
exaggeratedgreenhousewarmingeffectcausedbyextremelyhighatmosphericconcentrationsofcarbondioxide—perhapsmorethantentimesthepressureofourpresentatmosphere(thesamehighCO2concentrationsthatmayhaveacidifiedtheoceanandincreaseditssalinity).AsecondcleverscenariopositsthatEarth,initsearlyblack,thenblue,phases,
absorbedafarhigherpercentageoftheSun’senergythanthesurfacedoestoday.Todaytheoceansabsorbmoresunlightthantheland—aneffectpossiblyexaggeratedlongagobythehighconcentrationsofironintheearliestoceans.Thatincreasedsolarabsorptionwascoupledwithaprobabledearthoflight-scatteringclouds;today,plant-generatedparticlesandchemicalsplayamajorroleinnucleatingclouds,butbillionsofyearsagotherewerenoplantstotriggercloudformation.Yetanotherhypothesisplacesalargeamountofthepotentgreenhousegas
methaneintheearlyatmosphere.Acuriousconsequenceofamethane-richatmospherewouldhavebeenchemicalreactionshighintheatmosphere,whereultravioletradiationwouldhavetriggeredthesynthesisofarichvarietyoforganicmolecules,includingpossiblebuildingblocksoflife.Suchorganicmoleculesmighthavecausedathick,smoglikehaze,transformingtheblueEarthintoadistinctlyorangeworld,notunlikeSaturn’sbigmoonTitan.Andsowhilewedon’tyetknowtheexactcombinationoffactors,wehavemorethanenoughexplanationsastohowEarthkeptwellabovethefreezingpoint.Whatwecansaywithconfidenceisthat,onceformed,thisglobalocean
shapedtheplanet’soutermostlayers—insculptingtheland,intheevolutionoftheincreasinglydiversemineralkingdom,andintheoriginofthebiosphere.Waterstillworksitsmagicineveryfacetofourlives,astheconcentratorofmineralwealth,astheprincipalagentforsurfacechange,andasthemediumforalllife.
*ThoughoftenattributedtoDonaldRumsfeld’s2002speeches,thosequotesfirst
appearedseveralyearsearlierintheprefacetomy1997bookwithMaxineSinger,WhyAren’tBlackHolesBlack?
Chapter5
GrayEarth
TheFirstGraniteCrustEarth’sAge:200to500millionyears
Earthtodayisaworldofcontrasts—one-thirdland,two-thirdswater;seenfromspace,amélangeofblue,brown,green,andswirlingwhite.Notso4.4billionyearsago,whenwidelyscattered,symmetricalvolcanicconesofblackbasaltweretheonlymeagerparcelsofdrylandtopokeabovethebluemonotonyoftheshallowseas.Allthatwasabouttochangewiththeinventionofgranite—theruggedfoundationstoneofthecontinents.Earth’sstoryisasagaofdifferentiation—oftheseparationandconcentration
ofelementsintonewrocksandminerals,intocontinentsandseas,andultimatelyintolife.Timeandtimeagainthisthemehasplayedout.Theinnerrockyplanets—Mercury,Venus,Earth,andMars—formedwhenintensepulsesofsolarwindseparatedhydrogenandheliumfromtheheavierbigsixelements,sweepingthelightergaseouselementsoutwardtothedomainofthegiantplanetsJupiter,Saturn,Uranus,andNeptune.OnEarth,densemoltenironsettledtothecenter,asthemetalcoreseparatedfromtheperidotite-richmantle.Partialmeltingofperidotiteproducedbasalt,arockrichinsilicon,calcium,andaluminum,whichseparatedfromperidotitetoformEarth’sfirstthin,blackcrust.Asbasalteruptedexplosivelyontothesurface,waterandothervolatilesseparatedfromthebasalticmagmatoformthefirstoceansandatmosphere.Eachheat-drivenstepseparatedandconcentratedelements;eachstepledtoanincreasinglylayered,differentiatedplanet.Theriseofcontinentswasyetanotherimportantstepinthedifferentiationof
Earth.Astheoutercrustallayersofbasaltcooledandhardened,theyformedalidlike,heat-trappingcovertotheroilingmantlebeneath.Basalt,reheatedfrombelow,begantomeltatrelativelylowtemperatures,especiallyinthepresenceofwater—aschillyas1,200degreesFahrenheit.Asthetemperatureincreased,sodidthepercentofbasaltmelting—first5percent,then10percent,eventuallyupto25percentmelt.Inanechoofperidotitemelting,theresultingmagmawassharplydifferentincompositionfromitshostbasalticrock.Mostnotably,thisnewmeltwasmuchricherinsilicon,withasignificantlyenhancedcomponent
ofsodiumandpotassiumaswell.Water,too,concentratedinthishotfluid,asdiddozensofraretraceelements—beryllium,lithium,uranium,zircon,tantalum,andmanymore.Thisnewsilicon-richmagmawasmuchlessdensethanitsparentbasalt,soitinevitablypusheditswaytowardthesurface,formingthefirstgranite.Mostgraniteshostasimplemineralogyoffourdifferentspecies.Clear,
colorlesscrystalsofquartz—puresiliconoxide—aboundingranite;theirtoughgrainswoulderodetoproduceEarth’sfirstwhitesandybeaches.Twokindsoffeldspar,onerichinpotassiumandtheotherinsodium,gaveEarth’searliestgranitestheirmonotonousgrayish-whitecolor.Andsprinkledineverygraniteisafourth,darker,iron-bearingmineral—sometimesblockypyroxene,sometimessheetlikemica,sometimeselongateamphibole.Thenexttimeyouseeapolishedgranitecountertoporbathroomfixture,takealookforthissimplesuiteoffourminerals.Thepresenceofrarerelementsoftenleadstoscatteredtiniergrainsof
additionalminerals,suchaszircon,forexample,whichconcentrateszirconium.Recallfromthelastchapterthatminute,gemmyredzirconcrystalsextractedfromtheremoteJackHillsofAustraliaprovidehintsofanearlyocean4.4billionyearsago.Thosesamecrystals,whichappeartohaveformedunderrelativelycool,wetconditions,mayalsopointtothebeginningsofgraniteformationatthatearlystage.NotonlydotheJackHillszirconsbearthedistinctiveheavyoxygenisotopesignatureofacoolandwetorigin,butafew4-billion-year-oldcrystalsalsoholdinclusionsofquartz—amineralrarelyproducedbeforetheadventofgranite.Someexpertssuggestthattheseold,cool,quartz-bearingzirconcrystalsarethelastsurvivingremnantsoftheearliestgranitecrust.Withtheoriginofgranite,weseeforthefirsttimeasignificantdivergenceof
Earth’smineralevolutionfromthatofsomeofitsplanetaryneighbors.Graniteformationrequiresabundantbasaltneartheplanetarysurfaceaswellasintenseinternalheattoremeltit.ThesmallerplanetsMarsandMercury,aswellasEarth’sMoon,aregirdledbythenecessarybasalticveneer,buttheyaretoosmalltomakemuchgranite.Theylackthenecessaryinternalheat.Smallvolumesofgranitewereundoubtedlygeneratedontheseworlds,butnothinglikethedeeplyrootedgranitecontinentsofEarth.
Buoyancy
Earth’sprimordialcrustofblackbasalt,softenedbyheatfrombelowandwitha
uniformdensityaboutthreetimesthatofwater,nevercouldsupportmuchtopography.Afewvolcanicedificesmayhavesoaredamileortwoabovethemean,enoughforscatteredblackislandstoriseabovethesea,buttherewerenogreatmountainranges,noranydeepoceanbasinsbeforetheriseofcontinents.Granite,withasignificantlyloweraveragedensity(about2.7timesthatofwater),changedthatdynamic.Graniteinevitablyfloatsonbasaltandperidotite;itpilesupingreatmounds,risingmilesabovethesurfacelikeanicebergfloatingonwater.Ice,whichisabout10percentlessdensethanwater,providesafamiliar
analogue.Becauseofthisdensitydifference,about10percentofaniceberg’svolumesticksoutofthewater.Ajaggedtwo-hundred-foot-tallicebergwilltypicallyexposethirtyormorefeetabovethesurface;hence“thetipoftheiceberg.”Bythesametoken,graniteis10percentlessdensethanthebasaltonwhichitfloats.AstheEarth’spartiallymeltedbasaltcrustgeneratedlayeruponlayerofgranite,iceberglikeprotrusionsbegantoform.Amile-thickgranitebodymighthaveproducedasmallmoundthatprojectedalmostathousandfeetabovetheaveragelevelofbasalticcrust.Butovertime,accumulatingmassesofgranitecrustreachedthicknessesofmanymiles;accordingly,deeplyrootedcontinentallandmassesrosehigherandhigherabovetheoceans,withsomemountainrangessoaringmilesabovethewater’ssurface.Today’sRockyMountainchainintheAmericanWest,withgraniterootsasmuchasfortymilesdeep,boastsnumerouspeaksoverfourteenthousandfeet.ThisgrandspineoftheNorthAmericancontinentstandstallastestimonytothebuoyancyofgranite.In1970,whenItookmyfirstgeologycourseatMIT,thepowerofbuoyancy
indrivinggeologicalchangewasstilltextbookorthodoxy.(Weusedthe1965editionofBritishgeologistArthurHolmes’srichlyillustratedclassic,PrinciplesofPhysicalGeology.)“Isostasy,”itwascalled.Thedrivingforceof“verticaltectonics”was“isostaticreadjustment.”Aneatwoodcut,virtuallyunchangedfromnineteenth-centurygeologytexts,showedalineofrectangularwoodenblocksofdifferentheightsfloatinginwater.Tallerblocksprojectedhigheroutofthewater,justlikeamountain.Welearnedhowoceanbasinshadfilledwiththicklayersofsediments,andhowthosesedimentshadmeltedtoformmoregranitebodies.Welearnedhowmountainssubsequentlyrosefromthosebuoyantgraniticcores.Itallmadeperfectsenseatthetime,andit’sstilltheleadinghypothesisforhowEarth’searliestcrustformedmorethanfourbillionyearsago.EarlyinEarth’shistory,perhapsevenwithinthefirsttwohundredmillion
years,modestlandmassesofbuoyantgraygranitemusthavebeguntoform
abovehotspots,asdeepaccumulationsofbasaltwerepartiallymelted.Inthoseearlytimesverticaltectonicsandisostasymusthaveprevailed,justasArthurHolmestaught.Thosefirstisolatedcontinentalbitsofgranitewereutterlybarrenandwindsweptandbatteredbyintensewaves.Erodedrockfragmentsofquartzslowlyaccumulatedtoformmeagersandybeaches,whilefeldsparsweatheredtothinlayersofclay-richsoils.Thefirstgraniteislandswereisolated,modestinsize,andlowinprofile;theygavenohinttothescaleofthecontinentstocome.
ImpactRedux?
SohowdidearlyEarthshiftfromavolcano-dottedbasalticworldtoaplanetwithwide,graygraniticcontinents?Howdidthefirstfewlonelygraniteislandsexpandtothehemisphere-spanninglandmassesweseetoday?Earthscientistshaveneverbeenshyindevisinghypotheses.Oneofthemoreintriguingideaspositsacontinent-formingsequencetriggeredbythatfamiliaragentofchance:thecareeningasteroid.ForabillionyearsafterTheia’sobliterationandtheMoon’sformation,huge
impactsoccurredfromtimetotime.Thisisindisputable.Expertsestimatethatdozensoflargeasteroidsuptoahundredmilesacross—wanderingremnantsoftheoriginalplanet-formingera—musthavecollidedwithEarthduringitsformativeaeons.Imagineascenariofourbillionyearsago,asahotplumeofmagmaascendsbeneathayoungoceancrust.Manydozens,ifnothundreds,ofsuchplumesmusthaverisenfromEarth’sdeepinteriortotransferinnerheatbytheefficientprocessofconvection.Greatvolcanoesspewingbasalticlavashaveeruptedaboveeachplume,evenasremeltingofbasaltcrustgeneratedagraniticcomponentthatthickenedtheland.Thencomescatastrophe:athirty-mile-diameterasteroidslamsintothe
volcaniccluster,eradicatingeverytraceoflandwithinathree-hundred-mileradius.Theimpactproducesagiant,bowl-shapedmagmalakewhileshoweringtheadjacentsurfacewithblobsofstickymoltenlavaandshatteredrock.Thiscosmicinsultblocksthemantleplume,whichhastofindanewpathtothesurface.Accordingtothiscleverscenario,thepostimpactplumechangespathtopoke
upunderneathaminicontinent,withbasalticrootsandagrowinggraniteveneer.Onceitispositionedbelowsuchathick,heat-trappinglidofbasalt,thenewheatsourceproducesnewpulsesofgraniteinabundance,thusexpandingandthickeningtheland.ThisuntestablestoryisprobablypartofEarth’searliestcontinent-forming
narrative.Abillionyearsofverticaltectonicsenhancedbyasteroidcollisionswouldhavegeneratedanever-increasinginventoryofvolcanicoceanislandswithintermixedbasaltandgranitecores.Graduallylandarosefromthesea.Byfourbillionyearsago,largeislands,distributedatrandomacrosstheglobe,mayhaveoccupiedamodestpercentageofEarth’ssurface.Butthenalongcameplatetectonics,andEarth’snear-surfaceevolutionshifted
intohighgear.
DriftingContinents
ThediscoveryofplatetectonicsasEarth’sdominantgeologicalprocessisitselfastoryspanningmostofmodernscience.Thoughanticipatedbyatleastfourcenturiesofobservations,theideathatentirecontinentscouldsomehowmigrateacrossEarth’ssurfacewasdimandhereticalatfirst,comingintosharpfocusandwidespreadacceptanceonlyinafuriousinternationalrushofdiscoveriesinthe1960s.Butoncethefloodofevidencebegantopileup,theEarthsciencesexperiencedoneofthemostrapidparadigmshiftsinthehistoryofscience.Indeed,withinfiveyearsofmyundergraduatetenureatMIT,bythemid-1970s,everygeologytextbookhadtobecompletelyrewrittenastheoldorthodoxyofverticaltectonicswasallbutexpunged.Inretrospect,someoftheevidenceagainstverticaltectonicsshouldhavebeen
obvious.AstallastheRockyMountainsstandtoday,theyaredwarfedbytheseven-mileheightofMountEverestandthemightyHimalayanrange.Likewise,incontrasttothetwo-mileaveragedepthoftheoceans,Earth’sdeepestoceantrench,locatedofftheMarianaIslandsintheSouthPacific,plungestoanastonishingsevenmiles.Suchtopographicextremescouldnotpossiblybesustainedinanisostaticworld.Verticaltectonicscouldn’tbethewholestory.Subtlehintsoflateraltectonics—theroleofsidewaysmotionsinEarth’s
geologicalevolution—camewiththefirstaccuratemapsoftheNewWorld’scoastline.Bytheearly1600s,thestrikingconformitybetweentheeasterncoastlineoftheAmericasandthewesternshoresofEuropeandAfricawasplaintosee.Thesamesinuousshape,thesameembaymentsandbumps,theroundedcontoursofextremesouthwesternAfricaandthematchingsuggestiveeastwardcurlofSouthAmerica’stip—allpointedtosomeancientjigsaw-puzzle-likefit.Severalbizarrehypothesesattemptedtoexplainthetantalizingtransatlantic
continentalmatch.AstronomerWilliamHenryPickeringofHarvardUniversity,whosupportedGeorgeDarwin’stheoryoffissionoriginoftheMoon(asamoltenblobhurledintospacefromarapidlyspinningEarth),positedthat
simultaneouslyastheMoonwasrippedfromthePacificOcean,ontheoppositesideofEarththeAtlanticOceanopenedwide.OtherssawthehandofGodinthegreatAtlanticS.PerhapstheAtlanticcoastlinesweretheshoresofNoah’smightyflood,whichhadbeenunleashedafewthousandyearsagotocreatethegreatoceanand“dividethelands.”Systematicgeologicalsurveysmighthavehelpedresolvethequestion,but
fourhundredyearsagogeologyhadnotevenbeennamed,muchlesspursuedinanysystematicway.Miningandagriculture,thedrivingeconomicforcesbehindtheearliestgeologicalsurveysofthelateeighteenthcentury,werestrictlystateandnationalaffairs.Littleeffortwasmadetomatchupgeologicalformationsacrosspoliticalboundaries;norweretherichesofoneprincipalityseenaslinkedinanycoherentwaytothoseofanyother.Goldwasquiteliterallywhereyoufoundit.Insuchanationalisticmappingmilieu,matchingupgeologicalfeaturesacrossthegreatexpanseoftheAtlanticOceanwashardlyapriority.Thefirstdetailedtransatlanticgeologicalcomparisonswereundertakenbyan
unlikelyscholar,meteorologistAlfredWegener,whospentmuchofhiscareerintheArctic.(HediedatagefiftyduringaheroicwinterrescuemissiononthefrigidGreenlandicesheet.)Thoughhisprofessionallifewasdevotedprimarilytostudyingtheoriginsofweather,hismostmemorableandlastingworkrelatedtowhathecalled“continentaldrift,”anearlyandmuchdisparagedcontributiontolateraltectonics.TheinspirationforthisoddgeologicaldigressioncameduringWorldWarI,whenheservedasareservelieutenantintheGermanarmy.ShotthroughtheneckduringtheBelgiancampaign,Wegenerwasrelievedoffrontlinedutyandpermittedtodevotehisconvalescencetostudy.Wegener,likemanyofhispredecessors,wasstruckbytheapparentfitof
continentsacrosstheAtlanticOcean,thoughmanyscientistshaddismissedthematchascoincidence.WegenercastawiderfieldofviewandrealizedthatsimilarfitscouldbeseeninthevariedcoastlinesofEastAfrica,Antarctica,India,andAustralia.Indeed,allofEarth’scontinentscouldbeelegantlyclusteredtogethertomakeonesupercontinent,whichhedubbedPangaea(fromtheGreekfor“alllands”).Wegenerandahandfuloflike-mindedsupportersalsocitedevidencefromrecentlypublishedgeologicalsurveysofcoastalregionsofEurope,Africa,andtheAmericas—treatisesthatrevealedtantalizingcorrelationsacrossthewideexpanseoftheAtlantic.Greatminingdistricts,suchastheextensivegoldanddiamondreservesofBrazilandSouthAfrica,appearasasinglelargedepositwhenthecontinentsarejuxtaposed.Similarly,rocklayersbearingthedistinctivefossilfernGlossopterisandtheextinctreptileMesosauruslineupalmostexactly.Suchdetailedgeologicalandpaleontologicalcorrelations
couldnotbesimplecoincidence,heargued.Wegener’scontinentaldrifthypothesisfirstappearedinprintin1915.Three
subsequentGermaneditions,eachmoredetailedthanthelast,aswellasa1924EnglishtranslationentitledTheOriginsoftheContinentsandOceansandmanyothereditions,followed.Newdatapouredintosupporttheideathatcontinentshadoncebeenjoinedtogether.In1917acommitteeofpaleontologistscatalogedmorethanadozeninstancesofdistinctivefossil-bearingstratamatchingupacrosstheoceans—datatheyinterpretedasrequiringsomesortofancientlandbridges.SouthAfricangeologistJamesDuToit,whowasespeciallyenamoredofWegener’sideas,obtainedagrantfromtheCarnegieInstitutiontovisiteasternSouthAmerica.Herecordedmoreexamplesoftransoceanicmatchups:strikinginstancesofidenticalminerals,rocks,andfossils.Yetdespitetheaccumulatingdataforcontinentalalignment,theEarthscience
communitywasunmoved.Lackingaplausiblemechanismforcontinental-scalewanderings,manygeologistswereopenlycontemptuousofWegener’sconjectures.TheywerebolsteredinthesecriticismsbyNewton’sfirstlawofmotion,whichstatesthatnothinghappenswithoutaforce.Untilanepicforceofglobalscalecouldbeinvoked,continentaldriftwouldbeviewedaslittlemorethanacrackpotideabyageologicalamateur.CambridgephysicistHaroldJeffreyssummeduptheBritishviewpointin1923:“ThephysicalcausesthatWegeneroffersareridiculouslyinadequate.”GeologistsinAmericawereequallyunconvinced.RollinT.ChamberlinoftheUniversityofChicago’sgeologydepartmentblastedcontinentaldriftata1926symposium:“Wegener’shypothesisingeneralisofthefoot-loosetype,inthatittakesconsiderablelibertywithourglobe,andislessboundbyrestrictionsortieddownbyuglyfactsthanmostofitsrivaltheories.…IfwearetobelieveWegener’shypothesiswemustforgeteverythingthathasbeenlearnedinthelast70yearsandstartallover.”Evenso,afewEarthscientistsweresufficientlyintriguedbythefindingsof
Wegenerandhissupporterstodevisenovelmechanismsforcontinentalshifts.OneschoolofthoughtpositedthatEarthisshrinking,perhapsbycoolingorbycollapseofgas-filledvoidsinthedeepinterior,andthusportionsofthesurfacemustgraduallyfallinwardlikeabrokenarchway.Inthisuntenablemodel,thecontinentsonceboastedacontinuousexpanseoflandfromthewesterncoastsoftheAmericasallthewaytotheeasterncoastsofAfricaandAsia.Today’sAtlanticOceanwasviewedasagiantarchwayoflandthathascollapsedintothemantle.BasicEuclidiangeometryfoiledthisshrinkingEarthmodel:asimplearchwaycancollapse,buttransferthatideaontoasphereandthere’snowaya
continentalvolumecoveringtheareaoftheAtlanticOceancouldcollapseintoanything.AnothergroupproposedtheantitheticalviewthatEarthhasbeenexpanding,
inflatinglikeaballoonovergeologicaltime.Onceuponatime,therewasonlycontinentalcrust,whichhascrackedandsplitapartastheplanetinflated(bysomeaccountsfromthegenerationofdeep,hotexpandinggases).Indeed,ifyouplayanimaginaryvideotapeofasupposedlyexpandingEarthbackward,youcanarriveatastatewhereallthecontinentsslideneatlytogethertocoveraspherethatisaboutthree-fifthsthediameterofthemodernEarth.LackinganyotherwidelyacceptedformationmechanismfortheAtlantic,thishypothesispersistedinsomegeologicalcirclesfromthe1920stoaslateasthe1960s,whenacompellingnewideatookitsplace.
TheHiddenMountains
Fast-forwardtothepost–WorldWarIIyears,atimeoftremendoustechnologicalinnovationandoptimisminscience.Twodevelopmentsinantisubmarinewarfare,bothdeclassifiedandadoptedbyoceanographersinthe1950s,ledtotransformativediscoveriesaboutthedynamicEarth.Sonar,whichusessoundwavestomeasuredistanceanddirection,isa
century-oldtechnologyfamiliartoanyonewhohaswatchedHollywoodsubmarinemovies.YouhearaPING,whichisansweredashortwhilelaterbyasofter,echoingping.Asoundwavehasbouncedoffthesolidhullofasubmarine.(Theeffectontheviewerdependsonwhetherthemovie’spointofviewisthehunterorthehunted.)“PING......ping,”“PING....ping,”“PING..ping”:theechoescomefasterasthesubmarine’slocationispinpointed.Thetensemusicbuilds;depthchargesarereleased.Theexactsametechnologycanbeputtoscientificusetostudyoceandepth
andthusocean-floortopography.Eventhedeepestoceanvalleysandtrenchescanbeplumbedwithsoundwaves.Asearlyasthe1870s,Britishscientistsemployedcrudedeep-watersoundingsaboardHMSChallengerandreportedhintsofgreatmountainsonthefloorofthemid-Atlantic—atantalizingresultthatsomecontemporaryromanticsassociatedwiththelostcontinentofAtlantis.Primitiveechosoundingtechnology,firstdevelopedforicebergdetectionafterthe1912Titanticdisaster,enjoyedrapidimprovementduringWorldWarI,asGermansubmarinesbegantoprowl.The1920ssawthefirstsystematicapplicationofsonartomappingtheoceanfloorandtherapidrealizationthatgreatmountainrangeslayhiddenbeneathalltheEarth’soceans.However,the
geologicalimplicationsofthesepioneeringoceansurveyswerelittlenoted,andoceanographiceffortswerelargelycurtailedbytheGreatDepressionandtheloomingSecondWorldWar.Followingthewar,oceanographerswerearmedwithanewgenerationof
high-sensitivitysonardetectorsthatcouldnotonlymapthetopographyoftheentireoceanfloorbutalsodetectreflectedsoundwavesfromdeeperrocklayers.GeneralfeaturesoftheAtlanticOceanfloorwereeasilyconfirmed.Forexample,continentalshelvesdeepengraduallyasyoumoveawayfrommostAtlanticcoastlines,fordistancesuptohundredsofmiles.Theedgesofthesecontinentalshelvesaremarkedbyasuddendrop-offtoanabyssalplaintwomilesdeepandathousandmilesacross—afeaturemuchwiderandflatterthananyondryland.Andtheoceanisbisectedbyanextensivemountainrange,theMid-AtlanticRidge.Allthatwasinaccordwithearlierdiscoveries,butthethicknessoftheocean
crustcameasahugesurprise.Geologistshadpredictedthattheoceanswouldbelessdeeplyrootedthanland,withoceancrustgraduallythinningawayfromshore.Whattheyfound,insteadofthisgradualtransition,wasaremarkablyabruptcontrastfromthicktothin.Unlikethetensofmilesofcrustalrocksbeneaththecontinents,oceancrustwasonlyaboutfiveorsixmilesthick:thesharptransitionoccurredrightatthedrop-offattheedgeofthecontinentalshelf.Suchanarrowdemarcationbetweencontinentsandoceanswasatoddswithisostaticmodels.Yearafteryear,backandforthacrossthewideoceanthescientistssailed,
hundredsoftimes.Eachandeverycrossingyieldedthesameresult.Avastmountainchainmorethantwentythousandmileslong,thelargestonEarth,laybeneaththewaves,preciselybisectingtheAtlantic.ThesamesweepingcurvesofthecontinentalshorelinesweremimickedinthehiddenMid-AtlanticRidgecrest.What’smore,iftheedgeofthecontinentswastakenasthesharpunderwaterdrop-offtotheabyssalplain(asopposedtotheshiftingsandyshoreline),thenthematchbetweencontinentswasuncanny,asifabrokenchinaplatewerefitneatlybacktogether.Nolongercouldsciencedismisstheconformityofcoastlinesasmerecoincidence.AsscientistscompletedmoretraversesoftheAtlanticandcomparedmore
details,newpatternsemerged.TheMid-AtlanticRidgewasnoordinarymountainrange.Onland,mostmountainchainshavealineofthehighestpeaksdowntheiraxis,butattheverycenterlineoftheMid-AtlanticRidgewasawidetroughabouttwentymileswideandmorethanamiledeeperthantheadjacentpeakstotheeastorwest—afeaturewenowcallariftvalley.What’smore,the
ridgeanditsriftvalleydidn’tfollowasmoothandcontinuouscurvefromnorthtosouth.Rather,theriftvalleywasrepeatedlyoffset,ahundredmilesormoretotheeastortothewest,bysharplydefinedtransformfaults,placeswherethecrustwasbrokenanddisplaced,givingtheentireridgeajaggedandbrokenappearance.Whatwasgoingon?Suchsuggestivefindingscouldhaveeasilybeenburiedintheavalancheof
brilliantpostwarscientificdiscoveries.Inonesense,theywerejustmoredata.Buttheleadinvestigatorsontheocean-floorprojectwerenoordinarypublicists.BruceHeezenandMarieTharp,marinegeophysicistsatColumbiaUniversity’sLamontGeologicalObservatory,devisedanew,dramatictopographicmapofEarth’ssurface.Asonothertopographicmaps,theyrepresentedcontinentalelevationswithcolors—higherelevationsgradingfromgreensandyellowstobrownsandultimatelywhiteatthehighest,snowcapped-mountainelevations.Greatmountainranges—theHimalayas,theAndes,theAlps—stoodoutclearly.HeezenandTharp’sartisticinnovationwastohighlighttheimmensesubsurfacemountainrangesinexactlythesameway,albeitinvaryingshadesandhuesofblue—atechniquethatmadetheMid-AtlanticRidgeandotherdeep-seafeaturesstandoutasmonumentalonaglobalscale.AndbycenteringtheirexquisitemapontheAtlantic,theyhighlightedtheidenticalshapesofthecoastlinesandtheridgeinanunmistakableway.Bythe1960s,HeezenandTharp’smaphadachievediconicstatus.Whateverthecauseofthisparallelism,thefactthattherewassomegeneticlinkwasobvioustoall.(ThisstoryofBruceHeezen—pronounced“HAY-zen”—andhiswidely
laudedcontributionhasspecialmeaningtomeandmycareer,forwhenIarrivedatMITinthefallof1966,Iwassurprisedtofindthateventhemostseniorgeologyfacultymemberswerequiterespectfulandeagertoshakemyhand.Distinguishedpedigrees—evenoferroneoushomonymousvariety—arenotwithoutsomeadvantageinscience.)
TheExpandingSea
WiththerevelationoftheMid-AtlanticRidge,andthediscoveryofsimilarsubsurfacevolcanicridgesintheEastPacificandIndianoceans,scientiststackledthepossibilityoflateralcontinentalmotionswithrenewedintensity.Tobesure,continentsaren’tdriftingaimlessly,asWegener’scoinagemightsuggest,sogeologistslookedforsomehiddenforcethatcoulddramaticallyrearrangeEarth’ssurface.Discoveryfolloweddiscovery,asnewdatakeptarrivingtoconfoundthe
experts.In1956HeezenandhisLamontboss,seismologistMauriceEwing,documentedaremarkableassociationbetweenthepositionofthecentralriftvalleyoftheMid-AtlanticRidgeanda34,000-mile-longpatternofmoderateocean-floorearthquakesextendingaroundtheglobe.Somehowtheriftvalleysandearthquakesarerelated,soridgesmustbedynamic,changeablefeatures.Oceanfloorrocksalsosurprisedmanygeologists,whohadpredictedthatthe
Mid-AtlanticRidgewasatypicalmountainrangecappedbyresistantmarinelimestone,justliketheCanadianRockies.Butextensivedredgingalongtheridge,coupledwithobservationsoftheAtlantic’smanyislands,producednothingbutbasalt,andrelativelyyoungbasaltatthat.Itturnsoutthat,otherthanaveneerofsoftsediments,theocean’scrustismadealmostentirelyofvolcanicbasalt.Fromeasttowest,spanningmorethantwenty-fivehundredmilesofoceanfloor,basaltformsthepavement.What’smore,carefuldatingbasedonthesteadydecayratesofradioactive
elementsrevealsasimplepatternintheagesoftheserocks.BasaltcollectedfromtheriftvalleyrightatthecenteroftheMid-AtlanticRidgeisnewlyminted,lessthanamillionyearsold.Thefartheryougetfromtheriftvalley,eastorwest,theolderthebasaltis,untiltherocksnearthecontinentalmarginsaremorethanonehundredmillionyearsold.Whyshouldrocksatthecenteroftheoceanbeyoung,whilethoseontheoutskirtsaresomucholder?OnelogicalconclusionisthattheMid-AtlanticRidgeisalineofvolcanoesthatarespewingoutnewbasalticcrust.Butwheredidthemucholderrocksattheedgesoftheoceanbasincomefrom?Keydata,thesmokinggunofplatetectonics,camefromasecondsubmarine-
huntingtechnologycalledthemagnetometer.WorldWarIIsubmarinesarebighunksofiron-richalloys,sotheyaremagnetic.Thankstothedevelopmentofmagnetometers,submarine-huntingairplanescouldflyovertheoceansurfaceandpickupthemagneticanomalyofanearbyenemysubmarine.Followingthegreatconflict,geophysicistsinventednewtypesofmagnetometerswithgreatlyincreasedsensitivitytosmallchangesinthemagneticfield.Theyadaptedtheseinstrumentstobetowedbehindresearchships,justabovetheseabottom.Theirtargetwasocean-floorbasalt,whichcarriesaweakmagneticsignalin
theformofminutecrystalsoftheironmineralmagnetite.Earth’smagneticfieldisknowntovaryslightlyfromyeartoyear,inwhatiscalledsecularvariation.Asbasaltmagmacools,thesecrystalsfreezeinthedirectionofEarth’smagneticfieldliketinycompassneedles.OceanfloorbasaltthuspreservestheorientationofEarth’smagneticfieldontheexactdatewhentherockhardened.Thethrivingfieldofpaleomagnetismstudiestheseinvisiblemagneticforcefieldsthatare
lockedintobasaltandotherrocks.(Ondryland,ahodgepodgeofmagneticsignals,theconsequenceoffolding,faulting,andothergeologicalcontortionsofcontinentalcrustovertime,confusessuchpatterns.)Startingintheearly1950s,oceanographersdeployedmagnetometerscloseto
theseafloor,sweepingthemonlongtransectsacrossoceanridges.Theyhopedthattheirpaleomagneticmeasurementsmightyieldabetterpictureofsecularvariationontheoceanfloor.Whattheyfoundinsteadwasabizarremagneticpatternthatwasastonishinglyregularandintricate.ClosetothecentralriftvalleyinboththeAtlanticandthePacific,basaltdisplayedthenormalmagneticorientation,faithfullypointinginthedirectionofthemodernnorthmagneticpole.Butseveralmileseastorwestoftheriftvalley,themagneticsignalflipsafull180degrees:thenorthmagneticpoleisalmostexactlyoppositeitspresentposition,wherethesouthpoleissupposedtobe,andviceversa.Sailseveralmoremilesineitherdirection,andthemagneticfieldflips180degreesagaintothecorrectorientation.Overandover,dozensoftimes,themagneticfieldfrozenintherocksonanygiventransectisobservedtoflip.Additionalanalysisrevealedthreekeyfacts.First,therockswithreversed
magneticfieldformlong,narrownorth-southtrendingbandsthatexactlyparalleltheridgesinboththeAtlanticandthePacificoceans.Wherethecentralriftvalleyisbroken,offsetbytransformfaults,sotooarethemagneticbands.Second,thepatternofthesemagneticstripesissymmetricalabouttheridgeaxes:saileastorwestfromthecenter,andtheexactsamesequenceofnormalandreversestripes,somewiderandsomenarrower,occurs.Andthird,radiometricdatingofthesebasaltsfromridgesystemsaroundtheworldconfirmsthateachreversaloccurredsimultaneouslyoveranarrowandpreciselydefinedage.Magneticreversalsthusserveasakindofocean-floortimeline.Twological,ifmind-bending,conclusionsfollowed.First,Earth’smagnetic
fieldiswildlyvariable:itflips180degreesonaverageeveryhalf-millionyears,andithasbeendoingsoforatleastthepast150millionyears.Thereasonsforthisficklefieldarenowunderstoodinsomedetail.Ourplanetisagiantelectromagnet;itsmagneticfieldarisesfromswirlingelectricalcurrentsinEarth’sconvectingfluidoutercore.Heatdrivesthisconvection;densehotliquidattheinnercoreboundaryexpandsandrises,tobereplacedbycooler,denserliquidthatsinksfromabove.GeophysicistsemploysophisticatedcomputermodelstoshowthatEarth’srotationaddscomplicated,chaotictwiststotheconvection—motionsthatresultinaflipinthemagneticfieldeveryhalf-millionyearsorso.Earth’srotationalsoconstrainsthemagneticpolestospendmostoftheirtimealignedclosetothestablerotationaxis,butduringperiodsofcore
instability,themagneticfieldcanwanderwidelyandflip,perhapsinthespanofacenturyorless.Thesecondconclusionisthatmidoceanridgesproducenewbasalticcrustat
therateofaninchormoreeveryyear.Olderbasaltmovessideways,botheastwardandwestward,awayfromtheridge,asnewlavatakesitsplace.Theridgesystemsarethusgrandtwo-directionalconveyorbelts,spewingoutnewoceanfloor.FreshbasaltgeneratedattheMid-AtlanticRidgeisexpandingtheAtlantic,whichgrowsasmuchastwoincheswidereveryyear—roughlythirtythousandyearspermileofnewoceanflooronaverage.Playthetapebackward150millionyears,andtheAtlanticdidn’texist.Priortothattime,theAmericasmusthavebeenjoinedtoEuropeandAfrica,justasAlfredWegenerproposed.Oneofthemostinfluentialpresentationsofthisremarkablediscovery
appearedin1961intheGeologicalSocietyofAmericaBulletin.BritishgeophysicistRonaldMasonandAmericanelectronicsexpertArthurRaffoftheScrippsInstitutionofOceanographyinCaliforniahadcollaboratedforalmostadecade,compilingexhaustivemagneticsurveysoftheoceanfloorofftheWestCoastofNorthAmerica.ThecenterpieceoftheirpublicationwasadetailedmagneticmapoftheJuandeFucaRidge,aprominentridgefeatureonthePacificseafloorjustashortday’ssailfromportsinOregon,WashingtonState,andBritishColumbia.ThestarkblackandwhitemapofMasonandRaff—whiteandblackbands
representingnormalandreversedmagneticfields,respectively—displaysdozensofnorth-southtrendingstripes.Uniformityprevailswithinlargeblocksofoceancrust,eachhundredsofmileswide,eachwithasymmetricalpatternofstripesaboutacentralriftvalley.Butbetweenadjacentblocksthepatternisbroken,offsetbytransformfaultlinesandskewedlikeacubistpainting.Analysisofoffsetsalongoneofthesefaults,theMendocinoFractureZone,revealsaremarkablelateraldisplacementofsevenhundredmiles.EpicinternalprocessesmustbeatworktosodisruptEarth’scrust.Withsimilarevidencepilingupfromridgesystemsaroundtheglobe,
geologists,geophysicists,andoceanographersbegantotalktooneanotherinanewlyintegratedeffort.Correlationsofocean-floortopography,seismology,magnetism,androckagesallpointedtothesameconclusions.Oceancrustisbeingcreatedaroundtheworldatridgesystems,whicharedynamiczonesofvolcanicactivity.Therateofseafloorspreadingisrecordedinthesymmetricalpatternsofmagneticstripesandtheagesofbasalts.Afloodofinfluentialpaperstransformedthecollectivegeologicalmind-set,
andbythemid-1960salmosteveryonewasconvincedofwhathadoncebeen
heresy:thecontinentsaremoving.TheAtlanticOceanhasbeengrowingwidereveryyearformorethanonehundredmillionyears.
TheCaseoftheDisappearingCrust
Theearlyyearsoftheplatetectonicsrevolutionwereatimeofrapiddiscovery,changingparadigms,andequallypuzzlingnewquestions.Oneunansweredquestionstoodoutfromtherest:Howcouldamile-wideswathofnewbasalticcrustbeaddedeverythirtythousandyearsalongmorethanthirtythousandmilesofmidoceanridgesintheAtlantic,Pacific,andIndianoceans?Howdidallthatnewcrustfit?UnlessEarthwasgrowinglarger—andforabriefinterludeinthe1950sandearly1960s,asmallbutvocalgroupofgeologists,includingBruceHeezen,didadvocatetheuntenableexpanding-Earthscenario—theoldcrusthadtogosomewhere.Seismologistsfoundthesolution.Inthecoldwarclimateofthe1960s,nuclear
weaponsbecamethecentralfocus(andprimarysourceoffunding)ofseismology.Followingthe1962Cubanmissilecrisis,theUnitedStatesandtheSovietUnionagreedtoaLimitedTestBanTreaty,whichrestrictednuclearweaponstestingtoundergrounddetonations.Verificationofthetreatyreliedoncontinuousseismicmonitoringusinganextensive(read:expensive)arrayofvibration-sensitiveinstrumentsdeployedtothefarreachesoftheglobe.TheresultingWorld-WideStandardizedSeismographNetwork(WWSSN)linked120stationstoacentralcomputer-processingcenterinGolden,Colorado,homeofonebranchoftheUnitedStatesGeologicalSurvey.Forthefirsttimeever,itwaspossibletopinpointtheexactlocations,depths,magnitudes,andmotionsofsmallearthquakes(andbigexplosions)anywhereonEarth.ThefringebenefitsforEarthscienceweretremendous.Armedwiththeirnew
tools,geophysicistscouldsensethousandsofpreviouslyundetectableEarthmovementsandthusdocumentpreviouslyunrecognizedplanetwidepatternsofearthquakes.TheyfoundthatalmostallofEarth’ssuddencrustalmotionsoccuralongnarrowlinesofintenseseismicactivity—placeslikethemidoceanridges.Manyotherquakesoccurclosetochainsofvolcanoesnearthecontinentalmargins—forexample,aroundthePacificOcean’snotorious“ringoffire.”Theseviolence-proneregionsofthePacificrim,includingthePhilippines,Japan,Alaska,Chile,andotherdangerzones,formedacommonpattern.Ithadlongbeenknownthatrelativelyshallowearthquakes(thosefromdepths
ofafewmilesorless)originatejustoffshoreinthevicinityofdeeptrenchesontheoceanfloor,whiledeeperanddeeperearthquakes,someoriginatingmore
thanahundredmilesdown,occurfartherandfartherinlandfromthecoast.ThedeepestknownearthquakescommonlytakeplacebeneathchainsofdangerousexplosivevolcanoeslikeMountSt.HelensandMountRainierinWashingtonState—volcanoestypicallylocatedatconsiderabledistanceinland.Bythelate1960s,newdatafromWWSSNclarifiedthedetailsofthe
relationshipamongdeepoceantrenches,earthquakes,andvolcanoes.Thedistinctivepatternofquakedepthsincreasinginlandfromthetrenchespaintedapictureofhugeslabsofoceancrustplungingdownintothemantle,underneaththecontinents,alongwhatwerecalledsubductionzones.Oldbasalticcrust,whichismuchcolderandthusdenserthanthehotmantle,isliterallyswallowedupbyEarth.Assubductingbasaltsnagsandbucklestheadjacentcrustdownward,thedeepoceantrenchesform.Foreverysquaremileofnewcrustgeneratedatoceanridges,asquaremileofoldcrustdisappearsatasubductionzone.Thenewexactlybalancestheold.Asifaveilhadbeenlifted,thenewscienceofplatetectonicscameintosharp
focus.Oceanridgesandsubductionzonesdefinetheboundariesofaboutadozenshiftingplates,eachofwhichiscold(comparedwiththedeepermantle),brittle(hencesubjecttoearthquakefracturing),andonlyafewtensofmilesthickbuthundredstothousandsofmileswide.Theserigidplatessimplyslideacrosshotter,softermantlerocks.ThePacificOcean’sringoffiredefinesonelargeplate,Antarcticaanditssurroundingseasanother.TheNorthandSouthAmericanPlatesextendwestwardfromtheMid-AtlanticRidgeallthewaytothePacificcoastoftheAmericas,whiletheEurasianPlateextendseastwardfromtheMid-AtlanticRidgetothePacificcoastofEastAsia.TheAfricanPlate,stretchingfromtheMid-AtlanticRidgeonthewesttothemiddleoftheIndianOceanontheeast,displaysanintriguingaspectofEarth’sdynamicsurface:theAfricancontinentisbeginningtosplitapartasanewriftvalleyforms,markedbyastringoflakesandactivevolcanoes,aswellashighaltitudesthatregularlyproducethefastestdistancerunnersintheworld.SomedayAfricawillbecometwoplates,withanewexpandingoceaninbetween.Asoceanridgesproducenewplatematerialandsubductionzonesswallowup
theold,thescenarioisonceagaincomplicatedbyEuclid:Earthisasphere.Thegeometryofplategrowthandsubductiononasphererequiresthatsomeplatesscrapeagainsteachotheralongjaggedtransformfaultlines—hencetheoffsetbandsinMasonandRaff’sfamousmagneticmapoftheJuandeFucaRidge.TheviolentSanAndreasFault,whichhastriggeredmanymemorableCaliforniaearthquakes,isanothersuchsuture.Everydaymorestressbuildsalongthefault,asthemightyNorthAmericanPlatemovesinasoutheasterlydirectionrelative
tothemightyPacificPlate.EverydaytheseinexorableplatemotionsbringresidentsofLosAngelesandSanFranciscoclosertothenext“bigone.”Somuchforthesimplegeometryofplatetectonics.Whatoftheepicforces
thatmustpowerplatemotions?Whatcouldcauseentirecontinentstoshift,scrape,andcollideoverhundredsofmillionsofyears?TheanswerliesinEarth’sinnerheat.Earthishot,whilespaceiscold.Thesecondlawofthermodynamics,asweepingcoreconceptofthecosmos,statesthatheatalwaysflowsfromhottertocoolerobjects—heatmustgraduallydisperse,mustsomehowfindawaytoevenout.Recallthethreefamiliarmechanismsthatfacilitatethetransferofthermal
energy.Everywarmobjecttransfersitsheattothesurroundingsintheformofinfraredradiation;heatalsomoves,albeitmuchlessefficiently,bydirectcontactorconduction,andbyconvection,whenafluidmassflowsbetweenhotterandcoolerregions.Earthmustobeythesecondlawofthermodynamics.Buthowcanheatmoveefficientlyfromsearingcoretocoolcrust?Rockandmagmaimpedeinfraredradiation,whilesluggishconductionisnotmuchmoreefficient.Soconvectionofsoftened,taffylikehotmantlerocksisthekey.RocksatEarth’ssurfacearehard,brittlematerials,butdeepinsidethe
superheatedpressurecookerthatisthemantle,rockssoftenlikebutter.Overmillionsofyears,underthestressesofthedeepinterior,rocksdeform,ooze,andflow.Hotter,morebuoyantrocksgraduallyrisetowardthesurface,whilecooler,denserrockssinkintothedepths.Greatconvectioncells,eachthousandsofmilesacrossandhundredsofmilesdeep,overturnEarth’smantleinamajesticcyclehiddenfromview.Thepaceoftheseplanetaryshufflingsisequallyvast—itmaytakeahundredmillionyearsormoreforasinglerevolutionoftheconvectioncelltocompleteitself.Atfirst,perhapsformorethanabillionyears,mantleconvectionbeneath
Earth’suniformbasaltcrustmusthavebeenachaotic,swirlinghodgepodge.Hereandthere,hottermeltsoflower-densitygraniteroseindisorganizedpulsesandplumestowardthesurface,wheretheyaccumulated,disruptingthecolder,denserbasalt.Isolateddensechunksofthatcoldercrustslowlysankintotheinterior,inaglobal-scaleexchangeofheat.Overthenexthalf-billionyears,mantlechurningbecamemoreorganized.
Dozensofsmallerconvectioncells,eachwithrisingplumesandsheetsofmagmaanddescendingblocksofcrust,consolidatedintoahandfulofmajesticcycles,eachhundredsofmilesdeepandthousandsofmilesacross.New,hotbasalticcrustformedwheretheseconvectioncellsroseupwardalonggrowingseafloorridges,whileold,coldbasalticcrustplungedintothemantleatasteep
angle—subductionzones,inanEarthincreasinglydominatedbythenew,transformativeprocessesofplatetectonics.Incrosssection,Earth’sturbulentouterlayersmighthavelookedlikeacollectionofsidewayswhirlpools,eachrotationlastingahundredmillionyearsormore.Thenasnow,Earth’sevolvingsurfacereflectedtheepicprocessesoccurring
farbelow.Greatridgesofbasalticvolcanoesgrewaboveconvectionzonesofrisingmagma.Gashliketrenchesformedwhereoldsubductedcrustplungeddownwardintothemantle,bendingandbucklingtheadjacentoceanfloor.Subductionalsoacceleratedtheall-importantproductionofgranite.Ascold,wet,subductedbasaltcrustplungeddeeper,swallowedbackintoEarth,itheatedupandbegantomelt—notcompletely,butperhaps20or30percent.Thosegrowingvolumesofgraniticmagmarosetothesurface,producingchainsofgrayvolcanicislandshundredsofmileslong.Thestagewassettobuildthecontinents.
Revolution
Granitefloats,basaltsinks:that’sthekeytotheoriginsofcontinents.Magmasofgraniticcompositionaremuchlessdensethantheirparentbasalticrock,sothesefreshmeltsslowly,inevitablyrisetocrystallizeasnear-surfacerockmassesortoeruptfromvolcanoesthatspewlayersofcindersandashontothesurface.OverbillionsofyearsofEarthhistory,countlessgraniteislandshaveformedbythiscontinuousprocess.Platetectonicsnotonlyproducedthesegranite-rootedislandchains;italso
assembledthemintocontinents.Thekeyliesinthesimplefactthatgranitecannotsubduct.Thedensebasaltonwhichitfloatseasilysinksintothemantle,butgraniteislikeabuoyantcork.Onceformed,itremainsatthesurface,conserved.Assubductionproducesmoreislands,thetotalareaofgraniteirreversiblyincreases.Imagineasubductingplateofoceancrust,dottedwithunsinkablegranite
islands.Thebasaltsubducts,buttheislandsdon’t.Theymustremainatthesurface,toformastripoflandrightabovethesubductionzone.Astensofmillionsofyearspass,moreandmoregraniteislandspileuptoformawiderandwiderstrip,evenasnewvolumesofgranitemeltrisefromthesubductingslabtothickenandexpandthegrowingcontinent.Islandsaccretetoformproto-continents,whichaccretetoformcontinents,justasourSolarSystem’schondritesonceaccretedtoformplanetesimals,planetesimalstoformplanets.Theepiccycleofplatetectonicstransformsourworld.Earth’sthin,cold,
brittlesurfacecracksandshiftslikescumonapotofboilingsoup.Newbasalticcrustpoursoutofvolcanicridgesthatrevealwheredeepconvectioncellsrise.Oldcrustisswallowedupatsubductionzonesthatrevealwhereconvectioncellsdescend.Earth’smostviolentsurfacedisruptions—themostintenseearthquakes,themightiestvolcanoes—arebuttrivial,incidentalblipscomparedwiththevastlymoreenergeticglobal-scalemovementsofthedeepinterior.PlatetectonicsalsorevolutionizedtheEarthsciences.Inthepriordarkagesof
verticaltectonics,eachgeologicaldisciplinewasseparate,seeminglyunrelatedtoanyother.Beforetherevolution,paleontologistshadnoneedtotalktooceanographers;thestudyofvolcanoeshadlittletodowithoregeology;geophysicistswereunconcernedwithlife’soriginsandevolution;andtherocksofonenationwereofnoobviousrelevancetorocksofanother,muchlesstorocksonthedistantoceanfloor.PlatetectonicsunifiedeverythingaboutEarth.Nowlocationsofrarefossil
organismscanbematchedpreciselyacrossthevastspreadingoceans.Extinctvolcanicterrainsleadminerstovaluableoredepositshiddenintheircorrespondingsubductionzones,longsincesolidifiedintocontinentalrock.Geophysicalstudiesofshiftingcontinentspointtokeyinfluencesontheevolutionofplantsandanimals.PlatetectonicsrevealsEarthasanintegratedplanetarysystem,fromcrusttocore,atscalesfromnanotoglobal,withasingleunifyingprinciplethroughspaceandtime.Ittooktimefortheproductionofgranitetoshiftfromthedisorderly,vertical
tectonicpatchworkofplume-drivenislandstothecoordinatedassemblyofsubduction-drivencontinents.ButbythetimeEarthwas1.5billionyearsold,theconvectingmantle—theeighteen-hundred-mile-thickzonethatholdsmostofEarth’smassandheatenergy—hadirrevocablytransformedthesurfaceofourplanet.Unlikeblackbasalt,thegrowing,barrengranitelandmassesappearedwhitishgray,thetypicalblendedcolorofquartzandfeldspar.Andsoifyouwereatimetravelertothatancientworldthreebillionyearspast,youwouldfindsomefamiliarfeatures.Youcouldstandonproto-continentsdevoidofvegetation,withjaggedhillsandsteep-walledvalleys,notunlikesomeruggedcoastlinesofthehigharctic.Youwouldexperienceperiodsofviolentweather,punctuatedbydaysofsunnyblueskiesandpuffywhiteclouds.Youwouldfindtheoceansaturatedwithdissolvedminerals,includingcalciumandmagnesiumcarbonates,whichwereoccasionallydepositedascrystallayeringsonthebasaltseafloor.Youcouldliebythatcool,blueoceanonthefirstwhitesandybeaches,richinresistantquartzgrainserodedfromthegraygranite.Butyouwouldquicklysuffocateintheheavyatmosphere,richinnitrogenandcarbondioxide
butlackingtheslightestwhiffoflife-givingoxygen.
Theinventionofcontinents—landmassesconstructedofsturdygranitecrust—wasbutasideshowinEarth’sgrandevolutionarypageant.Graniteterrains,formedbydeepheatingandpartialmeltingoftheubiquitousnear-surfacebasalt,werelikegrowinggrayscabsontheotherwisepristine,submergedblackskinofourplanet.Gradually,thethickeningraftsofgranite,floatingastheydidontopofthedenserbasalticbasement,wereabletoriseaboveoceanlevelandthusprovidedtherootsofallthegreatcontinents—whatweperceivetodayinouranthropocentricviewasthesolidEarth.
Chapter6
LivingEarth
TheOriginsofLifeEarth’sAge:500millionto1billionyears
InfantEarthatagefivehundredmillionyearsgavelittlehintastohowveryprecociousitwouldsoonbecome.Earthboasteddramaticvolcanism,tobesure,butsodidseveralotherplanetsandmoonsinourSolarSystem.Earthwasgracedbyglobe-spanningoceans,butsowasMarsinthoseearlydays,whileJupiter’sgiantmoonsEuropaandCallistowereenvelopedbyice-coveredoceansmorethanfiftymilesdeep,thusholdingfargreaterproportionsofthepreciousliquidatthesurface.Platetectonicshelpedtotransformourplanet,butinthoseearlyyears,VenusandpossiblyMarshadtheirownversionsofconvection-driventectonics.NordidchemistrysetEarthapart.Basaltandgranitewerethefoundation
stonesforalltherockyplanets.Oxygen,silicon,aluminum,magnesium,calcium,andirondominatedtheircompositions.Earthhaditsshareofcarbonandnitrogenandsulfur,butotherworldsinourSolarSystemwereequallyendowedwiththosevitalelements.Byalmosteverymeasure,Earthfourbillionyearsagoappearedtobearatherordinaryplanet.ButEarthwassoontobecomeuniqueamongknownworlds.Admittedly,it
wasalreadyuniqueinthatatfivehundredmillionyears,nootherknownplanetormoonhadenduredsuchsweepingepisodesofchange;nootherplanethadaltereditsoutwardappearancesothoroughlyandsooften.Butthesemetamorphosesweredifferentonlyinscale,notinkind.Themostdynamicengineofplanetarychange—whatsetsEarthapart—wasyettoemerge.OnlyEarthbecamealive.TheoriginandevolutionofthebiospheredistinguishesEarthfromallotherknownplanetsandmoons.
WhatIsLife?
Whatdoesitmeantobealive?WhatisthisphenomenonthatmakesEarthsodifferentfromtherestoftheknowncosmos?Wemightwelltrytodescribelife
asasetofdistinctiveintertwinedtraits—acomplexstructure,coupledwiththeabilitytomove,togrow,toadapt,andtoreproduce.WemightpointtosuchdistinctivecellularattributesasamembraneorlongstrandsofthegeneticmoleculeDNA.Butnomatterhowlongthelistofdiagnostictraits,therealwaysseemtobeexceptions.Lichensdon’tmove.Mulesdon’treproduce.Chemistryprovidesafirmerfoundationfordefininglife,foralllivingthings
areorganizedmolecularsystemsthatundergochemicalreactionsofastonishingintricacyandcoordination.Everylife-formconsistsofdiscreteassemblagesofmolecules(cells)separatedbyamolecularbarrierfromtheoutside(theenvironment).Theseclevercollectionsofchemicalshaveevolvedtwointerdependentmodesofself-preservation—metabolismandgenetics—thattogetherunambiguouslydistinguishthelivingfromthenonliving.Metabolismisthevariedsuiteofchemicalreactionsthatalllife-formsuseto
convertatomsandenergyfromtheirsurroundingsintomorecellstuff.Liketinychemicalfactories,cellstakeinmolecularrawmaterialsandfuelandusethosehard-wonresourcestofacilitatemovement,repair,growth,and,fromtimetotime,reproduction.Andlikechemicalfactories,asopposedtoragingforestfiresorthenuclearchainreactionsofthefirstelement-generatingstar,cellsexquisitelycontrolandregulatethesereactionsbypositiveandnegativefeedbacks.Metabolismaloneisn’tenoughtodefinelife.Unliketheirnonliving
environment,cellscarryinformationintheformofDNAmolecules,andtheycancopyandpassthatmolecularinformationfromonegenerationtothenext.What’smore,theinformationcanmutate;moleculesareoftencopiedwitherrors,whichprovidegeneticvariations.Mutationsthuspromotechemicalnovelty—innovationsthatenablethepopulationofcellstocompeteagainstotherlessefficientpopulations,tosurviveduringtimesofenvironmentalchange,ortoexpandtheirfootholdintonewenvironmentalniches.Thusmetabolismandgeneticsmusttogethercharacterizelivingmatter.But
surprisingly,biologistshavefailedtodeviseasingle,universallyaccepteddefinitionoflife.NASA’sExobiologyProgram,taskedwithinvestigatingtheoriginsoflifeandthepossibilityofitonotherworlds,hasperhapscometheclosest.A1994NASApanelchairedbyGeraldJoyceoftheScrippsResearchInstituteagreedonastreamlinedsentence:“Lifeisaself-sustainingchemicalsystemcapableofundergoingDarwinianevolution.”Joyce,whoisaleaderinattemptstomakelifeinthelab(afuturisticfield
calledsyntheticbiology),recentlyachievedthisbenchmark—aremarkablebreakthrough,tobesure.Hedevisedatest-tube-boundcollectionofthousands
ofdiverseinteractingmoleculesthatisbothself-sustainingandevolving.Enclosedinglass,thisintricateprocessresultsinevolvingproportions,albeitexactcopies,ofthediversemoleculesthatwerealreadypresentfromthestartoftheexperiment.Joycerealizedthatachemicalsystemthatsimplychurnsoutmolecularduplicatesadnauseam,eveniftherelativeproportionofthosemoleculesevolvesovertime,islittlemorethanamolecularXeroxmachine.Naturallivingsystems,bycontrast,havetheabilitytomutateandthuspotentiallydocompletelynewthings—toexplorenewenvironmentalterritory,tosurviveunexpectedenvironmentalchanges,toperformnewtasks,tooutcompeteneighborsforresources.SoJoycehasrevisedhisdefinitiontoincludethecharacteristicofnovelty:“Lifeisaself-sustainingchemicalsystemcapableofincorporatingnoveltyandundergoingDarwinianevolution.”WhatisperhapsmostremarkableaboutthisdevelopmentisthatJerryJoyce,realizingthesubtletyoflife,modestlyamendedNASA’sdefinitionratherthanstakingaclaimtothehistoric,ifFrankensteinian,distinctionofbeingthefirsttocreatelifeinthelab.
RawMaterials
HowdidnonlivingplanetEarthinventtheintertwinedtraitsofmetabolismandgenetics?Mostofusintheorigins-of-lifebusinesssuspectthattheemergenceofthefirstcellwasaninevitablegeochemicalprocess.Earthpossessedalltheessentialrawmaterials.Oceans,atmosphere,rocks,andmineralswererichinthenecessaryelements:carbon,oxygen,hydrogen,nitrogen,sulfur,andphosphorus.Energy,too,wasabundant:solarradiationandEarth’sinnerheatprovidedthemostreliablesources,butlightning,radioactivity,meteorimpacts,andmanyotherformsofenergymighthavecontributed.(Andthereare,consequently,atleastasmanytheoriesoflife’soriginsastherearesourcesofelementsandenergy.)Ononepointjustabouteveryoneagrees:carbon,themostversatileelementof
theperiodictable,playedthestarringrole.Nootherelementhassuchrichmoleculardesignsorsuchdiversemolecularfunctions.Carbonatomspossessanunmatchedabilitytobondtoothercarbonatomsaswellastomyriadotherelements—notablyhydrogen,oxygen,nitrogen,andsulfur—withuptofourbondsatonce.Carboncanformlongchainsofatoms,orinterlockedrings,orcomplexbranchingarrangements,oralmostanyotherimaginableshape.Itthusformsthebackboneofproteinsandcarbohydrates,offatsandoils,ofDNAandRNA.Onlyversatilecarbon-basedmoleculesappeartosharethetwindefining
characteristicsoflife:theabilitytoreplicateandtheabilitytoevolve.Everymorseloffoodweeat,everymedicationwetake,everystructureofour
bodiesandthebodiesofeveryotherlivingthing,isloadedwithcarbon.Carbon-basedchemicalsareeverywhere:inpaints,glues,dyes,andplastics,inthefibersofyourclothesandthesolesofyourshoes,inthepagesandbindingandinkofthisbook,andinenergy-richfuelsfromcoalandoiltonaturalgasandgasoline.And,aswe’llseeinChapter11,ourgrowingrelianceoncarbon-basedfuelsandotherchemicalsisimplicatedintroublingshiftsinEarth’snear-surfaceenvironment—changesthatareoccurringatapaceperhapsunmatchedinmillionsofyears.Still,carboncannothaveundergonetheremarkableprogressionfrom
geochemistrytobiochemistrybyitself.AllofEarth’sgreattransformativepowers—water,heat,lightning,andthechemicalenergyofrocks—werebroughttobearinlife’sgenesis.
Step1:BricksandMortar
Nooneyetknowsexactlyhow(orwhen)theancienttransitionfromalifelesstoalivingworldtookplace,butbasicprinciplesareemergingfromfocusedresearchatdozensoflaboratoriesaroundtheworld.Biogenesismusthaveoccurredasasequenceofsteps,eachofwhichaddedchemicalcomplexitytotheevolvingworld.Firstthemolecularbuildingblockshadtocomeintoexistence.Thenthosesmallmoleculeshadtobeselected,concentrated,andorganizedintolife’sessentialstructures—membranes,polymers,andotherfunctionalcomponentsofacell.Atsomepoint,thecollectionofmoleculeshadtomakecopiesofitself,whiledevisingameanstopassgeneticinformationfromonegenerationtothenext.AndthenevolutionbyDarwiniannaturalselectiontookover;lifeemerged.Thefirstandbest-understoodstepinbiogenesiswastherampantproduction
oflife’smolecularbuildingblocks:sugars,aminoacids,lipids,andmore.Theseessentialchemicals,allbasedontheversatileelementcarbon,emergeanywherethatenergyinteractswithsimplemoleculeslikecarbondioxideandwater.Life’srawmaterialsformedwherelightningpiercedtheatmosphere,wherevolcanicheatboiledthedeepocean,evenwhereultravioletradiationbathedmolecularcloudsindeepspacebeforeEarthwasborn.TheseasofancientEarthbecameincreasinglyconcentratedinthestuffoflife,asbiomoleculesrainedfromtheskiesandrosefromthedepths.Modernorigins-of-liferesearchbeganin1953,withwhatremainstothisday
themostfamousexperimentinbiogenesis.ChemistHaroldUrey,aNobelPrize–winningprofessorattheUniversityofChicago,andhisresolutegraduatestudentStanleyMillerdesignedasimpleandeleganttabletopglassapparatustosimulateearlyEarth.GentlyboilingwaterproxiedforthehotHadeanocean,andamixtureofsimplegasesmimickedEarth’sprimitiveatmosphere,whileelectricsparkssimulatedlightning.Afterafewdays,theconfined,colorlesswaterturnedpinkish,thenbrown,withacomplexmixoforganicmolecules.Thetransparentglassbecamesmearedwithstickyblackorganicsludge.Miller’sroutinechemicalanalysesrevealedanabundanceofaminoacidsand
otherbio-buildingblocks.His1953paperinScience,announcingtheresults,generatedsensationalisticheadlinesaroundtheworld.Chemistssoonflockedtothestudyofprebioticchemistry.AndwhiletheexactcombinationofatmosphericgasesintheMiller-Ureyexperimentwascalledintoquestion,thousandsofsubsequentexperimentalvariationsonthethemeestablishedbeyondanydoubtthatearlyEarthmusthaveaboundedinlife’sessentialmolecules.Indeed,the1953sparkexperimentanditsprogenyweresosuccessfulthatmanyinthefieldthoughttheoriginsmysteryhadbeenlargelysolved.Thisinitialenthusiasmandsubsequentfocusmayhavecomeataprice.
Miller’smasterfulexperimentplacedorigins-of-liferesearchsquarelyinthecampoforganicchemistsandestablishedtheparadigmoflifeemergingfromaprebioticsoup—perhapsfroma“warmlittlepond”(echoingCharlesDarwin’sprivatespeculationsfromalmostacenturybefore).Fewexperimentalistsofthe1950sconsideredthestaggeringcomplexitiesofnaturalgeochemicalenvironments,alteredastheyarebydailycyclesofnight/day,hot/cold,wet/dry,andmore.Nordidtheyconsidertherangeofnaturalgradients—intemperature,forexample,asvolcanicmagmacontactscoldoceanwater;orinsalinity,asafreshstreamentersthesaltyocean.AndnoneofMiller’sexperimentsincorporatedrocksandminerals,chemicallydiversewithdozensofmajorandminorelements,andtheirreactiveenergeticcrystallinefaces.Earth’ssunlitsurface,theyassumed,waswherealltheactionmusthaveoccurred.Miller’sinfluencewasstrong,andheandhisfollowersdominatedtheorigins-
of-lifecommunityformorethanthreedecades.Afloodofpublicationsensued,newjournalsarose,andhonorsandawardswerebestowed,whilegovernmentfundingflowedtothe“Millerites.”Theninthelate1980s,thediscoveryofdeep-seablacksmokerecosystemsgaverisetoaviablealternativeto“primordialsoup.”Inthosedeepdarkzones,farfromthesunlitoceansurface,mineral-richfluidsinteractwithhotvolcaniccrusttogenerateocean-floorgeyserlikevents.
Jetsofscaldingwatercontactthefrigiddeepoceantocreateaconstantprecipitationofminerals(themicroscopicparticlesthatproducetheblack“smoke”).Lifeaboundsinthoseastoundinghiddenplaces,fueledbythechemicalenergyattheinterfacebetweencrustandocean.Thebattleoveroriginsparadigmsrevealsalotaboutthesociologyofscience.
Ontheonehand,theMiller-Ureyprocessproducedasuiteofbiomoleculesstunninglysimilartowhatlifeactuallyuses.Themixofaminoacids,carbohydrates,lipids,andbasesalmostlookslikeawell-balanceddiet.AsHaroldUreyquipped,“IfGoddidnotdoitthisway,thenHemissedagoodbet.”ButthetruebelieversoftheMillercampdidmorethanjustsupportthelightning-seededprimordialsoupidea;withavengeance,theypubliclyrejectedanyandallcompetingideas.TheeffectivenessoftheLaJollacabal’sobstructionismbegantodeclinewith
thestartlingdiscoveryofthoselivingblacksmokersdescribedabove,coupledwiththepowerfulinfluenceandfar-reachingambitionsofNASA.Theexistenceofblacksmokersatunderseaventsunderscoredagrowingawarenessthatlifeaboundsinextremeenvironments—inplaceswhereapreviousgenerationofbiologistswouldnothavelooked.Wenowknowthatmicrobesthriveinacidicstreamsflowingfromminewasteandboilingpoolsabovevolcaniczones.TheyekeoutalivinginsidefrozenAntarcticrocks,andtheypersistonstratosphericdustparticlesmilesaboveEarth’ssurface.VastmicrobialecosystemsmilesbelowEarth’ssolidsurface,wherecellsliveinthenarrowestofcracksandfissuresandsubsistonthemeagerchemicalenergyofminerals,maywellaccountforhalfofEarth’sbiomass—asmuchasallthetreesandelephantsandantsandpeoplecombined.Ifsuchextremophilelifecanthrive—ifasignificantfractionofEarth’slifesurvivesindeepenvironmentsprotectedfromtheviolentinsultsofasteroidsandcomets—whycouldn’tlifehaveoriginatedthere?NASA,whosesciencefundingistiedcloselytotheprospectofgreat
discoveries,jumpedatthispossibility.IflifeisconstrainedtoariseinaMiller-Ureyscenario,atthesun-drenchedsurfaceofawateryworld,thenEarthandpossiblyMars(initsearlieststages,itsfirstfivehundredmillionyears)aretheonlyplausiblelivingworldswithinourreach.Butiflifecanemergefromtheblack,hotdepthsofasubsurfacevolcaniczone,thenmanyadditionalcelestialbodiesbecometemptingtargetsforexploration.Marstodaymusthavedeephydrothermalzones;perhapsendureslivesthereevennow.SeveralofJupiter’smoonsarealsoripeforbiologicalinvestigation,asisSaturn’sorganic-rich,Earth-sizemoonTitan.Evensomeofthelargerasteroidsmayhavedeep,life-producing,hotwetzones.IflifearosedeeponEarth,thenNASA’ssearch(and
funding)forexobiologywillsurelylastformanydecades.MyCarnegieInstitutioncolleaguesandIarerelativelatecomerstotheorigins
game.Ourlab’sfirstNASA-sponsoredexperimentsin1996werespecificallydesignedtotestorganicsynthesisinblacksmokerregimes,wherehightemperaturesandpressuresprevail.LikeMiller,wesubjectedmixturesofsimplegasestoenergeticconditions—inourcase,heatandchemicallyreactivemineralsurfaces,justasyou’dfindinadeepvolcaniczone.LikeMiller,weproducedaminoacids,lipids,andotherbio-buildingblocks.Ourresults,nowduplicatedandexpandedinnumerouslabs,showbeyondadoubtthatasuiteoflife’smoleculescanbesynthesizedeasilyinthepressure-cookerconditionsoftheshallowcrust.Volcanicgasescontainingcarbonandnitrogenreadilyreactwithcommonrocksandseawatertomakevirtuallyalloflife’sbasicbuildingblocks.What’smore,thesesynthesisprocessesaregovernedbyrelativelygentle
chemicalreactionscalledreductionandoxidationreactions,orredoxreactions,suchasthefamiliarrustingofironorcaramelizingofsugar.Thesearethesamekindsofchemicalreactionsthatlifeusesinmetabolism,insharpcontrasttotheviolentionizingeffectsoflightningorultravioletradiation.Indeed,whileharshlightningboltsmayfacilitatetheproductionofsmallbiomolecules,theyjustaseasilyripthosebuildingblockstomolecularshreds.Tomanyofusintheoriginsgame,itmakesalotmoresenseforEarthtohavemadeitsprebioticmoleculeswithlessenergeticchemicalreactions,inmoreorlessthesamewaythatcellsdoittoday.StanleyMillerandhisfollowersdidwhattheycouldtosquelchour
conclusionsandabortourresearchprogram.Inaflurryofcriticalpublications,theyarguedthatthehightemperaturesofthevolcanicventswouldquicklydestroyanyusefulbiomolecules.“Theventhypothesisisarealloser,”Millercomplainedina1998interview.“Idon’tunderstandwhyweevenhavetodiscussit.”Theybasedtheirargumentsonmeticulousexperimentsinwhichbiomoleculesdegradeinboilingwater.ButthesesimplisticstudiesfailedtomimicthecomplexityofprimordialEarth;missingwerethedeepocean’sextremegradientsoftemperatureandcomposition,theturbulentflowandcyclingofvolcanicvents,thechemicalcomplexityofmineral-richseawater,ortheprotectivesurfacesofrocksonwhichbiomoleculesarenowknowntobind.Nonetheless,theoriginsfieldhasnowmovedbeyondtheMiller-Ureyscenario,andformanyexperts,Earth’sdeepdarkzonesaretodaytheprimaryfocusinthebiogenesisgame.AsImentionedearlier,everyancientenvironmentwithsourcesofenergyand
smallcarbon-bearingmoleculesprobablyproduceditsshareofaminoacids,
sugars,lipids,andothermolecularbuildingblocksoflife.Anatmospherelacedwithlightningorexposedtoharshradiationremainsintherunningasatheoryofbiogenesis;sodoblacksmokersandotherdeep,hotenvironments.Biomoleculesformduringasteroidimpacts,onsun-drencheddustparticleshighintheatmosphere,andindeep-spacemolecularcloudsexposedtocosmicrays.Everyyeartonsoforganic-richdustraindownonEarth’ssurfacefromouterspace,asithasformorethan4.5billionyears.Wenowknowthatlife’sbuildingblockslitterthecosmos.
Step2:Selection
Ahalf-centuryago,thegreatestchallengeinoriginsresearchwassynthesizingtherawmaterials:themolecularbricksandmortaroflife.Bythedawnofthetwenty-firstcentury,thatproblemhadlargelybeensolved;scientistsrealizedthatEarthmusthavebeengirdledbyadiluteconsomméoflife’svitalingredients.Muchofthefocushasnowshiftedtotheselection,concentration,andassemblyofbiobitsintomacromolecules—tothemembranesthatenclosethecell,theenzymesthatpromoteitschemicalreactions,andthegeneticpolymersthatpassinformationfromonegenerationtothenext.Twocomplementaryprocesseslikelyplayedarole.Oneisself-assembly,in
whichagroupofelongatedmolecules—lipids—clumpedtogetherspontaneouslytoformthemembranesthatencapsulatedthefirstcells.Lipidsfeatureskinnybackbonesofadozenormorecarbonatoms.Undercertainconditions,theytendtoself-assembleintomicroscopichollowballs;theelongatedmoleculeslineupsidetoside,likeseedsonadandelionhead.Inoneofthemostinfluentialoriginspublicationsinhistory,CaliforniabiochemistDavidDeamerdescribedhowheextractedasuiteoftheseversatileorganicmoleculesfromthecarbon-richMurchisonmeteorite(aconglomerationofchemicalsformedindeepspacelongbeforeEarth)andfoundthattheyrapidlyorganizedthemselvesintotinycell-likesphereswithaninsideandanoutside,notunliketinyoildropsinwater.AfewyearsagoDeamerandIfoundthatcarbon-richmoleculesformingunderhot,pressurizedblacksmokerconditionsbehaveinmuchthesameway.Theseandotherexperimentsrevealthatmembrane-boundvesiclesareaninevitablefeatureoftheprebioticworld;lipidself-assemblymusthaveplayedakeyroleinlife’sorigins.Mostotherbio-buildingblocksdon’tself-organize,buttheycanbecome
concentratedandarrayedonthesafe,protectivesurfacesofrocksandmineralsinwhat’sknownastemplate-directedsynthesis,thesecondofthetwoselection
processes.Ourexperiments,conductedattheCarnegieInstitutionoverthepastdecade,revealthatmanyoflife’smostvitalmolecularbuildingblockssticktovirtuallyanynaturalmineralsurface.Aminoacids,sugars,andthecomponentsofDNAandRNAadsorbontoallofEarth’smostcommonrock-formingmineralsinbasaltandgranite:feldspar,pyroxene,quartz,andothers.What’smore,whenseveralmoleculescompeteforthesamepieceofcrystalrealestate,theyoftencooperateandyieldcomplexsurfacestructuresoftheirownthatmaypromoteevenmoreadsorptionandmoreorganization.Weconcludedthatwherevertheprebioticoceancontactedminerals,highlyconcentratedarrangementsoflife’smoleculesarelikelytohaveemergedfromtheformlessbroth.HereIshouldissueacaveat.Inorigins-of-liferesearch(andprobablyinmost
otherdisciplinesaswell),scientistsgravitatetomodelsthathighlighttheirpersonalscientificspecialty.OrganicchemistStanleyMillerandhiscohortssawlife’soriginsasessentiallyaprobleminorganicchemistry.Geochemists,bycontrast,havetendedtofocusonmoreintricateoriginsscenariosinvolvingsuchvariablesastemperatureandpressureandchemicallycomplexrocks.Expertsinmembrane-forminglipidmoleculespromotethe“lipidworld,”whilemolecularbiologistswhostudyDNAandRNAviewthe“RNAworld”asthemodeltobeat.Specialistswhostudyviruses,ormetabolism,orclays,orthedeepbiospherehavetheiridiosyncraticprejudicesaswell.Wealldoit;weallfocusonwhatweknowbest,andweseetheworldthroughthatlens.I’mtrainedinmineralogy,soyoucaneasilyguesswheremyorigins
preferenceslie.Meaculpa.Manyotheroriginsresearchershavealsosettledonsuchaconclusion—indeed,morethanafewprominentbiologistshavealsogravitatedtominerals,becauseorigins-of-lifescenariosthatinvolveonlyoceansandatmospherefaceinsurmountableproblemsinaccountingforefficientmechanismsofmolecularselectionandconcentration.Solidmineralshaveanunmatchedpotentialtoselect,concentrate,andorganizemolecules.Somineralsmusthaveplayedacentralroleinlife’sorigins.
RightandLeft
Biochemistryiscomplex,withinterwovencyclesandnetworksofmolecularreactions.Fortheseintricatelylayeredprocessestowork,moleculeshavetohavejusttherightsizesandshapes.Molecularselectionisthetaskoffindingthebestmoleculeforeachbiochemicaljob,andtemplate-directedselectiononmineralsurfacesisnowtheleadingcandidateforhownaturedidit.
Perhapsthemostdauntingchallengeinmolecularselectionischirality,thepervasive“handedness”oflife.Manyoflife’smoleculescomeinmirror-imagepairs—left-andright-handedvariantslikeyourtwohands.Chiralpairsofmoleculesareidenticalinmanyrespects:theyhavethesamechemicalcompositions,thesamemeltingandboilingpoints,thesamecoloranddensity,andthesameelectricalconductivity.Butleft-andright-handedmoleculeshavedifferent,incompatibleshapes—afamiliarcharacteristicifyou’veevertriedtoputaleft-handedgloveonyourrighthand.Itturnsoutthatlifeisincrediblypicky:cellsalmostexclusivelyemployleft-handedaminoacidsandright-handedsugars.Chiralitymatters.Inthecuriouscaseoftheartificialfragrancelimonene,the
right-handedformsmellslikeanorange,whereastheleft-handedversionofthissimplering-shapedmoleculesmellslikealemon.Thesmellreceptorsinyournosearesensitivetochirality,soright-andleft-limonenetransmitslightlydifferentsignalstoyourbrain.Tastebudsarelesssensitivetothedifferencesbetweenright-andleft-handedsugars.Theybothtastesweet,butourbody’sfine-tuneddigestivesystemcanprocessonlytheright-handedforms.Theartificialsweetenertagatose,azero-calorieleft-handedsugarsubstitute,exploitstheseproperties.Thetragicstoryofthalidomidealsorestsonhandedness.Theright-handedversionofthisdrugalleviatedmorningsicknessinpregnantwomen,buttheleft-handedvariantthatinevitablytaggedalongcausedbirthdefects.TodaytheFDAimposesstrictrequirementsforchirallypuredrugs—regulationsthatsavelivesbutcostconsumersanestimated$200billionperyearinaddedmanufacturingcosts.Mostexperimentsthatsynthesizebiomolecules(includingMiller-Ureyand
hydrothermalexperiments)produceequalamountsofleft-andright-handedmolecules,andmostnaturalprocessestreatleft-andright-handedmoleculesexactlythesame.Indeed,thenonlivingnaturalworldisforthemostpartindifferenttothedistinctionbetweenleftandright.Butlifeabsolutelyrequiresthecorrectshape:left-handedaminoacidsandright-handedsugarsareessential.Theopposite-handedmoleculessimplywillnotdo.Soourresearchteamtackledthequestionofhowlifeselectedleft-handedaminoacidsalmostexclusivelyoverright,andright-handedsugarsoverleft.Ourrecentexperimentshaveexploredthepossibilitythatchiralmineral
surfacesplayedthestarringroleinselectinghandedmolecules,andperhapsintheoriginsoflifeaswell.In2000mycolleaguesandIrealizedwhatwasthensurprisingbutisnowobvious:chiralmineralsurfacesareeverywhereinnature.Thecommonestmineralsineveryrockandeverysoilaboundwithsurfaces
whereatomsformmolecular-scale“handholds,”someleftandsomeright.Inthenaturalworld,theseleft-andright-handedmineralsurfacesoccurinstatisticallyequalproportions,soEarthonaglobalscaledoesn’tappeartobebiasedforeitherleftorright.Buteachindividualmoleculedoescarewhereitwindsup.Ourexperimentsshowedthatcertainleft-handedmoleculescanaggregateononesetofcrystalsurfaces,whilethemirror-image,right-handedcounterpartmoleculesaggregatejustaseasilyonothersetsofmineralsurfaces.Ashandedmoleculesareseparatedandconcentrated,eachsurfacebecomesatinyexperimentinmolecularselectionandorganization.Onitsown,nosuchnaturalexperimentwithmineralsandmoleculesislikely
tohavegeneratedlife.Buttakecountlesstrillionsoftrillionsoftrillionsofmineralsurfaces,eachbathedinmolecule-richorganicbroth,andrepeatthosetinynaturalexperimentsoverandoverforhundredsofmillionsofyears.Earthmusthaveeventuallytestedvirtuallyeverycombinationofsmallmoleculessomewhere,sometime.Thetinyfractionofallthosemolecularcombinationsthatwoundupdisplayingeasierself-assembly,ordevelopedastrongerbindingtomineralsurfaces,orenjoyedgreaterstabilityunderthehightemperaturesandpressures,survived,possiblytogrow,possiblytolearnnewtricks.Wedon’tyetknowexactlywhichofthosemyriadpossiblecombinationsof
moleculesandmineralsledtolifelikeorganization,buttheprinciplesofmolecularselectionandorganizationarenowemerging.Biomoleculesweresynthesizedinabundance,andsomeofthosemoleculeswentontoformlargerandlargerclusters.Ourexperimentssuggestthatelectricchargeplayedabigrole.Somemoleculeshaveaslightpositivecharge;othershaveaslightnegativecharge;andstillothers(likewater)arepolarwithslightlypositiveandnegativeendstothesamemolecule.Minerals,too,havechargedsurfaces,somepositiveandsomenegative.Addallthesechargedpiecestogether,andtheyspontaneouslyorganize,withpositiveelectricchargealwaysattractedtonegativeelectriccharge.Andsovariedmolecularassembliesoccurredinvirtuallyeverywet,mineral-richenvironmentonprebioticEarth.
Step3:Replication
Arraysofchemicals,nomatterhowintricatelypatterned,arenotaliveunlesstheycanmakecopiesofthemselves.Life’smostdistinctivehallmarkisreproduction:oneconsortiumofmoleculesbecomestwo,twobecomefour,andonandoningeometricexpansion.Thegreatestenigmainthebiogenesisstoryremainstheemergenceofthatfirstsystemofself-replicatingmolecules.Clever
experimentsreplicateportionsofplausiblereproductivecycles,thoughwehaveyettomimiccompletelythatelusivebiochemicaltrickinthelab.Nevertheless,atsomepointinspaceandtime,anorganizedcollectionofmoleculesbegantoduplicateitselfattheexpenseofothermolecules(thatis,“food”).ImagineEarthatagefivehundredmillionyears—roughlyfourbillionyears
ago.Ithadabrothoforganicmolecules,ithadtrillionsupontrillionsofreactivemineralsurfaces,andithadhundredsofmillionsofyearstoplaywith.Mostofthemolecularmilieudidnothingofinterestanddisplayednousefulfunctions.Butasmallfractionoftheorganicmoleculesarrayedonmineralsurfacesproducedsomekindofstructurewithenhancedfunction—perhapsastrongersurfaceattachment,ormaybethemeanstoattractmoremoleculestothecommunity,orthetendencytocatalyzethedestructionofcompetingmolecularspecies,oreventheabilitytomakecopiesofitself.Thenaturalworldamplyrewardssuchinnovation,andonceestablished,lifequicklyinfestedeveryhabitablenookandcrannyoftheglobe.Butlet’stakeastepback.Whywouldacollectionofmoleculesspontaneously
startcopyingitself?Theanswerliesinthetwinevolutionarypillarsofvariationandselection.Systemsevolvefortworeasons.First,theydisplayvastnumbersofdifferentpossibleconfigurations—that’svariation.Second,someofthoseconfigurationsaremuchmorelikelytosurvivethanothers—that’sselection.Imagineaprebioticcollectionofhundredsofthousandsofdifferentmolecules,allmadeofcarbon,hydrogen,oxygen,andnitrogen,maybewithsomesulfurorphosphorusthrownin.Prebioticsynthesis(àlaStanleyMiller)andnaturalsamples(forexample,DavidDeamer’smeteorite)displaythisdegreeofmolecularvariation.Butnotallmoleculeswerecreatedequal.Somemoleculeswererelativelyunstableanddecomposed—theywerequicklyeliminatedfromthecompetition.Othersclumpedtogetherinuselesstarlikemassesandfloatedawayorsanktotheoceanfloor,wheretheycouldplaynofurtherrole.Butsomemoleculesprovedtobeespeciallystable,perhapsevenmoresowhentheycouldbindtoothersoftheirkindortoaparticularlytemptingmineralsurface.Thesemoleculessurvived,asthemolecularbrothwasrelievedoftheleastfit.Molecularinteractionsfurtherrefinedtheprebioticmix.Somegroupsof
moleculescooperativelystucktomineralsurfaces,thusenhancingsurvivaloftheclique.Othersmallmoleculesactedascatalysts,enhancingsomechemicalspeciesbypromotingformationofchemicalbonds,orspeedingthedestructionofotherchemicalspeciesbybreakingchemicalbonds.Themolecularbrothwasswiftlywinnowed,butultimatesecurityinsuchaworldwasnottobefoundineliminatingthecompetitionorjusthangingon.Theultimateprizeofsurvival
wouldgotothecollectionofmoleculesthatlearnedtomakecopiesofitself.Threecompetingmodelsattempttodescribethefirstself-replicating,quasi-
livingsystemofmolecules.Thesimplestofthesemodels(andthereforetheonemanyofusprefer)pointstoawell-knowncycleofafewsmallmolecules—theubiquitouscitricacidcycle.Itstartswithaceticacid,whichcontainsonlytwocarbonatoms.AceticacidreactswithCO2toformpyruvicacid(withthreecarbonatoms),whichinturnreactswithmoreCO2tomakethefour-carbonoxaloaceticacid.Otherreactionsproduceprogressivelylargermolecules,uptocitricacid,withitssixcarbonatoms.Thecyclebecomesself-replicatingwhencitricacidspontaneouslysplitsintotwosmallermolecules,aceticacid(twocarbonatoms)plusoxaloaceticacid(fourcarbonatoms),whicharealsopartofthemolecularloop.Onecycleofmoleculesthusbecomestwo,twobecomefour,andsoon.What’smore,manyoflife’sessentialbuildingblocks,includingaminoacidsandsugars,arereadilysynthesizedbysimplereactionswiththecoremoleculesofthecitricacidcycle.Justaddammoniatopyruvicacid,forexample,andyougettheessentialaminoacidalanine.EverylivingcellonEarthincorporatesthecitricacidcycle,soitmaywellbeaprimordialcharacteristic—achemicalfossildescendedfromtheveryfirstlife-form.Suchacycle,byitself,isnotalive.Butitdoeshavethepotentialtoreplicatetheinnercircleofmoleculesattheexpenseoflessfecundchemicals.Attheoppositeextremeofchemicalcomplexityistheself-replicating
autocatalyticnetwork,amodelchampionedbyStuartKauffman,whoconductedpioneeringtheoreticalstudiesatthefamedSantaFeInstitute.Theprebioticbrothmayhaveinitiallyincorporatedhundredsofthousandsofdifferentkindsofsmall,carbon-basedmoleculesfromvariedsources.Wenowknowthatsomeofthosechemicalscatalyzedreactionsthatmadenewmolecules,whileotherreactionsacceleratedthebreakdownoftheirneighbors.Anautocatalyticnetworkconsistsofacollectionofmolecules—perhapsthousandsofdifferentspeciesworkinginconcert—thatspeeduptheproductionofthemselves,whiledestroyinganymoleculenotinthenetwork.It’sthemolecularequivalentof“therichgetricher.”Again,aswiththecitricacidcycle,suchamolecularnetworkwouldnotbeconsideredtobealive,butinawayitdoespromotethecopyingofitself,anditisfarmorecomplexthanmostnonlivingchemicalsystems.Athirdscenario,theoneprobablyfavoredbythemajorityofbiologically
trainedoriginsresearchers,istheRNAworld—amodelbasedonahypotheticalmoleculeofRNAthatmakescopiesofitself.Tounderstandwhythisscenarioappeals,weneedtotakeanotherstepback,tothinkaboutlife’stwomostcriticalfunctions:metabolism(makingstuff)andgenetics(transferringinformationon
howtomakestufffromonegenerationtothenext).ModerncellsusetheladderlikemoleculeDNAtostoreandcopytheinformationneededtomakemoreproteins,buttheyusecomplexlyfoldedproteinmoleculestomaketheDNA.Sowhichcamefirst,DNAorproteins?Itturnsoutthatathirdkindofmolecule,RNA,playsthecentralroleinbothprocesses.RNAisanelegantpolymer—along,single-strandedmoleculeassembledfrom
smallerindividualmolecules(callednucleotides)likebeadsonastringorlettersinasentence.Fourdifferentmolecular“letters,”designatedA,C,G,andU,canlineupinanyimaginablesequence,likeacodedmessage.Indeed,theseRNAlettersholdgeneticinformation(justlikeDNA).Atthesametime,RNAcanfoldupintocomplexshapesthathavetheabilitytocatalyzekeybiologicalreactions(justlikeproteins).Infact,RNAmoleculesfacilitatethesynthesisofallproteins,bothbycarryinggeneticinformationandbycatalyzingproteinformation.Soofalllife’svariedmolecules,RNAistheonlyonethatseemsto“doitall.”TheRNAworldmodelrestsontheassumptionthatsomeasyetpoorly
understoodchemicalmechanismproducedvastnumbersofdifferentstrandsofRNA,orperhapsaninformation-richmoleculeverysimilartoit.Almostallthosevariedstrandsdidabsolutelynothing;theysimplysurvivedorgraduallydegraded.However,aselectfewstrandspossessedsomekindofself-beneficialfunction;theyfoldeduptobecomemorestable,ortheylatchedontoasecuremineralsurface,orperhapstheydestroyedtheirrivals—justanotherexampleofmolecularcompetitioninthebroth.ThekeyassumptionoftheRNAworldhypothesisisthatoneofthesemyriad
strandslearnedtheremarkabletrickofhowtomakecopiesofitself:itbecameaself-replicatingmolecule.Thisideaisn’tsofar-fetched.Afterall,RNAisalotlikeDNA,whichisabletomakecopiesofitself.What’smore,RNAiseasilymutated.Sothefirstself-replicatingRNAmolecule,howeverinefficientorsloppy,wouldsoonhavefounditselfcompetingwithlotsofslightvariantsofitself,someofwhichpulledoffthecopyingtrickalittlefaster,orwithalittlelessenergyexpenditure,orperhapsinslightlydifferentenvironments.SuchaprecociousRNAmoleculewouldseemtofulfillalltherequirementsforlife:itisaself-sustainingchemicalsystemcapableofincorporatingnoveltyandundergoingDarwinianevolution—inthiscase,molecularevolution.Perhapsittookalongtimeforthatfirstfunctional,crudelyself-replicating
molecularsystemtoemerge,whetheritwasacitricacidcycle,oranautocatalyticnetwork,orself-replicatingRNA.Butunimaginablenumbersofmolecularcombinationswerebeingtriedontrillionsoftrillionsofmineralsurfaces,acrossalmosttwohundredmillionsquaremilesofEarth’ssurface,for
manymillionsofyears.Andoneofthoseinconceivablyimmensenumbersofmolecularcombinations,someplace,sometime,worked.Itlearnedtoself-replicateandtoevolve.Andthatinventionchangedeverything.ExperimentsintheBoston-basedlaboratoryofHarvardbiologistJackSzostak
demonstratethepowerofselectioninmolecularevolution.Inmanyoftheirexperiments,Szostak’steamstartswithamixtureofonehundredtrilliondifferentRNAsequences,eachofwhichconsistsofarandomhundred-letterstringofA,C,G,andU.ThatimmensecollectionofdiverseRNAstrands,eachofwhichfoldsupdifferently,isthenconfrontedwithatask:forexample,bondingtightlytoanotherdistinctivelyshapedmolecule.Szostak’steampoursasolutionwithallhundredtrillionstrandsintoabeakerwithlittleglassbeads,eachofwhichhasbeencoatedwiththatdistinctivelyshapedtargetmolecule.ThesetargetmoleculesdangleoutintotheRNA-richsolutionlikelittlehooks.ThevastmajorityofRNAmoleculesdon’trespond;theyhavethewrongshapestointeract.ButatinyfractionoffoldedRNAslatchontothetargetsandsticktight.That’swherethefunbegins,becauseSzostak’scoworkerspourouttheold
solution(alongwithalmostonehundredtrilliondifferentnonfunctionalRNAstrands)andrecoverthefewstrandsthatbyvirtueoftheirfortuitousshapeshappentosticktothecoatedglassbeads.Then,usingstandardtricksofgenetictechnologythatmimicplausibleprebioticprocesses,theyprepareanewbatchofonehundredtrillionRNAstrands,butthistimeallthestrandsaresloppycopies—eachamutantofoneofthosefeworiginalfunctionalstrands.RepeatingtheabovestepsproducesanewpopulationoffunctionalRNAstrands,butsomeofthesesecond-generationvariantsbindmuchbetterthananyofthefirstgeneration.Someofthemutantdaughterstrandssignificantlyoutperformtheirparents.Repeatthewholeprocessafewmoretimes,andtheresultantRNAstrandsgetbetterandbetteratbinding,untilthebestmutantsareperfectlyfunctional:theylockontotheirtargetswiththehighestpossiblebindingenergy.Thewholeexperimenttakesafewdays—lessthanaweekfromrandom
strandstoaperfectbindingmolecule.Butifyouweretoaskateamoftheworld’smostbrilliantchemiststodesignsuchafunctionalRNAstrandfromscratch,theywouldfindthetasktobevirtuallyimpossiblewithanyknowncomputationalmethod.NocurrentmethodcanpredictexactlyhowalongRNAstrandwillfoldup,orhowitmightattachtoothercomplexlyshapedmolecules.Molecularevolution,notintelligentdesign,isbyfarthefastestandmostreliablepathtoachievingfunction.(That’swhywesayifGodcreatedlife,she’ssmartenoughtouseevolution.)
Life’sExplosion
Intheprebioticbroth,acollectionofmoleculeswitheventheslightestusefulfunctionhadanadvantage.ButthesemoleculargamesofwarpaledbesidetheadvantageconferredupontheRNAstrandthathadausefulfunctionandcouldmakecopiesofitself.Suchaself-replicatingmoleculeensureditsownsurvivalbyproducingmoreorlessidenticaldaughters.What’smore,themolecularcopyingprocesswasinevitablymessy,sosomeofthoseRNAcopiesweremutants.Andwhilemostmutationswerelethalorconferrednosignificantadvantage,afewfortuitousindividualsoutshonetheirparents,andthusthesystemevolved.Simplybychancecopyingerrors,theoriginalself-replicatingmoleculemusthaveproducedoffspringthattoleratedmoreextremeconditionsofpressureorheatorsalt,orreplicatedfaster,orfoundnewsourcesoffood,ordestroyedtheirlessfitneighbors.EvengreateradvantageswereenjoyedbythoseRNAstrandsthatfoundprotectiononamineralsurfaceorrefugewithinasafe,encapsulatingmembrane.Withoutcompetition,thefirstself-replicatingmoleculesengulfedEarth’s
nutrient-richzonesinageologicalinstant.It’sperhapscounterintuitivetothinkofamicroscopicobjecttakingover,butsay,justforargument,thatthefirstrelativelyinefficientself-replicatingmoleculetookaweektoduplicateonce.(Manymodernmicrobes,bycontrast,canreplicateinamatterofminutes.)Weekbyweek,twostrandsbecamefour,fourbecameeight,andsoforth.Atthatrate,itwouldhavetakenabouthalfayeartoformaclumpofonehundredmillionself-replicatingmolecules—anobjectjustlargeenoughtoseewiththenakedeye.Inanothertwentyweeks,themassofRNAwouldhaveexpandedtofillathimble.Andatthisrate,itwouldtakeyetanothertwentyweeksforalloflife’searliestmanifestationtofillagood-sizebathtub.Butcontinueddoublingeveryweekwouldhavequicklyproduceda
remarkabletransformation.TwentymoreweekswouldhaveseenmilesofRNA-infestedwaters,perhapsalongthecoastorinaninlandlakeoradeep-seaenvironment.Andwithintwoyears,assumingthatasingleinitialRNAstranddoubledeveryweek,Earthcouldboastasmuchasamillioncubickilometersoflivingstuff—enoughtoclogtheentireMediterraneanSea.Primitivesingle-celledorganismsthatfedonthechemicalenergyofrocks
couldn’thavehadmucheffectonEarth’sgeology—itsnear-surfacedistributionofrocks,forexample,orthediversityofminerals.Livingorno,fourbillionyearsagotheancientlandremainedbarrenblackandgray,surfaceweatheringwasslow,andtheearliestlifewouldhavecontributedalmostnothingtoaltertheglobe-spanningblueoceans.
Becausethefirstscrappymicrobeswouldhavemadelittlemark,wecan’ttellforsurewhenlifebegan.AfewofEarth’smostancientsedimentaryrocks,thoselaiddowninshallowoceanenvironmentsabout3.5billionyearsago,holdunmistakablemicrobialfossils.Domelikerockystromatolites,afewinchestoafewfeetacross,formedwherecoloniesofcellsprecipitatedthinlayeruponlayerofmineralsinshallowsettings.Microbialmatscoveredwideswathsofshoreline,consolidatingandpatterningsandintidalzones.Evenafewcarbon-richsphereswithdistinctivecell-likewalls—possiblemicrobialbodyfossils—havesurvivedtheaeons.Butnoolderincontrovertiblefossilshavebeenfound.Geochemicaltracesofcarbonandotherbioelementsfromintenselyalteredrocks3.85billionyearsoldaretantalizing,butbynomeanshavetheyconvincedthegeologicalcommunity.Sowhendidlifearise?Ifyourhunchisthatlifeemergesearlyandoftenon
anyviableplanetormoon,thenperhapsyou’dadvocateastablebiosphereby4.4billionyearsago—withinEarth’sfirst150millionyears.Alltheingredientswerethere:oceansandair,mineralsandenergy.GiantimpactsofasteroidsandcometswouldhavechallengedthesurvivalofthisHadeanlife,perhapsfavoringthosehardycellsthatlearnedtolivehotanddeepinprotectedrockyhomesbeneaththeoceanfloor.Perhapslifecameintobeingmorethanonce,perhapsmanytimes,beforeEarthsettledintoitscalmerpostadolescence.Ifso,thenthe3.5-billion-year-oldfossilswouldrepresentanecosystemalmostabillionyearsinthemaking.If,ontheotherhand,yoususpectthatlife’soriginsaredifficultandrareinthe
cosmos,thenadatecloserto3.5billionyearsagomayseemmorelikely.Perhapslifeissoimprobablethatabillionyearsofmineral-moleculeinteractionsthroughouthundredsofmillionsofcubicmilesofoceancrustwerenecessary.Perhapsthoseprecious,sparsefossilremainsoftheso-calledArcheanEonmarkthetruebeginningsofthebiosphere.
LivingEarth
Wheneverfirstlifeemerged,whetherbefore4.4billionyearsorafter3.8billionyearsago,thefactremains:itlittlealteredEarth’sancientsurface.ThoseearliestmicrobessimplylearnedchemicaltricksthatEarthalreadyknew.Fromourplanet’searliestdays,chemicalreactionshavetakenplaceatornearitssolidsurface.Thereasonboilsdowntothedistributionofelectrons:Earth’smantlehasonaveragemoreelectronsperatomthanthecrust.Themantleismore“reduced”andthesurfacemore“oxidized,”inthejargonofchemistry.When
reducedandoxidizedchemicalsmeet—forexample,whenreducedmagmaandgasesfromthemantlebreachthemoreoxidizedsurfaceinavolcaniceruption—theyoftenundergoanenergy-liberatingchemicalreaction.Intheprocess,electronstransferfromtheformertothelatter.Rusting,inwhichironreactswithoxygen,isafamiliarexampleofsucha
reaction.Ironmetalispackedfullofelectrons—somanyelectrons,you’llrecall,thatsomeofthemarefreetowanderthroughtheshinymetalandconductelectricity.Ironisthusanelectrondonor.Oxygengas,ontheotherhand,issostarvedforelectronsthatpairsofoxygenatomsmustpooltheirresourcestomakeanO2molecule,inwhichthemeagersupplyofelectronsissharedbybothatomslikerationsonadesertisland.Oxygenistheidealelectronacceptor.Sowhenironmetalmeetsmoleculesofoxygen,arapidexchangeofelectronsoccurs.Eachironatomgivesuptwoorthreeelectrons,whileeachoxygenatomtakesuptwoelectrons.Theresultofthisexchangeisanewchemicalcompound,ironoxide,plusalittlejoltofenergy.Inadditiontoiron,thecommonelectronsatedmetalelementsnickel,
manganese,andcopperweresubjecttooxidation.So,too,weremanyofthesimplecarbon-basedmoleculesthathadbeensynthesizedinprebioticprocesses,includingmethane(naturalgas),propane,andbutane.OxygengaswasscarceinEarth’searliestatmosphere,butotherelectron-hungrycollectionsofatoms,includingsulfate(SO4),nitrate(NO3),carbonate(CO3),andphosphate(PO4),werereadilyavailabletofillthatrole.Beforelife’semergence,redoxreactionsproceededatarelativelyleisurely
pace.Butthefirstmicrobeslearnedtoshuffleelectronsatanacceleratedrate.Inmanyplaces—primitivecoastlines,near-surfacewaters,ocean-floorsediments—livingcellsbecamethemediatorsofthesereactions.Communitiesofmicrobesmadetheirlivelihoodsbyspeedingupreactionratesoftherocks,usingtheresultantenergytoliveandgrowandreproduce.Earthhadmadeironoxidesfromthestart,tobesure,butthefirstmicrobesmadethemfaster.Intheprocess,lifebegan,eversoslowly,toalterEarth’ssurfaceenvironment.Microbesexploitedtheabundantenergyavailable,intheformofthereducedirondissolvedintheHadeanandArcheanoceans;theyoxidizedirontoformtherustyredmineralhematite—achemicaltransformationthatcanreleaseenoughenergytosupportanentireecosystem.MassiveArcheanbandedironformationsfoundinAustralia,SouthAmerica,andotherancientterrainsmaythusrepresenttheleavingsofanepicmicrobialbuffetthatlastedtensofmillionsofyears.Andsobegantheastonishingcoevolutionofthegeosphereandbiosphere.Evolutionbynaturalselectioncontinuedtodrivealltheseprocesses.A
microbialspeciesthatlearnedtouseitsironfoodsourcemoreefficiently,ortotoleratemoreextremeconditions,ortoexploitnewredoxreactions,hadadistinctadvantageandensureditsownsurvival.Consequently,mutatingpopulationsofmicrobesinventednewcatalyststhatpromotedtheseenergy-producingreactionsmoreefficientlythanthenonlivingenvironment.Theresults,hereandthere,werelittlemoundsoflimestoneandmodestdepositsofironoxides,alongwithagradualincreaseintheprocessingofnear-surfacecarbon,sulfur,nitrogen,andphosphorus.Nevertheless,theseearliestlife-formsdidlittlemorethanmimicthechemistrythathadalreadybegun(albeitmoreslowly)onthepreviouslynonlivingworld.
Light
Mostoriginsresearcherssuspectthattheearliestlife-formsreliedexclusivelyonthechemicalenergyofrocks—anabundantsource,tobesure,butonethatgreatlyrestrictedwherelifecouldthrive.Atsomepoint,afewmicrobesmovedbeyondtheirroleasmediatorsofchemicalreactionsintrinsictotheirenvironment.Theylearnedtocollectsolarradiation,whichwouldproveanabundant,cheapenergysourceforanysurfacedweller,anywhereontheplanet.Initsmostbasicform,photosynthesisusessunlighttomakebiomolecules
fromsuchubiquitousrawmaterialsascarbondioxide,nitrogen,andwater.Giventherightchemicalscaffolding,alloflife’sessentialbuildingblocks—aminoacids,sugars,lipids,andthecomponentsofDNAandRNA—canbemadefromatmosphericgasesandtheSun’sradiation.Unlikemoderngreenalgae,thosefirstmicrobialphotosynthesizersgeneratednooxygen.Indeed,modernanaloguesoftheseprimitiveorganismstendtoformabrownishorpurplishscumonstagnantponds.SomebiologistshaveevensuggestedthatimmensefloatingraftsofphotosyntheticmicrobesdiscoloredtheblueArcheanoceanswithunsightlybrownish-purplesplotches.Howwouldweknow?Suchmicrobeshavenohardpartstopreserveas
fossils;nordofloatingalgalmatsaltertherockrecordinanyobviousway.However,theremaybeawaytoteaseoutevidenceofthemostancientlight-lovingmicrobes.Photosyntheticcellsrelyinpartonhopanes—distinctivemoleculeswithfiveinterlockingcarbonrings(aconfigurationcloselyrelatedtothatofthesteroidssomuchinthesportingnewsthesedays).Aftermicrobesdieanddecay,theirtelltalemultiringhopanebackbonescansurviveforbillionsofyearsasamolecularresidueinfine-grainedoceansediments.Meticulouschemicalprocessingisrequiredtoextractandanalyzethesegeohopanesfrom
thebulkrock.Tentativeinterpretationsmustincorporatealitanyoftrickyassumptionsaboutpossiblesourcesofcontamination,bothancientandrecent.Thepaleontologicalcommunitygreetseveryreportofmoleculessurvivingforseveralbillionsofyearswithcaution,ifnotoutrightskepticism.Nevertheless,thechemicaltracesarethereandmaybethebestwindowontothistenuousancientbiosphere.(Moreonhowinchapter7.)
Byourplanet’sone-billionthbirthday,lifehadestablishedafirm,ifrelativelyinconsequential,footholdonitssurface.Foranotherbillionyears,Earth’smicrobiallifewouldgentlynudgenear-surfaceenvironments,firstbyspeedingalongredoxreactionsandthenthroughphotosynthesis.Asnearaswecantell,evenattwobillionyearsold,Earthwouldnothavedisplayedanysignificantlife-imposedmineralogicalnoveltyatornearitssurface.Cellswouldsimplymakemoreironoxides,morelimestone,moresulfatesandphosphatesthanmightotherwisehaveformed.Theywouldbuildlayereddepositsofiron-richmineralsinthedeeperoceanandcraftprotectiverockymoundsincoastalshallows—allphenomenathathadtranspiredonEarthbeforethedawnoflife,andonotherplanetsandmoonsintheSolarSystem.ButEarthanditsprimitivemicrobialpopulationwerepoisedtomakethemost
dramatictransformationintheplanet’shistory.Overthenext1.5billionyears,photosynthesizingmicrobeswouldlearnanewchemicaltrick—toexhalethehighlyreactive,dangerouslycorrosivegascalledoxygen.
Chapter7
RedEarth
PhotosynthesisandtheGreatOxidationEventEarth’sAge:1.0to2.7billionyears
Fast-forwardtothepresentday,andit’sclearthatlifehasirrevocablytransformedEarth’snear-surfaceenvironment—mostconspicuouslytheoceansandatmospherebuttherocksandmineralsaswell.Itwouldtakemorethanabillionyears,aftertheinnovationofthefirstlivingcell,forsuchatransformationtobegin.Inthatperiod,newvarietiesofmicrobesmayhavecreatedabrownishorpurplishscuminsomecoastalregions.Theremayevenhavebeenpatchesofgreenishslimedecoratingequatorialshoresandpopulatingshallowponds,asafewclevercellsexperimentedwithnewwaystoharnesstheSun’sradiantenergy.Butthecontinentswerestillbarren:noplantsclungtotherockylandscape,norwereanyanimalspresenttoeatthem.Youstillwouldhavediedquickly,inagony,ifyouwerestrandedinthisanoxicworld.Earth’ssurfacechangedfromdullgraytobrickred,inageologicalafternoon,
withtheinnovationofoxygen-producingphotosynthesisandtheconsequentriseofanoxidizingatmosphere.It’sdifficulttodocumentexactlywhenandhowquicklyslimygreenalgaeevolvedtotriggerthistransformation,calledtheGreatOxidationEvent.Ourbestguesscomesfromsubtlechangesintherockrecord,whichsuggestapulseofphotosynthesisshortlyafterEarth’stwo-billionthbirthday—about2.5billionyearsago.Afterthatmodeststart,thingshappenedrelativelyquickly:by2.2billionyearsago,atmosphericoxygenhadrisenfromzerotomorethan1percentofitsmodernlevel,foreverchangingEarth’ssurface.TheintriguingstoryofEarth’sinitialoxygenationisonlynowcominginto
focus,asunexpectednewclueshaveemergedandpromisingnewlinesofevidencehavebeenpursuedinearnest.Thepasthalfcenturyofpaleo-atmosphereresearchhasseenmanycompeting,sometimesdiametricallyopposedideas,butthescientificmethodisagreatwinnoweroftheuntenableandthefalse.Wedon’tyethavethewholenarrative,butwearegettingmuchcloser,andthepicturethatisemergingis(literally)breathtaking.
TestimonyoftheRocks
EvidencefortheGreatOxidationEventcomesfromagrowingcatalogofobservationsonrocksandmineralsthatdatefromavastchunkofEarth’shistory—roughly3.5to2.0billionyearsago.Ontheonehand,manyrocksolderthan2.5billionyearscontainmineralsthatareeasilydestroyedbythecorrosiveeffectsofoxygen,suggestinganoxygen-freeenvironmentpriortothattime.Geologistsfindunweatheredandroundedpebblesofpyrite(theironsulfidealsoknownasfool’sgold)anduraninite(thecommonesturaniummineral)inancientstreambeds—placeswheresuchmineralswouldquicklycorrodeandbreakdownintoday’soxygen-richsurfaceenvironment.Suchancientsandylayersalsohaveatelltalechemistry—unusuallyconcentratedinoxygen-avoidingelementslikecerium,whilestrikinglydeficientinotherslikeironcomparedwithtoday’ssoils.Thesechemicalquirksfurtherevinceanatmospheredevoidofoxygen.Bycontrast,rocksyoungerthan2.5billionyearscontainmanyunambiguous
signsofoxygen.Between2.5and1.8billionyearsago,massivedepositsofironoxidescalledbandedironformationsweredepositedinstaggeringabundance.Thesedistinctive,denseaccumulationsofalternatingblackandrustyredlayershold90percentoftheworld’sknownironorereserves.Manganeseoxidesalsosuddenlyappear,asthicklystratifieddepositsthatnowprovidetheworld’schiefrepositoriesofmanganeseores.Hundredsofothernewminerals—oxidizedoresofcopper,nickel,uranium,andmore—alsoappearintherockrecordforthefirsttimeaftertheGreatOxidationEvent.Yetinspiteofthisexpandedmineralogicalrepertoire,somescientistsremainedunconvincedthattheGreatOxidationEventwasreallyaneventatall.Perhapstherewasjustaslowandsteadyincreaseinatmosphericoxygen.Perhapsthespotty,erodedrockrecordisincompleteandmisleading.ThesmokinggunoftheGreatOxidationEventcamefromanunexpected
source—remarkablerecentdataontheisotopesofthecommonelementsulfur.The1990ssawdramaticincreasesintheresolvingpowerandsensitivityofmassspectrometers,whicharetheworkhorseanalyticalinstrumentsoftheisotopeworld.Thenewgenerationofmassspectrometersallowedscientiststoanalyzesmallerandsmallersamples,evenmicroscopicmineralgrainsorindividuallivingcells,withhigherandhigherprecision.Sulfur,oneoflife’sessentialelements,provedaparticularlytemptingtargetforstudy,astherearefourstablesulfurisotopesinnature:sulfur-32,-33,-34,and-36.Alloftheseisotopeshavesulfur’srequisitesixteenprotonsinthenucleus,butthenumberofneutronsvariesfromsixteentotwenty.Thedistributionofsulfurisotopesisusuallypredictableonthesimplebasisof
mass.Allatomswiggle,butlessmassiveisotopeswigglemore.Consequently,inanychemicalreactionlightisotopesaremorelikelytojumpaboutthanheavyones.Thisselectiveprocess,calledisotopefractionation,occursanytimeacollectionofsulfuratomsexperiencesachemicalreaction,whetherinasolidrockoralivingcell.Inthecaseofsulfur,anisotopeofmass32willtypicallyfractionatemorethananisotopeofmass34or36.What’smore,thefractionationratioisusuallydirectlyrelatedtothemassratiooftheisotopes:thefractionationofsulfur-36tosulfur-32isalmostalwaystwicethefractionationofsulfur-34tosulfur-32.ThisbasicphysicsfollowsdirectlyfromNewton’slaws:forceequalsmasstimesacceleration.Smallermassmeansabiggeracceleration,sounderagivenforce,sulfur-32wigglesmorethansulfur-34,whichwigglesmorethansulfur-36.AdecadeagogeochemistJamesFarquhar,workingatthescenic,oceanside
SanDiegocampusoftheUniversityofCalifornia,foundaprofoundandunexpectedchangeinthedistributionsofsulfurisotopesinrocksolderthanabout2.4billionyears.Morerecentrocksandmineralsalmostalwaysdisplaythemass-dependenttrendonewouldexpect;theirratiosofsulfurisotopesdependalmostexclusivelyontheratiosofmass.ButFarquharandhiscolleaguessawaradicallydifferentfractionationofsulfurisotopesinmanyrocksolderthan2.4billionyears—insomesamples,wilddeviationsofseveraltenthsofapercent(whichisalotinthiscase).Whatcouldhavecausedsuchamass-independentdeviationfromtheunassailableNewtonianlawsofmotion?Clevertheorists,backedbyexperimentalproofs,werequicktopointtoa
solutioninthenuancesofquantummechanics.Undertheinfluenceofultravioletradiation,isotopebehaviorcandeviatefromtheNewtonianideal.Itturnsoutthatisotopeswithanoddmassnumber,suchassulfur-33,canbeselectivelyaffectedbyultraviolet(UV)radiation.Ifamoleculeofsulfurdioxideorhydrogensulfidehappenstoincorporatesulfur-33,andifthatmoleculeencountersanultravioletray(mostlikelyhighintheatmosphere),itmayreactmorereadily.Thesulfur-33experiencesa“mass-independentfractionation”thatskewstheisotoperatios.ButwhythesuddenchangeonEarth2.4billionyearsago?Theanswerliesin
theUV-absorbingpropertiesofozone,amoleculeofthreeoxygenatomsthathasbeenmuchinthenewsthesepasttwodecades.TodayozonehighintheatmosphereprovidesanessentialbarriertotheSun’spotentiallylethalultravioletrays.Measurementstakenoverthepasttwodecadesrevealthatthishigh-altitudeozonelayerhasbeensignificantlydepleted,mostlikelybydestructivereactionswithhuman-producedchemicalscalledchlorofluorocarbons,orCFCs.(Freon,
onceusedinairconditioners,isthebest-knownexample.)Suchan“ozonehole”allowsmorecancer-causingUVradiationtoreachEarth’ssurface.ThegoodnewsisthataworldwidebanonCFCproductionseemstoberesultinginarapidrestorationoftheozonelayer.Beforetheriseofoxygengasandtheconsequentcosmicsun-blockingability
oftheozonelayer,sulfurcompoundshighintheatmosphereweresubjectedtoanonstopbathofultravioletradiation.Undersuchharshconditions,compoundswithsulfur-33experiencedmass-independentfractionation.AftertheGreatOxidationEvent,aprotectiveozonelayeraccumulatedtoabsorbmuchoftheSun’sUVradiationandthuseffectivelyshutdownthisoddisotopeeffect.AslabafterlabaroundtheworldverifiedandamplifiedFarquhar’sfindings,
thegreatmajorityofEarthscientistsacceptedtherealityoftheGreatOxidationEvent.Unlessscientistsdiscoverawayotherthanozonetoblockultravioletrays,thesulfurisotopedatapegthebeginningoftheGreatOxidationEventatabout2.4billionyearsago.
MakingOxygen
Sowheredidalltheoxygencomefrom?Thesedaysoneofthefirsttopicsinanyintroductorybiologyclassisphotosynthesis—theremarkableabilityofplantstocombinewater,carbondioxide,andsunlighttomaketheirtissues,whileproducingoxygenasaby-product.Wenowtakeitforgrantedthatplantsplaythiscentralroleinmakingourworldahabitableplace,butthediscoveryofphotosynthesiswasoneofthegreatestadvancesinscience.Andlikesomanyofscience’spivotaldiscoveries,itcameinpiecemealfashion.Thediscoveryofwater’srolecamefirst.Detailedmechanismsofplantgrowth
wereamysterytoscientistsoftheseventeenthcentury,butacommonassumptionheldthataplant’stissuesmustcomefromthemineral-richsoil,whichmustthereforebeconsumedasplantsgrow.FlemishphysicianJanBaptistaVanHelmont(1579–1644)putthisassumptiontothetestbyconductingasimpleexperimentinthe1640s.Inhisownwords:
ItookanEarthenVessel,inwhichIput200poundsofEarththathadbeendriedinaFurnace,whichImoystenedwithRain-water,andIimplantedthereintheTrunkorStemofaWillowTree,weighingfivepounds;andatlength,fiveyearsbeingfinished,theTreesprungfromthence,didweigh169pounds,andaboutthreeounces:ButImoystenedtheEarthenVesselwithRain-waterordistilledwater(alwayeswhentherewasneed)…At
length,IagaindriedtheEarthoftheVessel,andtherewerefoundthesame200pounds,wantingabouttwoounces.Therefore164poundsofWood,Barks,andRoots,aroseoutofwateronely.
VanHelmont’sdiscoverywasamajoradvance,thoughwater(aswenowknow)wasonlypartofthestory.AcenturylatertheEnglishclergymanandnaturalistStephenHalesfirst
suggestedthatplantsrelyonsomecomponentoftheairaswellaswater—traceatmosphericcarbondioxide.Wenowrecognizethatbothwaterinthesoilandcarbondioxideintheairareprincipalingredientsforphotosyntheticorganisms.(Ironically,itwasVanHelmonthimselfwhodiscoveredcarbondioxidegas,buthedidnotrealizeitscentralroleinplantgrowth.)Evenso,theroleofsunlightwasenigmatic,andittookanotherthreehundred
yearsforthedetailstoemerge.Advancesinnuclearphysicspavedtheway,asanewgenerationofparticleacceleratorscalledcyclotronsprovidedthefirststeadysupplyofthehighlyradioactiveisotopecarbon-11—asensitiveprobeofbiologicalreactions.Inthelate1930s,SamuelRubenandMartinKamenattheUniversityofCaliforniaatBerkeleyexposedplantstocarbondioxidewithacarbon-11“label.”Inthisway,theycouldusethetelltaleradioactivitytofollowcarbondioxideasitwastakenintoplanttissues,thoughthefleetingtwenty-one-minutehalf-lifeofcarbon-11madetheseexperimentsexceedinglydifficult.RubenandKamen’s1940discoveryofawaytomanufacturecarbon-14,a
muchmoresuitabletracerisotopewithaleisurelyhalf-lifeof5,730years,revolutionizedbiophysicalresearchandledtoarapidunderstandingofhowplantstakeadvantageofwater,carbondioxide,andsunlight.Inbrief,aclever(andveryancient)proteincalledrubisco—achemicalfoundinthetypeofpioneeringcyanobacteriathoughttodatebackthreebillionyearsormore—concentratescarbondioxideandwater,absorbstheSun’senergy,andassemblestherawmaterialsintoessentialbio-buildingblocks.Inthephotosynthesisreactionthatyieldstheoxygenwebreathe,algaeorplantsconsumesixmoleculesofcarbondioxideplussixmoleculesofwatertomakeonemoleculeofthesugarglucose,withsixmoleculesofoxygenasaby-product.Thischemicaltransformationisanotherexampleofouroldfriend,theredoxreaction(likerustingiron).Inthiscase,thecarbonatomsincarbondioxidegainelectronsandarethusreduced,whilewaterorsomeotherelectrondonorisoxidized.Inphotosynthesis,theSun’sraysprovidetheenergeticboosttoshiftelectrons.Asstraightforwardasthebare-boneschemicalreactionmightsound—carbon
dioxidepluswater(orsomeotherchemicalthatcancontributeelectrons)makes
sugarsandotherbiomolecules—thedetailsofphotosynthesisareimmenselycomplicatedandarestillbeingworkedout.Foronething,microbeshavefiguredoutquiteafewdifferentwaystoharvestsunlightandothersourcesofenergy.Mostoxygen-producingplantsandalgaetodayusethebrightgreenpigmentchlorophylltoabsorblightinredandvioletwavelengths.ButthroughoutEarth’shistory,avarietyofcellshaveemployedotherphotosyntheticpathwaysthatproducednooxygenatall.Alternatelight-absorbingpigmentshaveevolvedtodecorateredandbrownalgae,purplebacteria,andstrikinglybeautifuldiatomsandlichensinawiderangeofcolors.Afewinventivemicrobesevenpowertheirphotosyntheticreactionswithinfraredradiation—wavelengthsthatareutterlyinvisibletooureyesbutthatourbareskinsensesasheatenergy.Thecomplexoriginsofphotosynthesisarethesubjectoftheresearchof
biochemistRobertBlankenship,whoholdschairedpositionsinboththechemistryandbiologydepartmentsatWashingtonUniversityinSt.Louis.Blankenshipandhiscoworkers,includingformercolleaguesfromtheinfluentialastrobiologyteamatArizonaStateUniversity,searchforsignsofearlylife,bothonEarthandonotherworlds.Theirstrategyistoexaminethevariedphotosyntheticpathwaysofmanydifferentkindsoflivingmicrobes—purple,brown,yellow,andgreen—scrutinizingtheirgenomesforsimilaritiesanddifferences.Datacomefrommultipleaspectsoftheintricatephotosyntheticapparatus:thevariednatureofphotosyntheticpigments,theexactmolecularsequencesofprotein“reactioncenters”thatshiftelectronsfromonemoleculetoanother,themanywaysthosetransferredelectronsarethenusedtomakethecell’sbuildingblocks,andeventhemyriadstructuresof“antennasystems.”(Remarkably,cellshaveevolvedclustersofmoleculesthatoperateastinylight-collectingantennas.)Blankenshipfindsthatlifehasdevisedabewilderingdiversityof
photosyntheticstrategies.Life,itseems,exploitsanyaccessibleenergysource.Overandoveragainmicrobeshavefiguredoutnewwaystocollectlightforgrowthandreproduction—atleastfiveseparatepathways,extendingdeepintoEarth’sevolutionaryhistory.Manydetailsofthathistoryareobscure,butthemostancientandprimitiveoftheseenergy-gatheringchemicalreactions,possiblydatingtomorethan3.5billionyearsago,clearlyproducednooxygenatall.Ancestorsofthoseearlycellssurvivetodayandillustratethatthemostdeeplyrootedbiochemistrieswereanaerobic,neitherrequiringnoreventoleratingoxygen.TheresearchofBlankenshipandhiscoworkersnotonlyrevealsthewide
rangeofthesediversechemicalstrategiesbutalsopointstoatendencyfor
microbestoshuffleandswaptheirlight-collectinggenes,co-optingtheirrivals’photosyntheticpathwayslikeindustrialtradesecrets.Indeed,themodernschemeofphotosynthesisusedbyvirtuallyallplantsappearstobeacombinationoftwomoreprimitiveschemes(prosaicallynamedPhotosystemIandPhotosystemII).ContemporaryorganismscanthuspiggybackcomplexbiosynthesisreactionsandcollectandusesunlightfarmoreefficientlythanthoseinearlierstagesoflifeonEarth.
MoreOxygen
Evenwithoutphotosynthesis,Earth’ssurfacewouldhaveexperiencedaleisurely(andcorrespondinglytrivial)oxidationthroughtheslowlossofhydrogenmoleculesintospace.Highintheatmosphere,H2Omoleculesarevulnerabletothedestructivepowersofultravioletradiationandcosmicrays,whichcanfragmentwaterintohydrogenplusoxygen.Water’satomsrearrangeintoothersimplemolecules,mostlyH2andO2,aswellastracesofozone,O3.Theresultingswift-movinghydrogenH2molecules,unlikethemuchheavierlumberingO2andO3moleculesofoxygen,areabletoescapetheincessantpullofEarth’sgravityandflyoutintothevoidofspace.ThroughoutEarth’shistory,smallamountsofhydrogenhavebeenlostinthisway,leavingbehindagradualaccumulationofexcessoxygen.Eventodaytheprocesscontinues,asaquantityofhydrogenroughlyequaltotheatomsinafewOlympic-sizeswimmingpoolsescapestospaceeveryyear.Bythesameprocess,smallerMars,withmuchlessgravitytoholditshydrogen,hasshedmuchofitswater.Over4.5billionyears,mostofMars’snear-surfacehydrogenhasthusescapedtospace,whileironmineralsnearthesurfacehaverustedtogivetheplanetitspresentredcolor.Evenso,thetotalamountofoxygeninMars’sthinatmosphereistrivial:wereitalltocondenseontothesurface,thelayerofliquidoxygenwouldbelessthanathousandthofaninchthick.InexorableoxygenproductionbyhydrogenlossmighthaveturnedEarth’s
surfacerustyredoverasimilarmultibillion-yearperiod,butitcan’thavehadmucheffectonEarth’searlyenvironment.Evenwiththemostextremeestimates,lessthanoneatmosphericmoleculeinatrillionwasO2beforetheGreatOxidationEvent.(Todayit’soneinfive.)Thattrivialamountofoxygenwouldhavebeensnappedup,asfastasitcouldbegenerated,atEarth’ssurfacebyhugequantitiesofironatomsjustwaitingtobeoxidizedintheoceansandinthesoils.EvenifEarthhadremainedlifelessandeventuallysportedreddish
weatheredzonesinolder,stablepartsofthecontinents,suchasuperficialcoatingofrougewouldhavebeenpurelycosmetic.Life,too,mayhavecontributedasmallinventoryofoxygenpriorto
photosynthesis.Infact,cellshavelearnedatleastfourdifferentwaystomakeoxygenfromtheirsurroundings.Oxygenicphotosynthesisisthebigonetoday,butotherbiochemicalpathwaysmayhaveplayedsmallrolesinancienttimes.Lifescavengesenergyfromitsenvironmentanywayitcan.Theeasiestway
togainenergywhilereleasingoxygenistostartwithamoleculethatisalreadyoxygenrichandhighlyreactive.Thusanumberofmicrobeshavelearnedtoexploitmoleculesofperoxide(H2O2,producedbyreactionshighintheatmosphere)togenerateO2plusenergy.Admittedly,suchmolecularspecieswouldhavebeenscarcebeforetheriseofatmosphericoxygen,andsuchmicrobialmechanismscan’thaveplayedmuchofaroleinmodifyingEarth’searlyenvironment.AteamofmicrobiologistsinHollandrecentlyreportedamorerelevant
oxygen-producingscenario:theydiscoveredremarkablemicrobesthatgainenergybydecomposingnitrogenoxides.EarlyinEarth’shistory,theseso-calledNOXchemicalswereproducedinsmallamountsthroughreactionsofnitrogengaswithminerals—duringlightningstorms,forexample.Today,owingtowidespreaduseofnitrogen-richfertilizers,manylakes,rivers,andestuariesareheavilypollutedwithNOXcompounds,whichpromotelargemicrobialblooms.Thenewlydiscoveredmicrobesareabletodecomposenitrogenoxidesintonitrogenplusoxygen,thenusetheoxygento“burn”naturalgas,ormethane,andthusenjoyajoltofenergy.Suchacleverchemicalstrategymightproveespeciallyusefulonanitrogen-rich,oxygen-starvedworldlikeMars.
FossilEvidence
Ofalltheoxygen-producingmechanisms,photosynthesisistheundisputedchampion,buthowearlydidphotosynthesisandtheproductionofoxygenbegin?Paleontologists,whoscrutinizethefragmentarytangibleremainsofancientlivingworlds,seetheconnectionsbetweenlifepastandlifepresentmorevividlythananyotherscientists.Perhapsit’snotsurprising,therefore,thattheywereamongthefirsttofindevidenceforanoxygenatedEarthmorethantwobillionyearsago.Intheirsearchfortheearliestphotosynthesis,thefossilhuntersnaturallyfocusonEarth’soldestrocks.Fossilevidenceforancientphotosyntheticcellsisspottyatbest.Preciousfew
microbialremainsmakeitthroughbillionsofyearsofburial,heating,squeezing,andchemicalalteration.Whatdoessurviveiscookedandcrushed,ofteninwaysthatnecessitateacolorfulimaginationtoachieveanybiologicalinterpretation.Coloniesoffossilmicrobesoftenappearaslittlemorethanscatteringsofsmallblacksmudges,soit’snotsurprisingthateveryreportofmicrobesmorethantwobillionyearsoldhasbeenmetwithcautiousskepticism,ifnotoutrightridicule.Formuchofthepastfourdecades,oneofthemostardentdefendersof
paleontologicalrigorhasbeenJ.William(Bill)Schopf,professorofpaleontologyattheUniversityofCalifornia’sLosAngelescampus.Basedonhisstudiesofincreasinglyancientmicrobialfossils,Schopfhasdevelopedachecklistoftraitsnecessaryandsufficienttoconfirmtheclaimoflife.Byfirstfocusingonmorerecent,well-preserved,andunambiguousspecimens,Schopfisthescientistwhomostconvincinglypushedthefossilrecordfurtherandfurtherback,morethanthreebillionyearsintotheremoteArcheanEon.Schopf’scriteriaarestraightforwardandreasonable:fossilmicrobesmust
comefromproperlydatedsedimentarylayerslaiddowninenvironmentswheremicrobescouldhaveoncelived.Fossilsmustshowuniformityofsizeandshape—consistentspheresorrodsorchains,unlikethekindofshapelessblackblobsandstreaksthatarefoundinmanyoldrocks.SchopfandhisstudentsalsoemployedstatisticstoremovesomeofthesubjectivityinherentinobservationsofEarth’soldestsedimentaryrocks.Thisquantitativecatalogofessentialtraitsforanysuiteofmicrobialfossils
servedSchopfwell.Hewasabletopublishunassailabledescriptionsofnewfossilfinds,whileraisingdoubtsaboutsomeofthemorequestionableclaimsofancientlifebycompetingresearchers.Hismostnotablechallengecamein1996,whenNASAscientistsannouncedthatmicrobialremainshadbeenfoundinaMartianmeteorite.AtadramaticNASA-hostedpressconferenceinAugustofthatyear,Schopfwasthelonedissentingvoice.Withthinlyveiledcontempt,hepointedoutthattheMartian“fossils”weremuchtoosmall,lackedsupportingchemicalandmineralogicalevidence,andwereinthewrongkindofrocktoboot.(InspiteofSchopf’spersuasivearguments,PresidentClintonlaudedthediscovery,whichmayhaveledtoasignificantbumpinNASAfundingforastrobiology—moneythatwoundupsupportingmanyofusintheoriginsgame,includingSchopf.)Ironically,Schopfwouldsoonmeetthesamekindofwitheringcriticismfor
anearlierclaimhe’dmadein1993,whenheannouncedhisdiscoveryofEarth’soldestmicrobialfossilsfromtheApexchert,arockformationalmost3.5billionyearsoldinnorthwesternAustralia.Photographsofsuggestiveelongatedblack
structureswithcell-likesegmentationseemedcompellingenough.Thestory,publishedinahigh-profileSciencepaper,supplementedthefossilphotoswithartisticlinedrawings(to“aidtheeye”),whichwereplacedsidebysidewithphotosofcyanobacteria,similar-lookingmodernphotosyntheticmicrobes.Schopfevenclaimedthathisfossilswereprobablyoxygenproducers.Withinafewyears,hismostconvincingphotoshadbecomeamongthemostreproducedpaleontologicalimagesofalltime,adorningnumeroustextbookswithcaptionsthatrepeatedthe“earliestfossil”claims,oftenwiththesuggestionthatthemicrobeswerephotosynthetic.It’saruleinsciencethatextraordinaryclaimsrequireextraordinaryevidence.
It’salsotruethatextraordinaryclaimsusuallyreceiveextraordinaryscrutiny.AllofSchopf’sfossilspecimensresideattheBritishMuseuminLondon,preservedascarefullycataloged,thintransparentsectionsofrockmountedonglassslides.In2000OxfordpaleontologistMartinBrasierbeganadetailedreexaminationoftheApexchertmaterialandcametoaverydifferentconclusion.Schopf’s“thinsections”ofApexchertactuallyturnedouttoberatherthick,at
leastcomparedwiththesizeofamicrobe.BrasierandhiscolleagueswereeventuallyabletolocatemostofthetinyobjectsthatSchopfhadphotographedandpublished,buttheyweresurprisedtorealizethatmanyofthephotographswereatbestmisleading.EachofSchopf’snow-classicphotosrepresentsasinglemicroscopicfocalplane—athintwo-dimensionalslicethroughhissmudgythree-dimensionalblackobjects.Brasierandhisteamemployedanewerphotographictechniquethatcreatedathree-dimensionalmontageofimagesandthusrevealedamuchmorecomplexstory.OnlyiftheysetthemicroscopefocustotheexactdepthofSchopf’sphotographscouldtheyreproducethenow-classicimagesofApex“fossils.”Butraiseorlowerthefocusslightly,andwhatappearedatfirsttobeaconvincing,elongatedstringofmicrobialcellsmorphedintoawavysheetorirregularblob,sometimeswithfolds,branchings,orsquiggles.AccordingtoBrasier’sobservations,the“chainsofmicrobes”aremisleadinglychosencrosssectionsthroughcomplexthree-dimensionalstructuresthatbearlittleresemblancetoanythingbiological.TheembarrassingchallengebyBrasierandcolleagues,“QuestioningtheEvidenceforEarth’sOldestFossils,”appearedintheMarch7,2002,issueoftheprominentperiodicalNature.Schopffoughtbackwithhisownarticle,“Laser-RamanImageryofEarth’s
EarliestFossils,”publishedback-to-backwithBrasier’sinthesameissue.SchopfandhiscolleaguespresentednewanalysesoftheApexchert’scarbon-richblackblobsandshowedthemtohaveisotopiccompositionsandatomic
structuresconsistentwithbiology.Heboldlyrepeatedthe“oldestfossil”rhetoric,thoughheseemedtobackoffhisinterpretationthatthemicrobeswerephotosynthetic.Nevertheless,theseedsofdoubthadbeensownonSchopf’sclaims,andthebarhadbeenraisedinthehuntfortheearliestsignsoflife.(Inalate-breakingsequel,MartinBrasierandhiscoworkersfromAustralia
nowclaimthattheyhavefoundthe“oldestfossils”—microbialremnantsfromthe3.4-billion-year-oldStrelleyPoolformation,discoveredonlytwentymilesfromSchopf’sslightlyolder,butstilldisputed,smudges.Fewobserversexpectthisdevelopmenttobethelastwordinthedebate.)
TheSmallestFossils
Imaginewhathappenswhenacolonyofmicrobesdies.Almostalwaysthetinybagofchemicalsthatoncewasalivingcellfragmentsanddisperses;bigbiomoleculesbreakdownintosmallermolecularpieces,mostlywaterandCO2.Othermicrobesmayeatthetastiestbits,whileindigestiblemoleculesdissolveintheoceans,orevaporateintotheair,orbecometrappedinrock.Usuallyafterafewyears,nothingisleft,fortimeisnotkindtosuchfragilemolecularremains.Underextraordinarycircumstances—ifdeadcellsarequicklyburied,ifthere’s
nocorrosiveoxygenaround,ifthehostrocknevergetstoohot—afewofthehardiestbiomoleculescansurvive,albeitinaratheralteredform.Mostlikelytopersistaremoleculeswitharuggedbackboneofuptoabouttwentycarbonatoms,sometimesarrangedinasimplelongchain(perhapswithafewcarbonatomsstickingofftothesidehereandthere),sometimesinagroupofinterlockingrings(notunliketheOlympiclogo).Thesediagnosticbiobitsarelikeultrasmallskeletons.Theyrepresentwhat’sleftofmuchlargercollectionsoffunctioningmoleculesthathavebeendegradedandstrippedofeverythingbutthemostresilientcore.Ifyoucanfindsuchamolecularskeletoninanancientsedimentaryrock,and
ifyoucanbesurethatit’snotcontaminationfromnearbyyoungerstrataortheubiquitousleavingsofmorerecentlydeceasedcells(frommodernsubsurfacemicrobes,forexample,orevendeadskinfromyourthumb),thenyoumightbeabletoclaimdiscoveryofachemicalfossil—theactualatomsofaonce-livingmicrobe.HencethefascinationwithSchopf’sblackblobsintheApexchert.Manymodernmolecularpaleontologistsleadafascinatingdoublelife.Onthe
onehand,theymaychoosetoenduretherigorsofthefieldgeologist,hikingmilesacrossarduousterrain,schleppinghundredsofpoundsofpromisingrock
fromremoteoutcropsinbakeddeserts,frozentundra,andhighmountains.EveryyearsmallteamssetoutforWesternAustralia,SouthAfrica,Greenland,andcentralCanadainsearchofnewspecimens.Otherslaboratdrillingrigsinthehopesofsecuringdrillcoresofpristineancientrock,uncontaminatedbyweatherandvegetation.Suchexpeditionscanmeanmonthsofhardship,danger,anddeprivation.Theseadventurescontrastwithmonthsoftediousanalysisconductedin
ultracleanlabs,wheretheslightestbreathorthumbprintcanirrevocablycontaminateapreciousthree-billion-year-oldrocksample.Ittakestimeandpatience,exquisitecare,andanarsenalofsophisticatedanalyticalapparatustoextractindividualmoleculesfromarock.Oneleadingpractitionerofthistwenty-first-centuryartisAustralianpaleontologistRogerSummons,whohassetupshopatMIT’sdepartmentofEarthandplanetarysciences.ThereheheadstheSummonsLab,acrackteamofadozenmolecularfossilhunterswhostudyEarth’soldestrocks.Adozenyearsago,whileworkingatAustralianNationalUniversity,
Summonsledagroupofscientistswhomadeheadlineswhentheystudiedpromisingsedimentsfromthe2.7-billion-year-oldPilbaracratoninWesternAustralia.Summonsandhiscolleagueshadaccesstoauniquedrillcore,asequencealmosthalfamilelongthatincludedatantalizingsectionofblack,carbon-richshale—thekindofsedimentaryrockmostlikelytoholdmolecularfossils.ThesePilbararockswereofspecialinterestbecausetheyappearedtobeessentiallyunalteredbyheatanduncontaminatedbysurfacelifeorgroundwater.Ifevertherewasarockwhereoldbiomoleculesmightsurvive,thatwasit.TheAustralianresearchersfocusedonhopanes,theelegantclassofhardy
biomoleculesmentionedinchapter6.Hopanesplayimportantrolesinstabilizingprotectivecellmembranesand,becauseoftheirrarityoutsideoflivingcells,areperhapsthemostconvincingofallthemolecularbiomarkers.Eachhopanehasadistinctivebackboneoffiveinterlockingrings—fourlittlehexagons(eachdefinedbysixcarbonatoms)andafifthpentagon(withfivecarbonatoms)ontheend.Eachringsharestwocarbonatomswithitsneighbors,foratotaloftwenty-onebackbonecarbonatomsinall.MeticulousstudiesatSummons’sAustralianlabledtotwohigh-profile
papers,bothpublishedinAugust1999.ThefirstoneinScience,withSummons’sPh.D.studentJochenBrocksasfirstauthor,describedthediscoveryofhopanesinthe2.7-billion-year-oldPilbararocks—whatwouldhavebeentheoldestknownmolecularfossils,breakingthepreviousrecordbyabillionyears.Discoveriesofhopanescanrevealalotaboutancientecosystems,fordifferent
speciesuseanumberofdifferentkindsofhopaneswithextracarbonatomsstuckonatvariousplacesaroundtherings.BrocksandhiscolleaguessuggestedthatthePilbarahopaneswerediagnosticofratheradvancedcellscalledeukaryotes—cellsthatcontainanucleusthathousesDNA.Atthetimeofpublication,theoldestknowneukaryotefossilcellswereonlyaboutonebillionyearsold,whereasprimitivemicrobesofthetypethoughttoexistbeforeabouttwobillionyearsagodon’thaveanucleus,sothisinterpretationwasmetwithsurprise,ifnotdownrightdisbelief.Ifthediscoveryistrue,thenonlytwoconclusionsarepossible.Eithereukaryotesappearedmuch,muchearlierthananyonehadthought(andlife’sevolutionwascorrespondinglyaccelerated),orelsehopanesevolvedmuchearlierthaneukaryotes.Ineithercase,ourunderstandingoflife’shistorywouldhavetoberevised.Thesecondarticle,publishedinNaturewithSummonsasleadauthor,made
theequallystartlingclaimthatnearby2.5-billion-year-oldblackshalesfromMountMcRae,amodest3,300-footpeakinWesternAustralia,containavariantofthehopanefive-ringmoleculewithanextracarbonatomstickingoffthesideofthefirstring.These2-methylhopanoidmoleculesareknownonlyfromphotosyntheticcyanobacteria,whichareEarth’sprincipaloxygenproducers.SummonsconcludedthatphotosynthesiswaswellunderwayonEarthby2.5billionyearsago.Suchachronologywasconsistentwiththeknownriseofoxygenataboutthattime,butthesuggestionthattheoriginofphotosynthesiscouldbesoughtinpreservedmolecularfragmentsopenedexcitingnewdoorstopaleontology.Noteveryonewaspersuaded.LikeBillSchopf’searlierclaimsof“Earth’s
oldestfossils,”RogerSummons’sextraordinaryhopanefindingshavebeenmetwithsomeopposition,includinggravedoubtsnowraisedbyJochenBrocksabouthisowndoctoralwork,aswellasallotherstudiesofpurportedbiomarkersmorethantwobillionyearsold.Younghopanesareeverywhere,theskepticssay.Thedeepsubsurfaceisteemingwithmicrobeslivingintherocks,socontaminationduringmorethantwobillionyearsofEarthhistoryisunavoidable.Thehopanesandotherbiomoleculesarenodoubtthere,butwhocansaywhenorhowtheygotthere.Staytuned:suchdebatesarefuntowatch,andtheyalmostalwaysleadtonewdiscoveries.
SandFlatsofTime
Whereelseisapaleontologisttolook?Ofthemanycluesinthefossilrecordrelatedtothehistoryofphotosynthesis,microbialmatsmaybeatoncethemost
obviousandthemostoverlooked.Todaytheyform,theworldover,inshallowcoastalwatersandalongthebanksofslow-movingriversandstreams,wherealgaecanintertwinefilamentsinthick,tangledlayers.Thesetough,clothlikematsensurethatthealgaehaveaccesstoawet,sunlitenvironment,whilebeingprotectedfromtheinevitableerodingactionoffloodsandwaves.Inspiteoftheirwidespreaddistribution,thepaleontologicalcommunityallbutoverlookedfossilmicrobialmatsbeforeNoraNoffke’sdiscoveries.Formorethanadecade,I’vehadtheopportunitytoassistNoraNoffke,a
professorofgeobiologyatOldDominionUniversityinNorfolk,Virginia,andtheworld’sleadingauthorityonancientmicrobialmats.Armedwithakeeneye,auniqueperspective,andsteelydetermination,shehaschosensomeofthemostforbiddingareasintheworldtoconductherfieldwork.VenturingintoremoteandhostileplacesinSouthAfrica,WesternAustralia,Namibia,thescorchingMiddleEast,andfrigidGreenland,shehasunearthedpaleontologicalwondersforwhichnoonehadpreviouslythoughttolook.OverandoveragainNorahasrecognizedevidencethatmicrobialmatsgrewonmanyofEarth’smostancientsandyshores.Thereasonmicrobialmatfossilsaresosignificantisthattheymustarisefrom
somekindofphotosynthesis.Themicrobesthatlefttheirfragmentaryremainsinblackchertsandblackshalescouldhavecomefromdeepzones,farfromsunlight.Astrongcasecanbemadethattheshallow-waterstromatolitesfrom3.5billionyearsagosupportedaphotosyntheticlifestyle,thoughthesemineralizedmoundscouldalsosimplyhavebeenprotectivehigh-risesinanotherwiseharsh,wave-sweptenvironment.Butmicrobialmatsmusthavebeenphotosynthetic.Whywouldacolonyofmicrobesgotoallthetroubleoffixingitselfinarough,shallowtidalzoneifitwasn’tafterthesunlight?ToplaceNoraNoffke’scontributionsincontext,considerotherreallyold
fossils.Formuchofthepasthalfcentury,paleontologistslookingforEarth’soldestlifehavefocusedonthreekindsofrockformations.Firstaretheblackcherts,likeBillSchopf’scontroversial3.5-billion-year-oldApexchert.Blackchertsfirstmadepaleontologicalheadlinesintheearly1960s,whenHarvardpaleobotanistElsoBarghoornrecognizedancientmicrobialfossilsinthe1.9-billion-year-oldGunflintchertfromnorthernMinnesotaandwesternOntario.Barghoornscrutinizedthin,transparentsectionsofthefine-grained,silica-richrockandrealizedhewasseeingancientmicrobialbodyfossilsinexquisitedetail.WithgeologistStanleyTyler,whohadfirstobservedenigmaticspherelikeobjectsintheGunflintadecadeearlier,Barghoorndescribedastonishingsuitesofunambiguouscells—amicroscopicecosystemofspheres,rods,andfilaments,
someintheprocessofdividing.Indeed,inspiteofdecadesofsubsequentclaimsforolderfossils,somepaleontologistsstillpointtotheGunflintchertasholdingtheoldestabsolutelyunambiguousfossilsofphotosyntheticcellsonEarth.Carbon-richblackshales,asecondrocktype,ofthekindstudiedbyRoger
Summonsandhiscolleagues,areperhapsthebestsourceofancientmolecularfossils.Blackshalesaredeep-wateraccumulationsofmudandorganicdebris,sowecanbeconfidentthattheyentombtheremainsofancientmicrobes.Asaconsequence,thicksequencesofblackshalesfromAustralia,SouthAfrica,andotherlocalitiesbillionsofyearsoldarereceivingpainstakingchemicalscrutiny,layerbymicroscopiclayer.Asnewer,moresensitiveanalyticaltoolscomeonline,somecapableofdetectingsinglemolecules,importantdiscoveriesaresuretofollow.Thethirdintensivelystudiedtypeofancientfossil-bearingformationisthe
stromatolites,thoselayereddomelikestructuresofmineralsdepositedbyearlylife.Paleontologistsmighthavebeenstumpedbytheoriginofthesemounds,usuallypreservedinlimestone,wereitnotformodernlivingstromatolitereefsinshallowseas,mostfamouslyinthescenicandremoteWorldHeritageSiteofWesternAustralia’sSharkBay.Theseoddsedimentaryfeaturesarisewhenaslimysurfacecoatingofmicrobes—photosyntheticmicrobes,inthecaseoftoday’slivingreefs—produceslayeruponlayerofminerals.Hundredsoffossilstromatolitelocalitieshavebeenidentifiedaroundtheworld,someinrocksolderthanthreebillionyears.Blackchert,blackshale,andstromatolites.TothisshortlistofEarth’soldest
fossiliferousformations,NoraNoffkehasaddedafourthrocktype:sandstone.It’sunderstandablewhysandstoneswereoverlooked.Mostfossilsarepreservedinfine-grainedrockslikechertorshale,orinlimestonereefs—hencethefocusonblackchert,blackshale,andstromatolites.Sand,bycontrast,isrelativelycoarse,withmineralgrainsmuchlargerthanmostmicrobes.What’smore,sandtendstoconcentrateatthebeach,intheturbulenttidalzone,wheremostsignsoflifearequicklyerased—eroded,washedaway,anddispersed.ButNoffkehasspenttwodecadesstudyingmoderntidalflatsandtheirrichecosystemsandfoundthattough,fibrousmicrobialmatsimposedistinctivestructuresonshallow,sandyshorelines.Theyimprintacrinklytexturetothesandsurface,notunlikeawrinkledtablecloth;theybindandtrapsedimentgrainsinathick,resilientmassofalgalstrands;theyalterthepatternofripplemarksinthesand;andtheyfragmentinstorms,tearingintodistinctivegeometricchunksandrollinguplikelittlePersiancarpets.Mostsandstoneoutcroppingsappearsmoothorgentlyrippled,devoidof
anythingobviouslybiological.ButonceNoffkelearnedtospotthedistinctivewrinkledandcrackedsurfacescharacteristicoffossilizedmicrobialmatsinancientrocks,shespiedsubtlefeaturesalmosteverywhereshelooked.In1998sheidentifiedthediagnosticcrinklytexturesonthesurfacesof480-million-year-oldrocksoftheMontagneNoireintheFrenchAlps.In2000,aftermovingtoHarvardUniversityforpostdoctoralwork,shepushedtherecordevenfurtherback,identifyingsimilarpatternsin550-million-year-oldrocksofNamibia.Thefactthatmicrobialmatsexistedahalf-billionyearsagowasnotparticularlynewsworthy;allpaleontologistswouldhaveagreedthatmicrobialmatsmusthavebeenaroundtodecoratecoastalregionsmuchearlierthanthat.ButnoonebeforeNoffkehadtakenthetimetoscrutinizemodernmatsystems,thenrecognizethesimilartracespreservedasunambiguousfossilsinancientrocks.In2001Noffkemadethefirstofaseriesofgroundbreakingmicrobialmat
discoveriesinformationsmorethanthreebillionyearsoldfromSouthAfricaandAustralia—atimelongbeforethepresumedGreatOxidationEvent.SuchfeaturesaredifficulttospotintheoverheadglareofthemiddaySun,butlateintheafternoonattheendoflongandoftenfruitlessdaysofsearching,asthesunlightrakesacrossbarrenrock,thetelltalewrinkledsandstonesurfacesstandinstarkrelief.“Thestructuresseemedtopopouteverywhere,”sherecallsofonethrillingdiscovery,consummatedatthefinalhourofthefinaldayofonearduousAfricanfieldexcursion.Norafirstcametomein2000atthesuggestionofherHarvardmentor,
paleontologistAndyKnoll.AndyandIhadbeenfriendssincegraduatestudentdaysinthe1970s;foratime,ourcareershadtakenusindifferentscientificdirections,butourmutualinterestinastrobiologyhadrenewedourconversations.KnollrealizedthatNoffke’scaseforancientmatswasbasedalmostentirelyonsurfacefeaturesthat,whilesuggestive,attimesrequiredaspeculativeimagination.Theaveragepaleontologist,lackingNoffke’sextensiveexperiencewithmodernmats,couldeasilyoverlookordismissoddripplemarksorwrinkledrocksurfaces.SoKnollencouragedhertobolsterhercaseformatsbyaddinganalyticaldataontheminerals,biomolecules,andisotopespreservedinherdistinctivelycrenulatedlayers.Maybetracesofancientcarbonorconcentrationsofcharacteristicmineralscouldprovideasmokinggunforsomeoftheoldest,yetambiguous,matlikefeatures.IhadworkedwithotherKnollstudents,soIgotthecall.TheveryfirstspecimensthatNoffkesentprovedtobeanobjectlessoninwhy
suchanalysesareimportant.Shehadfoundthinwigglyblacklayersinasandysedimentthreebillionyearsold—whatwouldhavebeentherecordoldest
microbialmatatthetime.Sheneededconfirmationthattheblackfeatureswerecarbon-richwithlife’srequisiteisotopicsignature,withabout3percentlessoftheheaviercarbon-13isotopecomparedtotheaveragecrust.ShehadalreadywrittenapaperforScience,readytosubmit,awaitingonlythatsingleconfirmatoryvalue.TherocksampleswererushedfromCambridge,Massachusetts,totheGeophysicalLaboratorybyFedExunderthehighestpriority.Iwasunderthegun.FortunatelymycolleagueMarilynFogel,whoisthecarbonisotopeexpertat
theCarnegieInstitution’sGeophysicalLab,waswillingtohelp.Marilynlookedatthesampleandtoldmewhattodo:crushtherockandgrindittoafinepowder,putafewmicrogramsofpowderintoeachofseveraltinycupsofpuretinfoil,weighthesamples,andfoldeachfoilcupintoatinyballthesizeofaBB.Thesesamplesandcarbonisotopestandardswerethenfed,onebyone,intoafurnacethatvaporizesanycarbon-containingcompoundstocarbondioxidegas.Thegasflowsintoasensitivemassspectrometerthatseparatesandmeasurescarbon-12andcarbon-13.Itonlytookafewhourstoobtainthetelltaleratio.Norawashopingforsomethinginthe−25to−35range,typicalofother
microbialmats.Butthemachinespatoutadifferentstory.Theisotopicratiowascloseto0,avaluethathadnothingtodowithbiology.Rather,itwascharacteristicofinorganiccarbon,thekindthatrisesfromthemantleinfluidsandisdepositedasthinveinletsofblackgraphite.Bottomline:theblackfeaturesinNoffke’ssampleswerecarbon-rich,buttheywereunambiguouslynotbiological.Withthatobjectlessoninmind,weproceededquicklytoanalyzethin,
blackenedfeaturesinmanyotherpromisingancientsedimentsthatNorahadaccumulatedfromhervariedfieldareas—fromSouthAfrica,fromAustralia,fromGreenland.Timeandagainwemeasuredcarbonisotopesinthe−30rangebefittingmicrobialmatsandfoundotherconvincingevidencethatmicrobeshadflourishedalongthesandyshorelinesofancientEarthmorethanthreebillionyearsago.Andunliketinyblacksmudgesortracesofbiomolecules,youcouldseeNoffke’sevidenceinthefield,atthescaleofanoutcrop.Herevidence,youcouldholdinyourhand.Butacorequestionremains:didthematmicrobesproduceoxygen,ordid
theyusesunlightforsimplerphotochemistry?Microbesevolvedavarietyofsun-harvestingstrategies,notallofwhichproduceoxygen.Sothedetailsofhowthosethree-billion-year-oldmat-formingorganismsmadetheirlivingswillremainahottopicforsometimetocome.
TheMineralogicalExplosion
ThegrandarcofthestoryofEarth’soxygenationiswidelyaccepted.Priorto2.5billionyearsago,Earth’satmospherewasessentiallylackinginO2.Theriseofphotosyntheticmicrobescauseddramaticcumulativechangesbetweenabout2.4and2.2billionyearsago,whenatmosphericoxygenrosetogreaterthan1percentoftoday’sconcentration.ThisirreversiblechangetransformedEarth’snear-surfaceenvironmentandpavedthewayforevenmoredramaticchanges.Asthepreviousaccountsdemonstrate,thedetailsofthistransitionhave
becomethefocalpointsofmanyscientists’careers.Inrecentyears,mylongtimecolleagueDimitriSverjenskyandIhavejumpedintothefraywithastrikingandsomewhatcounterintuitiveclaim:mostkindsofmineralsonEartharetheconsequenceoflife.Forcenturies,thetacitassumptionhasbeenthatthemineralkingdomoperatesindependentlyoflife.Ournew“mineralevolution”approach,bycontrast,stressesthecoevolutionofthegeosphereandbiosphere.Wesuggestthatfullytwo-thirdsoftheapproximatelyforty-fivehundredknownmineralspeciescouldnothaveformedpriortotheGreatOxidationEvent,andthatmostofEarth’srichmineraldiversityprobablycouldnotoccuronanonlivingworld.Inthisview,suchmineralfavoritesassemipreciousturquoise,deepblueazurite,andbrilliantgreenmalachiteareunambiguoussignsoflife.Thereasonsforthismineralogicaldependenceonthelivingworldaresimple.
Thesebeautifulminerals,alongwiththousandsofotherspecies,areformedintheshallowcrustbytheinteractionsofoxygen-richwatersandpreexistingminerals.Subsurfacewatersdissolve,transport,chemicallyalter,andotherwisemodifytheupperfewthousandfeetofrock.Intheprocess,newchemicalreactionsoccurforthefirsttime,producingnewsuitesofminerals.SverjenskyandIhavecatalogedlonglistsofthemineralsthataregeneratedthisway,derivingfromcopper,uranium,iron,manganese,nickel,mercury,molybdenum,andmanyotherelements.Priortotheriseofoxygen,suchmineral-formingreactionssimplycouldnothaveoccurred.“WhatoftheredplanetMars?”ourcolleaguesask.Isn’ttherustedsurfaceof
ourplanetaryneighborevidencethatMarshasbeenoxidizedandcouldpossessadeepmineraldiversitysimilartoEarth’s?No,weargue.ThecrucialdifferenceisthatMars,andpresumablyothersmallplanetslikeit,didn’texperiencethedynamiccirculationofoxygen-richsubsurfacewatersthatproducesEarth’sastoundingmineraldiversity.TheremaybestoresofgroundwateronMars,asrecentdatahavetantalizinglysuggested,butthatwaterisfrozen.TheonlyreasonMarsisredisthatithaslostmostofitsnear-surfacehydrogen(andthus
mostofitswater).Thelittlebitofoxygenproducedbyhydrogenlossmakesthesurfacered,likeathincoatingofpaintafractionofaninchthick,butthatoxygencan’tpenetrateverydeeplyintotheMartiancrust.OurnewframingofEarth’smineralogicalpastchallengedsomepriorviews.
Ina2007Sciencearticle,provocativelytitled“AWhiffofOxygenBeforetheGreatOxidationEvent?,”geochemistArielAnbarandhiscoworkersmeticulouslydocumentedtraceelementsthatoccurinasequenceof2.5-billion-year-oldblackshalesfromMountMcRaeinWesternAustralia.Thesefinelylayeredsediments,depositedintheoffshoreenvironmentofanancientocean,appearmonotonoustotheeyebutcontainchemicalsurpriseswhensubjectedtoclosescrutiny.Mostnotably,athirty-foot-thicksectionnearthetopoftheshaleissignificantlyenrichedinmolybdenumandrhenium—chemicalelementsthatdon’tgenerallyappearinsedimentaryrocksunlesstheyareoxidized.Intheirmoreoxidizedforms,molybdenumandrheniumdissolveeasilyfromigneoushostrock,flowingdownriversandintotheocean,wheretheycanbeincorporatedintoblackshaleontheoceanfloor.Everyoneagreesthattheseenrichmentsofmolybdenumandrheniumare
tellingussomethingabouterosion2.5billionyearsago.Molybdenite,thecommonestmineralofmolybdenum(andonethatoftenincorporatesrhenium,aswell),isexceptionallysoftandeasilyabraded.Perhapsmolybdenite-bearinggranitewasexposedonanancientmountainslope.Perhapsmechanicalweatheringproducedmicroscopicbitsofmolybdenitethatwerecarriedtotheseaandsettledtotheblackmuddybottom—sedimentsthatwouldbecomeburiedandsolidifiedtoformtheMountMcRaeShale.Anbarandhisteamreachedadifferentconclusion;theyproposedthata
“whiffofoxygen”fromearlyphotosyntheticcellswastheagentthatdidthetrick.Perhapsalocalconcentrationofslimygreencellscreatedamicroenvironmentwithenoughoxygentomobilizemolybdenumandrhenium.Afterall,wehaveunambiguousevidencefortheglobalriseofoxygen2.4billionyearsago,sowhynotlocally100millionyearsearlier?SverjenskyandIcounterthatthereareplentyofwaysbesidesoxygentomove
molybdenum,rhenium,andotherelements.Commonatmosphericmoleculescontainingsulfurornitrogenorcarboncouldhavedonetheelectron-acceptingtrickjustaswellintheabsenceofanyO2.Suchisthenatureofscientificdebateasnewideasandargumentsaremetwithalternateclaimsandcounterarguments.
Whatevertheexacttimingoftheriseofoxygen,byEarth’stwo-and-a-half-billionthbirthday,itssurfacehadchangedonceagain.Thefirstdramaticchanges
occurredonland,asEarthrusted.Oxygen-drivensurfaceweatheringbegantobreakdowniron-bearinggraniteandbasaltintobrick-redsoils.Asthelandaged,itsubtlyshiftedhuefrompredominantlygrayandblacktotheruddycolorofrust.Fromspace,Earth’scontinentstwobillionyearsago—thoughstillsignificantlysmallerthantoday’slandmasses—mighthaveappearedsomethinglikethemodernredplanetMars,butwithblueoceansandswirlingwhitecloudsthatprovideddramaticallycolorfulcontrasts.Rustwasonlythemostobviousofmanyprofoundmineralogicalchanges.Our
recentchemicalmodelingsuggeststhattheGreatOxidationEventpavedthewayforasmanyasthreethousandminerals,allofthemspeciespreviouslyunknowninourSolarSystem.Hundredsofnewchemicalcompoundsofuranium,nickel,copper,manganese,andmercuryaroseonlyafterlifelearneditsoxygen-producingtrick.Manyofthemostbeautifulcrystalspecimensinmuseums—blue-greencopperminerals,purplecobaltspecies,yellow-orangeuraniumores,andothers—speakpowerfullyofavibrantlivingworld.Thesenewlymintedmineralsareunlikelytoforminananoxicenvironment,solifeappearstoberesponsible,directlyorindirectly,formostofEarth’sforty-fivehundredknownmineralspecies.Remarkably,someofthesenewmineralsprovidedevolvinglifewithnewenvironmentalnichesandnewsourcesofchemicalenergy,solifehascontinuouslycoevolvedwiththerocksandminerals.Oxygen,amagicallytransformativeelement,playsthestarringroleinthis
drawn-outhistory.Hungryforelectrons,oxygenatomsreactvigorouslywithallmannerofminerals,thusweatheringawayrocksandformingnutrient-richsoilsintheprocess.Whenconcentrationsofatmosphericoxygenfirstrosetosignificantlevelsmorethantwobillionyearsago,allphotosyntheticlife-formslivedintheoceans.Thelandswereabsolutelybarrenoflife.Butoxygenpavedthewayforlife’seventualexpansionacrosstheglobe.Todayweexperienceoxygeninthemostintimateexchange.Withevery
breathwetake,atinyportionoftheairbecomesapartofus,evenasatinypartofusbecomestheair.Asdayspass,ourbodiesmeltawayandformagaininmoment-by-momentchemicalreactionswithoxygen.Ourtissuesarereplacedoverandoveragainthroughoutourlives,Earth’sfinitestoreofatomsrecyclingamongair,sea,land,andallitslivingforms.Mostoftheatomsthatformedyourinfantbodyatbirtharenowdispersed,asyourpresentatomswillbeagain,ifyouhavethegoodfortunetoliveafewmoreyearsonthisoxygen-richplanetaryhome.
Chapter8
The“Boring”Billion
TheMineralRevolutionEarth’sAge:2.7to3.7billionyears
AustraliangeologistRogerBuick,adynamic,wiryfirebrandoftheearly-Earthsciencecommunity,oncesummeduptheperiodsandwichedbetweenthePaleoproterozoicEra(punctuatedbytheGreatOxidationEvent)andtheNeoproterozoicEra(whichwouldseeglobe-spanningglaciersdominatethesurfaceandlifebegintoevolveininterestingways)withthesestarkwords:“ThedullesttimeinEarth’shistoryseemstohavebeentheMesoproterozoic.”Thatsupposedlyuneventfultime,thebillionyearsbetween1.85billionand
850millionyearsago,isthesubjectofthischapter.Thisvastinterval,dubbedtheintermediateocean(ormoresardonically,theboringbillion,bysomescientificwits),appearstohavebeenatimeofrelativebiologicalandgeologicalstasis.Noobviousdramatictransformativeeventstookplace.Atfirstblush,therockrecordrevealsnoepic,game-changingimpactsorsuddenclimateperturbations.Theinterfacebetweentheocean’smoreoxidizednear-surfacelayerandtheanoxicoceandepthsmayhavegraduallygottendeeperanddeeper,butnofundamentallynewlife-formsseemtohaveemerged;norisitgenerallythoughtthatmanynewrocktypesormineralspeciesarose.Atleastthat’stheconventionalwisdom.Butboringisariskyterm.Ioncemadethemistakeofcallinglipids,therich
andvariedclassoflife’smoleculesthatincludesfatsandoilsandwaxes,boring.Thisremark,madeduringapubliclectureandinignoranceofthenuancesoflipidchemistry,wasamistakeontwocounts.First,lipidsare,infact,amazinglydiverse.Theyplayallsortsofinterestingrolesinregulatinglife’schemicalreactionsandcraftingitsintricatenanoscalestructures.Lipidsdividetheinsidesfromtheoutsidesofmostlivingthings.Withoutthem,lifeasweknowitwouldnotbepossible.ThesecondreasonmyremarkwasamistakewasthatImadeitunwittinglyinthepresenceofanattentive,humorlesschemistwhohadspentherentireresearchcareerstudyinglipids.Sherightlytookmetotaskandsentmelotsofhighlytechnicalliteraturetosettherecordstraight.Mypenancewastoreadthesedetailed(andsomewhatboring)tomes.
Thepointisthatboringmayspeakmoretoourprofoundstateofignorancethantoanyintrinsicdullness.Earth’sboringbillionmay,infact,beanalogoustohumancivilization’sso-calledDarkAges—thatdynamicintervalofgreatinnovationandexperimentation,inexorableandirreversiblechange,thegatewaytothemodernlivingworld,yetoncelargelyignoredbyscholars.Ourself-inflictedignorancemaybeself-reinforcingaswell.Ambitiousstudents,whoseektoestablishtheiracademicreputationsinthebrieftemporalwindowofgraduateschoolandpostdoctoralfellowships,areunlikelytofocusonageologicalerawhennothingmuchisthoughttohavehappened.Butthegeologicalstrataofthatenigmaticagemustcontainsurprisesforthe
astutescholar.Hintsofdramatictransformationsmustliehiddeninrockswhosestoryislargelyunread.SomeofEarth’smostvaluableorereserves—vastdepositsofleadandzincandsilverfromZambiaandBotswanainAfrica,fromNevadaandBritishColumbiainNorthAmerica,andfromtheCzechRepublicandsouthernAustralia—seemtoclusterinrocksofthatage.Otherlocalitiesrichinexoticmineralsofberyllium,boron,anduraniumalsoappeartohaveflourishedataboutthattime.EmergingevidencesuggeststhatEarth’scontinentsmayhaveclumpedtogetherintoasinglegiganticsupercontinentduringtheboringbillion,thenbrokenapart,thenclumpedtogetheragainintheplanet’smostmajesticsurfacecycle.Andthroughoutthatbillion-yearinterval,abundantmicroorganisms—beautifullypreservedtodayasfossils—crowdedcoastalshallowsandoffshoreenvironments.SurelywehavemuchtolearnaboutEarth’sdarkages.
AHistoryofChange
DramaticchangehasbeentheoneconstantinoursagaofEarth’sevolutionuptonow,acouplehundredmillionyearsafteritstwo-and-a-half-billionthbirthday.Thesolarnebulacoalesced,andtheSunformed.Thedustarounditmeltedintochondrules.Chondrulesclumpedintoplanetesimals,andplanetesimalsintotheproto-Earthandotherterrestrialbodiesthousandsofmilesindiameter.TheimpactofTheia,thesubsequentformationoftheMoon,theincandescentmagmaoceanhardeningtoablackenedbasalticcrustpockmarkedbythousandsofexplosivevolcanoes,thehotseathatsooncoveredalmostallthesolidsurfacesothatonlythetopsofthetallestvolcanicconesweredry—allthesedramaticeventstranspiredwithinahalf-billionyears.EveninthelesstumultuoustwobillionyearsthatfollowedtheaccrualofEarth’suniqueocean,ourplanet’ssurfacewasconstantlyinflux,asgraniteemergedfrombasaltmeltsandproto-
continentsgrewontheconvectioncellsthatdroveplatetectonics.Itwasonsuchadynamic,variableworldthatlifeemerged,evolved,and
eventuallylearnedtomakeoxygen.ConstantchangewasEarth’shallmark.Likeaprecociousartist,ourplanethadreinventeditselfoverandoveragain,ateverystagetryingsomethingnew.How,then,couldourdynamicplanethavefounditselfmiredinanaeonof
stasis?ThesimpleansweristhatEarthwasn’tstaticeventhen.Changewas
incessant,thoughperhapsnotasdramaticasaMoon-formingimpactortheGreatOxidationEvent.Theboringbillionsawtheinventionofdistinctiveprocessesthatformednewtypesofrocksandvaluableoredeposits,aswellasthefirstappearanceofmanynewmineralspecies.Andmostcritically,geologicalevidencefromaroundtheworldisrevealingthatitwasatimeofcoordinatedglobalplatemovementthatwouldestablishnewpatternsstillinplacetoday.
TheSupercontinentCycle
Earth’sfamiliargeographyofoceansandcontinentsisephemeral,geologicallyspeaking.TheAmericas,Europe,andAfricaframingthemightyAtlanticOcean;thegreateastwardsweepoftheAsiancontinent;theexpansivePacificOceanwithitsplethoraofsouthernislandsandthecontinentofAustralia;andthepolarworldofAntarcticaarebutamomentaryconfiguration.Thestatelyprocessofplatetectonicsnotonlyformsthecontinentsbutalsoceaselesslyshuttlesthemacrosstheglobe.Landandwaterhavebeensubjecttoextrememakeoverstimeandtimeagain.Anelitebandofgeoscientistshaslearnedtoteaseoutourworld’sancient,
aliencartographyandhasproducedremarkable,ifapproximate,mapsoftheonceandfutureEarth.Theyhavemanycluestoworkwith.First,weknowhowthecontinentsaremovingtoday—howfastandinwhatdirections.YearbyyeartheAtlanticopenswider,Africasplitsintwo,andaswewatchamazed,IndiasmashesintoChina,crumplingtheimpactzoneintothejaggedHimalayanMountains.Itallhappensinslowmotion,ofcourse,butsteadily,aninchortwoayear;overthespanofahundredmillionyears,evenasnail’spacecanproducemonumentalchanges.WeplaytheimaginaryvideotapeofEarth’sgeographybackwardandforward,andcanguessthefeaturesofourplanet’scapriciousface.Evenasfarbackasahalf-billionyearsago,therichfossilrecordofanimalsandplantscanhelpscientistssketchapicture,especiallywhenthefloraandfaunaof
widelyseparatedcontinentsfollowdivergentevolutionarypathways.ThevariedmarsupialsofAustralia,forexample,andthelargeflightlessbirdsofNewZealandtellacompellingstoryofzoologicalisolation.Pushingbackmorethanfivehundredmillionyears,thepicturebeginstofade;
wemustseekotherkindsofclues.Ofspecialimportisthefossilmagnetismlockedintovolcanicrocks.Wetendtothinkofourplanet’smagneticfieldintermsofanorth-southorientation,familiarfromthealignmentofacompassneedle,butitismorecomplexthanthat.MagneticfieldlinesintersectEarth’ssurfaceatanangle,calledthedip.AtEarth’sEquator,thedipisclosetozero—nearlyhorizontal—butathigherlatitudes,thedipbecomessteeperandsteeperuntilatthepolesitisalmostvertical.Exactingmeasurementsoftheancientmagneticfieldfrozenintoavolcanicrockthuscanrevealboththenorth-southorientationandthelatitudeofthecontinentwhenthoserockssolidified.Remarkably,suchsubtleevidenceshowsthatsomerocksnowattheEquatorwereonceclosetoEarth’spolesandviceversa.FossilevidenceofformertropicallagoonsinAntarcticaandfrozentundrainequatorialAfricareinforcessuchfindings.Thesedimentaryrockrecordaddsvitaldata.Differentkindsofsedimentsaccumulateindifferentenvironments:shallowseas,continentalshelves,tundra,glaciallakes,tidallagoons,andswampseachhostadistinctrocktype.Bolsteredwiththeseclues,expertsinpaleogeographyhavemanagedtocrafta
coherentanddefensiblepictureofEarthbackatleast1.6billionyears,wellintotheboringbillion,withinformedspeculationsextendingtoevendeepertime,totheformationofthefirstcontinents.Ittookalongtimeforplatetectonicstomaketheoriginalcontinents.Atthetippingpointofsubduction,attheveryfaultlinewheredenseslabsofEarth’searliestbasalticcrustplungeddownintothemantledepths,unsinkablebitsoflow-densitygraniteislandspileduponeafterthenexttomakelargerandlargerstable,long-lastinglandmasses.Theseancientchunksofwhatarenowthecontinentsgobythenamecraton,atermderivedfromtheGreekwordfor“strength.”Cratonsarestrong;onceformed,theylastalongtime.Earthtodaypreserves
perhapsthreedozenmoreorlessintactcratons,someasoldas3.8billionyearsandranginginsizefromahundredtomorethanathousandmilesacross.Thesediversepieces,eachevocativelynamed—SlaveandSuperiorinNorthAmerica,KaapvaalandZimbabweinAfrica,PilbaraandYilgarninAustralia—haveexperiencedbillionsofyearsofmigrationacrosstheglobe.Jumbledtogetherandrippedapart,alongwithlotsofsmallerancientfragments,theysurviveasthecontinents’foundationstones.ThreesuchcratonsformmostofGreenland,
whilemuchofcentralCanadaandthenorthernpartsofMichiganandMinnesotacompriseaclusterofahalf-dozenothers.LargeportionsofBrazilandArgentinaareunderlainbyseveralcratons,asarebigchunksofnorthern,western,andsouthernAustralia,Siberia,Scandinavia,alargepieceofAntarctica,separateregionsofeasternandsouthernChina,mostofIndia,andseveralswathsofwestern,southern,andcentralAfrica.Allofthesecratonsbegantoformmorethanthreebillionyearsago—atimebeforemodern-styleplatetectonics,whenonlyatinyportionofEarth’ssurfacewasdryland.Consequently,allcratonscarryaprecious,ifsomewhatwarpedandscrambled,recordofEarth’svibrantadolescence.Cratonsarethekeys—theRosettastonesofearly-Earthhistory.Theoceans
can’thelpusdecipherearlyEarth.Thankstotheincessantconveyorbeltofplatetectonics,whichproducesnewbasalticcrustattheoceanridgesandswallowsitupagainatconvergentboundaries,theoldestoceancrustisnotmuchmorethantwohundredmillionyearsold.Anythingolderthanthatmustbepreservedonthecontinentsornotatall.Theperipateticcratonshaveanastonishinglycomplexhistory.Propelledby
themotionsoftectonicplates,theyhavebeenshuttledabout,collidingwithoneanothertoformcompositecratonsandsupercratons,whichinturnoccasionallyclumpedintosinglegiantlandmasses—continentsorsupercontinents.Eachcollisionproducedanewmountainrangealongthesuturezone;eachrangeprovidescompellingevidencefortheancientassemblyoflargerlandmasses.Thesupercontinents,inturn,riftedapartandfragmentedintoseparate,ocean-
boundedislandcontinents.Eachtimeacontinentrifted,awideningoceanformedbetweenthedivergingpiecesandatelltalesuiteofsedimentswaslaiddown:shallow-watersandstoneandlimestoneatfirst,thendeeperwatermudandblackshale.Suchsedimentarysequencespointtoepisodesofcontinentalfragmentation.Overandover,supercontinentshavebeenforgedandthenrippedasunder.It’sanimmensejigsawpuzzlewithanunknownpicture,wherepiecesconstantlychangetheirpositionsandtheirshapes.
Whatdoesallthishavetodowiththeboringbillion?Everything.Foraperioddevoidofflashysignsofactivity—athwack-free,treelesstimebeforeelaboratefloraandfaunaarrivedinthegeologicalrecord—wemustturntopaleogeographerstocomprehendwhatitlookedlike.Todecipherthedetailsofthecratons’multibillion-yeardanceacrosstheglobe,thesegeologiststrektothemostremoteplacesonEarth,maptherocks,collectsamples,andsubjectthemtoabatteryoflaboratorytests.
Atthecoreofeverycratonarereallyoldrocks,typicallythreebillionyearsoldormore.ThesefragmentaryparcelsofEarth’smostancientcrustrepresent,intotal,onlyasmallproportionoftheplanet’scontinentalmass.Theyhaveinvariablybeenbakedbyheatandpressure,alteredbythedissolvingpowersofsubsurfacewaters,andcontortedbycrustalstresses.Evenso,thenatureoftheoriginalrocks,whethergranitelikeintrusionsorsedimentarylayerings,canoftenbededuced.Moreover,it’sfortunatethatcratonsaren’tstatic.Throughouttheirhistories,newpulsesofmagmapenetratetheold,formingigneousrockbodiesinveinsandpods.Newsedimentarydepositsforminlandinlakesandrivers,aswellasalongshallow,sandycoastlines.Distinctiverocktypesandcharacteristicstructuresalsoformwhenevercratonssmacktogetherorripapart—eventsthatsuggesttherelativemotionsoftwolandmasses.Carefulstudiesofthesevariedyoungerformationscandiscernasuiteofrocktypesthatspanstheentirehistoryofacraton.Thenthefunbegins.Theyoungerrocksprovidehintsaboutthechronologyofcratonmovements.
Igneousrocksholdtinymagneticmineralsthat,whentheysolidify,lockintheorientationofEarth’smagneticfield.Carefulpaleomagneticstudiescanidentifynotonlytheorientationoftheformernorthandsouthpolesbutalsotherocks’approximatelatitudewhentheycooled.Thosedata,whilenotexactlyGPScoordinates,dorecordtherelativepositionsofcratonsthroughtime.Sedimentaryrockscomplementthesedata,fortheycanhosttelltalecluesaboutclimateandecology.Sedimentsdepositedinrapidlyweatheringtropicalzonesdiffermarkedlyfromthoseintemperatelakesortheglacialdepositsofhigherlatitudes.Somesedimentaryrocksalsoincorporatetinygrainsofmagneticmineralsthatholdcluestopolarpositions.TogarnerevenavaguesenseofEarth’schangingface,everyoneofthethree
dozencratonsisbeingscrutinizedbyarmiesofgeologists.Decadesofmeticulousfieldworkandlabstudiesarebeingundertaken.Datafromeverypartoftheplanetarebeingintegrated.Thenallthecratonswillhavetobejuxtaposedlikebumpercarsonaglobe—andtheimaginarymovieoftheirwanderings,startingfromtheknowngeographyofthemodernworld,willbeplayedslowlybackward.Inevitably,thatmoviebecomesfuzzierandmorespeculativethefurtherbackwego.Nevertheless,thepicturethatisslowlyemergingisextraordinary.Accordingtothelatestinterpretations,Earthhasexperiencedarepeatedcycleofatleastfivesupercontinentassembliesandbreakups,extendingbackperhapsthreebillionyears.ThestoryofEarth’searliestlandmassesisstillemerging,andmorethanafew
controversiesswirlaroundthetopic.Noonehashadthenervetodrawmore
thanasketchmapofEarth’ssurface3billionyearsago,atleastnotyet,butonewell-vettedideahasnamedthefirstcontinent-sizelandmassUr,formedabout3.1billionyearsagofromearlierscatteredcratonicbitsofwhatarenowSouthAfrica,Australia,India,andMadagascar.(Anevenearlierpostulatedlargelandmass,Vaalbara,mayhaveexistedabout3.3billionyearsago,butevidenceisslim.)AccordingtocomparisonsofpaleomagneticdatafromalltheseUr-formingregions,whatarenowseparatecratonsweresuturedtogetherformostofEarthhistory—theirglobalperambulationsappeartohavebeenvirtuallyparallelandthusprobablylinked.Indeed,magneticdatasuggestthatthecontinentofUrpersistedforalmost3billionyearsandbegantosplitapartonlyabout200millionyearsago.Theearliesttruesupercontinent,dubbedKenorlandorSuperia(after
associatedrocklocalitiesinNorthAmerica),isthoughttohaveformedabout2.7billionyearsagofromUrandlotsofothersmallerpieces.Eachtimeonecratoncollidedwithanother,asuturezonewasformed,whileepiccompressionalforcespushedupanewmountainrange.Aplethoraofsuchfeaturescanbedeterminedfromrocks2.7to2.5billionyearsold,suggestingasequentialgrowthofthesupercontinent.PaleomagneticdatarevealthatKenorlandwasatlowlatitude,probablystraddlingtheEquator,formostofitsrelativelybriefexistence.WiththoseearlytractsoflandcameEarth’sfirstepisodesoflarge-scale
erosionandthefirstgreatpulsesofsedimentsintotheshallowoceanmargins.Mostearly-Earthmodelerspositanancientatmospherequitedifferentfromthatoftoday.Oxygenwastotallyabsent,whilecarbondioxidelevelsmayhavebeenhundredsorthousandsoftimesgreaterthaninourtime.Rainwouldhavefallenasdropsofcarbonicacidthatateawayatthelandandtransformedhardrocktosoftclays.Riverscarriedtheirmuddycargointotheshallowcoastalslopesoftheencirclingoceans,wherethickdeltalikewedgesofsoftsedimentsaccumulated.Byabout2.4billionyearsago,aboutthesametimethatoxygenbeganto
accumulateintheatmosphere,Kenorlandexperiencedtheflipsideofsupercontinentformation.GeomagneticdatarevealthedivergenceofUrfromothercratonsasKenorlandbeganitsprotractedfragmentation.ThosecratonicpuzzlepiecesscatteredfromtheEquatortothepoles.Newlyopenedshallowseasbetweenthedivergingpiecesledtothickdepositsofshallowmarinesediments.Thesupercontinentcyclehadbegun.
HailColumbia
Theadditionofthesupercontinentcycletotheannalsofgeologyhasmadetheboringbillionalotlessboring.Thenextsupercontinentepisode,onemoresharplyinfocusthanKenorland,thankstoyoungerandmuchbetterpreservedsuitesofrocks,beganabouttwobillionyearsago,atatimewhenEarthboastedatleastfiveseparatecontinent-sizelandmasses.LargestamongthesewastheLaurentiansupercraton,aconglomerationofatleastahalf-dozencratonsthousandsofmilesacrossthatencompassesmuchofwhatisnowcentralandeasternNorthAmerica.(SpecialistsinancientlandmassessometimesrefertothisclusteringofcratonsastheUnitedPlatesofAmerica.)TheoriginalcontinentUrsoldieredonasthesecond-largestlandmass,separatedfromLaurentiabyasubstantialocean.ThemuchsmallerBalticaandUkrainiancratons,whichformthecoreofwhatisnoweasternEurope,andcratonsrepresentingpartsofwhatarenowSouthAmerica,China,andAfricawerealsolargeislandsapproachingcontinentsize.BythetimeEarthwas1.9billionyearsold,thesevariedlandshadcollidedatconvergentplateboundaries,raisingnewmountainbeltsandformingasupercontinentvariouslynamedColumbia,Nena,Nuna,orHudsonland.(ThenameColumbia,basedonpersuasivegeologicalevidencefromthevicinityoftheColumbiaRiver,alongtheWashington-Oregonborder,seemstobeusedmostoften.)Thisvastbarrenland,roughlyestimatedtohavebeeneightthousandmileslongfromnorthtosouthandthreethousandmileswide,incorporatedalmostallofEarth’scontinentalcrust.Thecomplexitiesofretroactivelyarrangingthirty-pluscratonicfragmentsinto
oneextinctsupercontinentaredaunting.Notsurprisingly,morethanonemodelusuallyviesforacceptance.InthecaseofColumbia,tworatherdifferentstoriesemergedalmostsimultaneouslyin2002.Ontheonehand,geochemistJohnRogersoftheUniversityofNorthCarolinaandhiscolleague,IndiangeologistSantoshMadhavaWarrier(basedatKochiUniversityinJapan),proposedthatLaurentia,whatisnowmostofNorthAmerica,formedthecoreofColumbia.AccordingtoRogersandSantosh,thecontinentofUrwassuturedtothewestcoastofLaurentia;portionsofSiberia,Greenland,andBalticawerepositionedtothenorth;andpartsofwhatarenowBrazilandWestAfricalaytothesoutheast.Inthesameyear,GuochunZhaooftheUniversityofHongKongandseveralcolleaguesdevisedasomewhatdifferentconfiguration,inwhichBalticaissuturedtotheeastcoastofLaurentia,whileeasternAntarcticaandChinaareattachedonthewest.GiventhegreatageofColumbiaandthepreliminarynatureofthesereconstructions,theagreementbetweenthetwoscienceteamsisremarkablygood.Nevertheless,wecananticipatemanydebates,aspostulatedcratonlocationsareshuffledandtweakedfordecadestocome.
Inanycase,theassemblyofColumbia,commencing1.9billionyearsago,setthestagefortheboringbillion.WhatevertheconfigurationaldetailsoftheColumbiansupercontinentactuallywere,wecanbefairlyconfidentthatmuchofitsinteriorwashot,desiccatedterrain,withabsolutelynovegetationandgreatexpansesofrustydesert.Seenfromspace,Earthwouldhaveappearedasastrangelylopsidedworld,withitsonegreatenreddenedlandmasssurroundedbyanevenmoreexpansive(andasyetunnamed)bluesuperocean.WithallthecontinentsconcentratedtogetherneartheEquator,onlymodestamountsoficewouldhavedecoratedthepoles.Oceanlevelswouldhavebeencorrespondinglyhigh,perhapshighenoughtoinvadesomecoastalregionswithshallowinlandseas.TheequatorialColumbiansupercontinentisthesupposedstartingpointforthe
dullesttimeinEarthhistory,butwhatmakesitsodull?Whatdoesstasisreallymean—whatparameterswerestable?Wasitglobalclimateandrainfall?Wasitthenatureanddistributionoflife?Wasitthecompositionoftheoceanoratmosphere?Whatmeasurementshavebeenmadetoestablishthisallegedstasis?Conversely,whatuncertaintiesremainunaddressed?
Stasis
Mostgeologygraduatestudentssimplyignorerockformationsbornbetween1.85billionand850millionyearsago.Fouryears,whileworkingtowardaPh.D.,tryingtomakeasplashandlandatenure-trackjob,istooshortatimetospendonageologicalerawithsuchaquestionablerap.ButLindaKahwasnotlikemostgraduatestudents.HerundergraduatementoratMITwasJohnGrotzinger,aleaderinthestudyofEarth’soldestrocksfrombefore2billionyearsago.HerPh.D.adviseratHarvardwasAndyKnoll—therenownedpaleontologistwhoencouragedNoraNoffkeinhermicrobialmatresearch.Kahcouldn’thelpbutnoticethatEartholderthan1.8billionyears(describedbyGrotzinger)wasstrikinglydifferentfromEarthyoungerthan0.8billionyears(describedbyKnoll).Somethinginterestingmusthavehappenedduringtheboringbillion,andKahwasdeterminedtofindoutwhat.AndsoshehasdevotedherselftounderstandingtheMesoproterozoicEra—theimmensespanofEarthhistoryfrom1.6to1.0billionyearsago—atimeencompassingmostoftheboringbillion.EveniftheMesoproterozoicwasinfactatimeofstasis,abillionyearsof
equilibriumwouldberemarkable.ChangeisthecentralthemeofEarth’sstory.Theoceansandatmosphere,thesurfaceanddeepinterior,thegeosphereand
biosphere—allaspectsofourplanethavechangedincessantlyovertheaeons.HowcouldEarthhaveexperiencedabillionyearswithoutanydramaticevents,withnosignificanttransitionsinthenear-surfaceenvironments,withnogreatnoveltyinthelivingornonlivingworld?Wastherereallyabillion-yearperiodwhenallthefeedbacksofclimateandlifewereinsuchperfectharmoniousbalance?Howcouldsuchathinghavehappened?AtaleisurelybreakfastmeetingneartheUniversityofTennesseecampus,
LindaKahpatientlyexplainsEarth’sdramaticandrepeatedtransformations(and,consequently,whytheMesoproterozoicwasn’tboringatall).Shecomespreparedwithastackofblanksheetsof8½-by-11-inchwhitepaper;whileshetalks,shedrawsneatillustrativediagramsincontrastingblueandredink.“Ihadtheseideasadecadeago,”shesays,describingthefruitsofhergrueling
fieldworkinthehostileMesoproterozoicterrainsoftheMauritaniandesertinnorthwesternAfrica.She’dlovetogobacknow,butariseinbanditryandkidnappingmakessuchafieldexcursionproblematic,foolhardy.InsteadshewillbeamemberofthenextMarsroverscienceteam,asaferchoice.Kah’sscientificstoryisdevotedtoplatetectonicsandthemesstheymakeof
Earth’spast—theconstantshuffling,colliding,splitting,andsuturingoflandmassesthatcauseourglobetolookradicallydifferenteveryhundredmillionyears.Evenduringthethree-hundred-million-yearintervalthatintroducedtheboringbillion—whenthesupercontinentColumbiapersistedmoreorlessintact—platetectonicsdidn’tstop.Anotablefeatureofsupercontinentsisthattheycontinuetogrowgraduallyattheedges,asoceanplatesplungeundertheirmarginsandnewvolcanoesrisenearthecoasts.Themodern-dayexpansionofthePacificNorthwestcoast,wheresuchmajesticvolcanoesasMountRainier,MountHood,andMountOlympiaarestillactive,isbutonerecentexampleofthisage-oldphenomenon.SoitwaswiththeexpandingmarginsofColumbia.EvenmorecontinentalcrustwasaddedtoEarth’sinventorywhenColumbia
experiencedepisodicriftingandbreakupintosmallercontinentsandislands.About1.6billionyearsago—thebeginningoftheMesoproterozoic—thesplittingoffofthecontinentUrtothewestfromLaurentiaandtherestofColumbiatotheeastledtoamajorintercratonicseaandthedepositionofamassivesedimentarysequencereachingthicknessesofmorethantenmiles.Thisheroicdeposit,calledtheBelt-Purcellsupergroup,todayformsprominentoutcropsovermuchofwesternCanadaandthenorthwesternUnitedStates.Thus,evenassupercontinentssplitapartanderoded,newcontinentalrockswerebeingproducedfromtheold.ThisriftingofColumbiaintotwodiverginglandmasseshadother
consequences.Laurentia,Ur,andtheothercontinentswerestillallcenteredmoreorlessabouttheEquator,whichmeansthattherewerestillnocontinentsatthepoles,whichmeansthattherewasstillnothickicebuildupatthepoles,whichmeansthatoceanlevelswerestillrelativelyhigh.Indeed,shallowseasembayedvastswathsofLaurentia’snewwestcoast;likelylessthanaquarterofEarth’ssurfacewasdry.Foratimeperhapsexceedingtwohundredmillionyears,Earth’stotallandareawasdramaticallyreduced,whilethicksedimentarydepositswereaccumulatinginshallowwatersaroundtheglobe—depositsnowpreservedasatelltalesedimentaryrecord.Noicealsomeansnoglaciers.Theintervalfrom1.6to1.4billionyearsagoholdsnoneofthecharacteristicglacialremains—pilesofice-roundedcobblesandboulders,sandandgravel—thatarefoundinmostothergeologicalages.SothedullMesoproterozoicsawagreatdealofchange,evenifthosechangesweregeological“businessasusual.”
SupercontinentReprise:TheAssemblyofRodinia
Theboringbillionsawtheformationofnotonebuttwosupercontinents.ThescatteredfragmentsofColumbiadriftedapartforperhaps200millionyears,butacontinentcandivergeonaglobeonlyforsolongbeforeitstartsconvergingonceagain.About1.2billionyearsagoUr,Laurentia,andotherMesoproterozoiccontinentsbegantoreassembleintoanewlandmasscalledRodinia(aftertheRussianwordfor“motherland”or“birthplace”).Therockrecordoffar-flunglocalitiesinEurope,Asia,andNorthAmericapreservesanassociatedworldwidepulseofmountain-buildingeventsbetween1.2and1.0billionyearsago;eachnewmountainrangeelevatedasconvergingcratonscollidedandcrumpled.TheexactgeographyofRodiniaisstillamatterofdebate,butgeologicaland
paleomagneticdata,coupledwiththearrangementofcratonsontoday’sglobe,placesignificantconstraints.MostmodelssituatetheentiresupercontinentneartheEquator,withLaurentia—whatisnowmostofNorthAmerica—atthecenter,andlargepiecesofalltheothercontinentsstuckontothenorth,south,east,andwest.Accordingtoseveralreconstructions,BalticaandchunksofwhatarenowBrazilandWestAfricalaytothesoutheast,withotherpiecesofSouthAmericatothesouthandfragmentsofAfricatothesouthwest,thoughdetailsoftherelativepositionsofAustralia,Antarctica,Siberia,andChinaareasyetunsettled.ThedistinguishingcharacteristicofRodiniaisitsabsenceofcertainkindsof
rocks.Unlikeanyotherintervalofthepast3billionyears,fewifany
sedimentarydepositshavebeenpreservedfromtheperiodbetweenabout1.1billionand850millionyearsago.Thishiatusmeanstherewereprobablynoshallowseasbetweencontinentsofthetypethathostedthe1.6-billion-year-oldBelt-Purcellsupergroup.Theconclusion:allthecontinentsmusthavefitneatlytogether.Nordoesitappearthattherewereanylargeinlandseas,ofthekindthatoncefloodedallofcentralNorthAmericaandlaidthesedimentaryfoundationsfortheGreatPlainssome100millionyearsago.Bythismodel,equatorialRodiniahadahot,dry,desertlikeinterior,muchlikeAustraliatoday.Foralmost250millionyears,thesedimentaryrockcycleseemstohaveallbutshutdown.LindaKahmakesherpointsmethodically,butit’sclearthatsheispassionate
aboutherchosengeologicalinterval.Inspiteofthemeagerrockrecordnearitsend,thegreattimeintervalfrom1.85billionto850millionyearsagowitnessedmanynotablechangesasaconsequenceofthecratonicdance.Twosupercontinentswereassembledduringtheboringbillion,eachproducingadozenmountainrangesbycratoniccollisions.Inbetweenthosetwogatheringsofthelands,astheColumbiansupercontinentriftedapart,someofEarth’smostimpressivesedimentarysequenceswerelaiddown.MuchofEarth’slandwassubmergedandthenbecamedryagain.Theratesofsedimentationvariedbyordersofmagnitude.Icecapsdisappearedandreappeared.That’salotofchangefora“boring”aeon.Butthere’sanothersidetothestory.
TheIntermediateOcean
Whatevertheexactgeometryoftheglobe,everyoneagreesthattheRodiniasupercontinentmusthavebeensurroundedbyanevenlargersuperocean,abodythathasbeennamedMirovia(aftertheRussianwordfor“global”).GeochemistswhostudyEarth’spasthavereachedtheconclusionthatiftheMesoproterozoicErawasboring,thenMiroviaistheprincipalreasonwhy.TheGreatOxidationEvent,whichsetsthedynamicperiodfrom2.4to1.8
billionyearsagoapartfromallothersinEarthhistory,wasprimarilyatimeofchangesinatmosphericchemistry.Earth’satmospheretransformedfromhavingessentiallynooxygentohavingapercentortwo.That’samonumentalchangeasfarasthenear-surfaceenvironmentgoes,buttoEarth’soceans,suchachangewasinsignificant.Thekeyliesinrelativemasses.Theoceanscontainmorethan250timesthe
massoftheatmosphere.Anysmallchangeinatmosphericchemistry,evena1percentincreaseinoxygen,takesaverylongtimetobereflectedintheoceans—perhapsaboutabillionyears.
Geochemistswhowanttounderstandthehistoryoftheoceansscrutinizeahostofchemicalelementsandtheirisotopes.Priorto2.4billionyearsagotheoceanswererichindissolvediron,astatethatcouldbemaintainedonlyifthewatercolumnwasutterlydevoidofoxidants(whichwouldhavecausedironoxidestoprecipitate)andalsolowinsulfur(whichwouldquicklyleadtotheformationofpyriteandotherironsulfideminerals).WiththeatmosphericchangesoftheGreatOxidationEvent,someofthatironwasremovedasironoxidesinshallowwater,eitherdirectlybyoxygenorindirectlybyreactionwithoxidizedweatheringproductsfromtheland.Oxygenintheatmospherealsoledtotherapidweatheringanderosionofsulfur-bearingminerals,whichflowedintotheoceansandconsumedmoreiron.Thesechemicalchangestriggeredamassivedepositionofbandedironformations,orBIFs—thethickocean-floorsedimentswithlayeruponcolorfullayerofironmineralsthatnowconstitutemostoftheworld’sironorereserves.TheBIF-makingprocesswasgradual,andtheoceansheldalotofiron,soBIFdepositioncontinuedforanothersixhundredmillionyears.Bythetimeoftheboringbillion,theoceanswerestillanoxic,buttheyhadlostmostoftheirdissolvediron.Fast-forwardabillionyears:photosyntheticalgaecontinuedtoproduce
oxygen,whichbegantotakeovertheoceans;bysixhundredmillionyearsago,mostofEarth’soceanswereoxygenrich,toptobottom.Whathappenedinbetween,thecruxoftheboringbillion,isknownastheintermediateocean.In1998geologistDonaldCanfieldoftheUniversityofSouthernDenmark
proposedthatsulfur,notoxygen,playedthemajorroleinEarth’sintermediateocean.(Manyscientistsnowrefertoasulfur-dominatedMesoproterozoicoceanastheCanfieldOcean.)Hisprovocativethesis,entitled“ANewModelforProterozoicOceanChemistry,”appearedintheDecember3issueofNature(afteralmostayear’sdelaybyinitiallyreluctantreviewers)andsoontransformedthewaymanyofusthinkabouttheoceanthroughdeeptime.Thecoreideaissimple.TheGreatOxidationEventproducedenoughoxygen
toinfluencethedistributionofmany“redoxsensitive”elements,includingiron,butnotnearlyenoughtooxygenatetheoceans.Ontheotherhand,enhancedweatheringandoxidationofthelandintroducedlotsofsulfateintotheoceans.Hencetheintermediateoceanbecameenrichedinsulfur,whileitwaspoorinoxygenandiron,asteadystatethatpersistedforabillionyears.
HangingOn
Thefossilrecordreinforcestheviewofaneversoslowlychangingintermediate
ocean.Somerocksdepositedbetween2and1billionyearsagopreservemicroscopicfossilsofunprecedentedquality.The1.9-billion-year-oldGunflintchertofNorthAmerica,the1.4-to1.5-billion-year-oldGaoyuzhuangformationofnorthernChina,andthe1.2-billion-year-oldAvzyanformationfromtheUralMountainsinRussiacontainminutefossilmicrobessosharpandclear,someintheintimateactofdividing,thattheylookjustliketheirmodernlivingcounterparts.Yetsuchstrikingimprovementinthequalityofsomefossilsonlyreflectstheirlessalteredhistory,notanythingintrinsicallynovelaboutthattimeinEarthhistory.Theprotractedanoxic,sulfidicintermediateoceanwasbothgoodnewsand
badnewsforlife.Ontheplusside,theinfluxofsulfateprovidedanexcellentenergysourceforsomemicrobes—theymadetheirlivingbyreducingthesulfatetosulfide.Hintsfromthefossilrecord,includingdistinctivemolecularbiomarkers,sulfurisotopedata,andevensomewell-preservedmicrobesinchert,allpointtoathrivingMesoproterozoiccoastalpopulationofgreenandpurplesulfurbacteria.Thesesulfur-eatingmicrobes,whichpersisttodayinsomeanoxicenvironments,produceorganicsulfurcompoundsthatsmelljustawful—likeatragicallyfailedsepticsystem.LindaKahjokes,“TheMesoproterozoicwasthesmelliesttimeonEarth,”ina
riffonRogerBuick’s“dullest”line.“Whenwasitsmelly?”Iask.“Ithinkitwassmellythewholetime,”shereplies.Thebadnewsforlifewasitsrelianceonnitrogen.Nitrogengas(N2)is
abundant,constituting80percentoftoday’satmosphere.Theproblemisthatlife’sbiochemistrycan’tusenitrogengas;instead,liferequiresnitrogeninitsreducedform,calledammonia(NH3).Consequently,lifehasevolvedacleverprotein,anenzymecallednitrogenase,thatconvertsnitrogentoammonia.Butthere’sacatch.Thenitrogenaseenzymereliesonaclusterofatomscontainingsulfurplusametal,eitherironormolybdenum,butneithermetalwaspresentintheintermediateocean.TheironhadbeenremovedduringtheformationofBIFs,sothatwasn’tanoption.Molybdenum,ontheotherhand,issolubleonlyinoxygen-richwater,asintoday’soceans.Duringtheanoxictimeoftheintermediateocean,molybdenumwastobefoundonlynearweatheringcoastlinesinrelativelyshallowwater—environmentswherethosesulfurbacteriaaresuspectedtohavethrived.SoitwasthatCanfield’sseminalpaperwasfollowedbyastreamof
publicationslinkingMesoproterozoicgeochemistryandpaleontology—twodisciplineswhosepractitionersseldomspoketooneanothertwentyyearsago.
Theconclusion:theintermediateoceanharboredmicrobiallife,butsuchlifecouldthriveonlynearthecoasts.Sulfur-reducingbacteriacoexistedwithoxygen-producingalgae.Forabillionyears,lifehungon,buttherewerefewbiologicalinnovations.
TheMineralExplosion
Entermineralogy,anotherfieldthathaslongbeentaughtinamannerstrangelydivorcedfromthegrandstoryofEarth—asseparatefromgeochemistryandpaleontologyasthosefieldshadbeenfromeachother.It’saninexplicablebias,foreverythingweknowofEarth’sdistantpastcomesfromevidencelockedinminerals.Yetthemajorityofmineralogistsrarelytalkabouttheagesorevolutionoftheirsamples.Rather,formorethantwocenturies,mineralogicalresearchhasfocusedontheirstaticphysicalandchemicalproperties.Investigationsofhardnessandcolor,chemicalelementsandisotopes,crystalstructureandexternalformhavedominatedtheliteratureofmylivelihood.I,too,wasoncefirmlycommittedtothistwo-hundred-yeartradition.Forthe
firsttwodecadesofmyresearchcareer,Iisolatedperfecttinycrystalsofcommonrock-formingminerals,squeezedthemtounimaginablepressuresbetweentwobrilliant-cutdiamondanvils,zappedthecompressedsampleswithX-rays,andmeasuredsubtlechangesintheiratomicarrangements.MycolleaguesandIignoredgeologicaltimeandgeographicalplace,aswecaredlittleabouttheageorlocationofourmicroscopicsamples.Wecalledourselvesmineralphysicistsandalliedourselveswiththenonhistoricalsciencesofchemistryandphysics.Hadweboughtintothesubtleprejudiceagainstmeregeological“storytelling”?Thismind-setreflectsmineralogy’soriginsinminingandchemistry,colored
perhapsbyasubliminalbeliefthatthefieldsofphysicsandchemistryaremorerigorousthanthecreative,qualitativeyarnsofthegeologists.(EarthscientistsoftenwonderifthatbiasmighthaveanythingtodowithwhythereareNobelPrizesinphysicsandchemistrybutnotingeology.)Consequently,fewmineralogistshavethoughtabouttheastonishingchangesinEarth’snear-surfacemineralogythroughtime.WhenIjoinedwithsevencolleaguestopublishourarticle“Mineral
Evolution”in2008,ourobjectivewasinlargeparttochallengethistraditionalperspective—toreframemineralogyasahistoricalscience.OurventureintothemineralogicalhistoryofEarth,aswellasthatofotherplanetsinourSolarSystemandbeyond,positsthatEarth’smineralogyhasevolvedthrougha
sequenceofstages,eachofwhichsawchangesinthediversityanddistributionofminerals.Hencethenarrativearcofthisbook,inwhichplanetsprogressfrommineralogicalsimplicitytocomplexity,fromonlyaboutadozenmineralsinthedustandgasthatmadeourSolarSystemtomorethanforty-fivehundredknownmineralspeciesonEarthtoday—two-thirdsofwhichcouldnotexistinanonlivingworld.Itwasahighlytechnicalarticle,publishedinthespecialtyjournalAmerican
Mineralogist,usuallyreadonlybyhard-coreprofessionals.Buttheinternationalmediaquicklypickedupthepropositionthatlifeandmineralscoevolved.TheEconomistandDerSpiegel,ScienceandNature,andahandfulofpopularsciencemagazinesallseizedonoureducatedguessesaboutaplanet’schangingmineraldiversity.TheNewScientistevenpublishedaclevercartoonshowingfour“stages”ofmineralevolution,fromaswimmingcrystalwithfinstoan“evolved”crystalwithawalkingstick.Whatnoneofthemacknowledgedwasthattheseprovocativeguesseswereallspeculative.IsMarsreallylimitedtofivehundredmineralspecies?Arenonlivingworldsreallyunlikelytoexceedfifteenhundredspecies?Isittruethatittookaliving,oxygenatedworldtotripleEarth’smineralogicaldiversity?Wehadpresentedthesestatementsashypotheses;thequesttotestthemhadyettobegin.Andwhocouldhavepredictedthatthemostfruitfulplacetolookwouldbe
therocksoftheboringbillion?
Toputquantitativefleshonthebonesofthemineralevolutionhypothesis,onemustexamineindividualgroupsofminerals.Fortunatelytheworldboastsexpertsinmanydifferentmineralgroups.That’swhyIcontactedEdGrew,aresearchprofessorinEarthsciencesattheUniversityofMaine.Edisawiry,intensescientistwhohasdevotedhislifetothemeticulousstudyofthemineralsthatincorporateberylliumandboron—rareelementsthatoccasionallyconcentrateinbig,beautifulcrystals.Heknowsall108officiallyapprovedberylliummineralslikeoldfriends.Eachhasacharacter;eachplaysageologicalrole.SoIaskedhimtothinkabouttheirstoriesthroughtime.Whendidtheyfirstappear?Whatprocessesledtotheirdiversification?Haveanyberylliummineralsbecome“extinct”?Noonehadevertriedtoanswersuchquestions.It’shardenoughtocatalogeverymineralofagivenelement,buttoteaseoutwheneachspeciesfirstappearedordisappearedisamonumentaltask.Forberyl,thecommonestberylliummineral(mostprizedinitsdeepgreenvariety,emerald),therearethousandsoflocalities.It’sadauntingchallengetotrackdowntheoldestberyl.Afterayearoftoil,EdGrewproducedalandmarkgraphshowingthe
cumulativenumberofberylliummineralsthroughtimebasedonthousandsofreportedoccurrences.Asexpected,ittookaverylongtimeforthefirstberyltoappear—almost1.5billionyears.TheelementberylliumispresentatonlyabouttwopartspermillioninEarth’scrust,soittakestimeforhotfluidstoselectandconcentratethattraceberylliumintoanenrichedfluidthatcanprecipitateberylcrystals.Foranotherbillionyears,onlyabouttwentydifferentberylliummineralsappeared.Byourfledglingtheory,abigpulseofnewmineralsshouldhaveoccurredduringtheGreatOxidationEvent,between2.4and2.0billionyearsago,butthat’snotwhatEdfound.Instead,thebiggestincreaseoccurredabitlater,morethandoublingthenumberofknownspeciesbetweenabout1.7and1.8billionyearsago.Thatspan,rightatthebeginningoftheboringbillion,wasatimeofassemblyfortheColumbiansupercontinent.Perhapsberylliumwasconcentratedintonewmineralsduringtheintensemountain-formingeventsassociatedwithcontinentalcollisions.EdGrewfollowedupwithanevenmoreimpressivesurveyofthe263known
boronminerals.Tourmaline,mostprizedinitsgorgeoussemipreciousred-greenvariant,isfoundinsomeofEarth’soldestrocks,butthatwasitfornearlyhalfabillionyears.Insamplesfrom2.5billionyearsago,ameaslytwentyorsodifferentboronspecies—lessthan10percentofthemoderntotal—arerecognized.Aswithberylliumminerals,Edobservedadoublingofthenumberofboronspeciesinrocksoftheboringbillionera,thistimeinanintervalbetweenabout2.1and1.7billionyearsago—anintervalthatbracketstheformationoftheColumbiansupercontinent.Again,thisrapidincreaseinmineraldiversityraisesalotofquestions—aboutwhenthepostoxidationdiversificationactuallytookoff,abouttheassemblyofsupercontinents,andabouttheriseofmineralogicalnoveltyduringtheboringbillion.Forournextforayintomineralevolution,wetookontheninetyknown
mineralsofthescarceelementmercury—astudythatfurthercomplicatesthepicture.Likethemuchmoreabundantelementiron,mercurycanoccurinthreechemicalstates—asanelectron-richmetal(thefamiliarsilveryliquidofoldthermometers),aswellasintwodifferentoxidizedforms.Accordingly,weanticipatedasharpincreaseinmercurymineraldiversityfollowingtheGreatOxidationEvent,butthepicturethatemergedisratherdifferent.Aswiththehistoryofberylliumandboronminerals,theearliestmercurymineral—itscommonestore,thebrilliantredmineralcinnabar—tookmorethanabillionyearstoappear.Additionalspeciesfollowedinpulses:adozennewmineralsduringtheassemblyofKenorland;morethanahalf-billionyearsofstasis;anotherhalf-dozenduringtheassemblyofColumbia.Evidently,ascontinents
collide,theresultingmountain-formingepisodesunleashafloodofmineralizingfluids—processesthatgeneratenewminerals.However,thediscoverythatsuchmineralizationwasrestrictedtointervalsofsupercontinentformationwasabigsurprise.Andthenanevenbiggersurprise:foranepicintervalfrom1.8billionto600
millionyearsago—agapexceedingeventhespanoftheboringbillion—nothing.NotevenduringthetimeofRodinia’sassemblyabillionyearsagodidanewmercurymineralappear.Wenowsuspectthatthesulfide-ladenintermediateoceanistoblame.Cinnabar,mercurysulfide,isamongtheleastsolubleofallores.Anymercuryatomsthatwashedintotheancientsulfidicseaswouldhaveimmediatelyreactedwithsulfurtoformsubmicroscopicparticlesofcinnabar,toslowlysettletothebottom,tothwartfurthermercurymineralization.Onlyinthelast600millionyears,astheoceansbecameoxygenrich,asthelandsbecamecoveredwithlife,didthepopulationofmercurymineralsexplode.
Mysteries
So,wastheexplosionofnewmineralsaconsequenceofthesupercontinentcycle,oneoftheboringbillion’ssignaturephenomena?Orwasitsimplyadelayedreactiontotheriseofoxygen?Andwhatoftheelementmercury:isthesulfur-richoceanthewholestory?Andwhatnewandunexpectedresultswillstudiesoftheotherfiftyorsomineral-formingelementsreveal?What’sclearisthatthere’salotmoretolearn,forwe’veonlyjustrecentlyhadourheadsturnedbytherichsubtletiesofthatbillion-yearspan.Thispoorlydocumentedinterval,from1.85billionto850millionyearsago,
sharestheinexorableprocessesofchangethathavecharacterizedeverystageofourplanet’sevolution.By850millionyearsago,Earth’snear-surfaceenvironmenthadchangedirreversibly.Theincreasinglyoxygenatedoceanmarginsteemedwithalgaeandothermicroorganisms,includingthestinkysulfurbacteria,whilethelandswerepoisedtoburstwithnewlife.Ifnothingelse,themysterious,not-so-boringbillionteachesusthatEarthhas
thepotentialtosettleintostasis,abenignbalanceofitsmanycompetingforces.Gravityandheatflow,sulfurandoxygen,waterandlifecanfindandmaintainastableequilibriumforhundredsofmillionsofyears.Butthere’salwaysabut.Giveanyoneoftheseforcesanudge,andEarthcanbecomeunbalanced,reachingatippingpointwithconsequencesdifficulttoanticipate—rapidchangesthatmaydisruptthenear-surfaceenvironmentinamatterofyears.Andthatiswhathappenednext.
Chapter9
WhiteEarth
TheSnowball-HothouseCycleEarth’sAge:3.7to4.0billionyears
TheProterozoicEon,spanningalmosthalfofEarth’shistoryfrom2.5billionto542millionyearsago,wasalongperiodofsharpcontrasts.Itsnewsworthyfirst500millionorsoyearswitnessedthegreatflourishingofphotosyntheticalgaeandtheconsequentriseofatmosphericoxygen,thetransformationofonceiron-richoceansbythedepositionofmassivebandedironformations,andthebiologicalinnovationofeukaryotecellswiththeirDNAsequesteredinanucleus—cellsthatweretheprecursorsofallplantsandanimals.ThemiddlebillionyearsoftheProterozoic—theso-calledboringbillion—
wereamuchmoreplodding,steadilychangingtimethatwasalsoverysmelly.Bycontrast,thefinalthreehundredmillionyearswereperhapsmostdynamic
ofall,withcontinentalbreakupandassembly,radicalclimateswings,epicshiftsinoceanandatmosphericchemistry,andtheriseofanimallife.IhopeI’veestablishedthatEarthsystemsarecomplexlyinterconnected.Air,
water,andlandappeartousasseparatespheres,whichchangeoververydifferentscalesoftime.Weathervariesdaily;oceanschangeovermillennia;rockscycleovermillionsofyears;supercontinentstakehundredsofmillionsofyearstoassembleandbreakapart.YeteveryEarthsystemaffectseveryotherinwaysbothobviousandhiddenfromview.Ahouseservesasauseful,ifimperfect,metaphorforourhomeplanet.When
youareconsideringbuyingahouse,youwanttoknowmanythings—whenitwasbuilt,forexample,aswellastheageandgeometryofitsvariousadditionsandrenovations.Youwantdetailsaboutyourhouse’sbuildingmaterialsandtheirinstallation,fromthefoundationtotheroof.Youneedtolearnabouttheplumbingsystemanditssourceofwater,aswellastheairhandlingsystem—thefurnaceandairconditionerandtheirsourcesofenergy.Thesmarthomebuyeralsoasksaboutpotentialrisks:fromfireandcarbonmonoxide,termitesandcarpenterants,radonandasbestos,leaksandmold.Geologists,likewise,studyEarth’soriginsandmajortransitions,thenatureofrocksandminerals,the
movementofwaterandair,thesourcesofenergy,andtheriskfromgeologicalhazards.AhousealsomimicssomeofEarth’scomplexbehaviors,asdifferentsystems
areinterconnectedinsometimessurprisingandunexpectedwaysthroughnegativeandpositivefeedbackloops.Onacoldwinterday,asthetemperatureinsidethehousedropsbelowyourcomfortlevel,thethermostatrespondsbyturningonthefurnace,andthetemperaturerises.Oncethehousewarmsup,thefurnaceshutsoff.Onhotsummerdays,theairconditionermirrorsthatresponsebytrippingoniftheinsidetemperaturerisestoohigh.Earth,too,operatesthroughmanysimilarnegativefeedbackloopsthathelptheplanetmaintainitsmoreorlesssteadyconditionsoftemperature,humidity,andcompositionatandnearthesurface.So,forexample,warmingoceansresultinmoreclouds,whichreflectsunlightbackintospaceandcooltheoceans.Likewise,risingconcentrationsofatmosphericcarbondioxidecauseglobalwarming,whichacceleratesrockweathering—aprocessthatgraduallyconsumesexcesscarbondioxideandleadsinturntocooling.Housesalsooccasionallydisplayreinforcingor“positive”feedback,
sometimeswithbadconsequences.Ifyourheatingsystemfailsonacoldwinterday,thepipescanfreezeandburst,causingcoldwatertofloodyourhome,makingthehouseevencolderandlesslivable.ManyoftheuncertaintiesregardingEarth’schangingclimatetodayfocusonpotentialpositivefeedbackloopsandtheirtippingpoints.Risingsealevelswillleadtocoastalflooding,whichmightresultinmoreevaporationandrainfall,whichcausesmorecoastalflooding.Awarmingoceanmightcausethewidespreadmeltingofmethane-richiceatandbeneaththeoceanfloor,whichcouldaddthegreenhousegasmethanetotheatmosphereandcauseevenmorewarming,whichcouldreleaseevenmoremethane.WehaveonlytolookattherunawaygreenhouseeffectofoursisterplanetVenus,withitsthickcarbondioxideatmosphereand900-degreeFahrenheitsurfacetemperatures,toseethepotentiallycatastrophiceffectsofuncheckedpositivefeedback.Theboringbillion,totheextentthatitreallywasboring,wasaconsequence
ofmanyefficientnegativefeedbacksthatheldchangeincheck.Inspiteoftheglobe-spanningmigrationsoflandmassesandtherepeatedassemblyandbreakupofsupercontinentsduringthatlengthyinterval,Earth’sclimateseemstohavebeenfairlystable.Therewerenogreaticeages.Thechemistryoftheanoxic,sulfur-richoceandidn’tchangemuch;nordidlifeevolveinanystrikingnewways.Somenewmineralvarietiesappeared,butnomajortippingpointsalteredtheair,land,orsea.
AllthatwasabouttochangewiththebreakupofRodinia.
Breakup
InsharpcontrasttoEarth’senigmaticquietintervalfrom1.85billionto850millionyearsago,thenextfewhundredmillionyearsexperiencedsomeofthemostremarkablyrapidandextremenear-surfacefluctuationsinourplanet’shistory.About850millionyearsago,mostofEarth’smanycontinentalmasseswerestillclumpedtogetherneartheEquatorinthedryandutterlylifelessRodiniasupercontinent.TheimmenseoceanofMirovia,perhapspunctuatedbyonlyafewisolatedarcsofvolcanicislands,surroundedthisdenuded,rusty-redmegacontinent.Theinhospitableatmospherecontainedonlyasmallfractionoftoday’soxygenbudget,toolittletoformmuchofanultravioletprotectiveozonelayer.Atimetravelerwithareliablesupplyofoxygenandsunblockmighthavesurvivedalongthecoastonablanddietofalgae,butlifewouldhavebeennopicniconthatdesolateNeoproterozoicworld.Rodinia’sunbalancedjuxtapositionoflandandseawasnotdestinedtolast.
FormostofEarth’shistory,climatehadbeenmoderatedbynegativefeedbacks.IthadvariedthroughoutEarthhistory,tobesure,butthefluctuationsseldomreachedlife-threateningextremes.Beginningabout850millionyearsago,however,severalchangesdisruptedtheformerequilibriumandpushedEarthtoaclimatictippingpoint.ThemostimportantwasthegradualbreakupofequatorialRodinia.Thefirstrift850millionyearsagowasmodest,astheCongoandKalaharicratons(whatarenowpartsofsouthernAfrica)begantoseparatetothesouthwestoftheotherwiseintactsupercontinent.About800millionyearsago,asecondsmallriftzoneisolatedtheWestAfricancraton,whichmovedsouthfromthemainlandmass.Rodinia’sfragmentationwasinfullswingby750millionyearsago,atimewhenextensivechainsofvolcanoesandbasalticlavaflowsrevealmajorcracksinthecrust.Thesupercontinentsplitinhalf,asagreatnorth-southriftzoneseparatedUrtothewestandacontinentalclusterofLaurentia,Baltica,Amazonia,andothersmallercratonstotheeast.Withriftingcamethousandsofmilesofnewcoastlineandassociatedpulsesof
rapidcoastalerosion.DynamicsedimentarybasinsformedintheintercratonicseasandmarkedanendtothelonghiatusinEarth’srockrecord—thatvirtualcessationofsedimentaryrockdepositionthathadbegunintheMesoproterozoicEraandlastedforalmostaquarterofabillionyears.Microbiallifeflourishedinthisshifting,fragmentingworld.Erodedlandscontributedmineralnutrientstophotosyntheticalgae,whichhadlongbeenlimitedbytheocean’smeagerstore
ofphosphate,molybdenum,manganese,andotheressentialelements.Paleontologistsimagineatimeofshallow,sandytidalzoneswiththickmatsofslimygreenfilamentsandoffshorewaterschokedwithsmellyalgalrafts.TectoniceventsfurtherconspiredtoalterEarth’soceans,atmosphere,and
climate.Atmosphericoxygenroseinpartbecauseoftheprofligatecoastalalgalblooms,butalsobecauseincreasedproductionofalgalbiomassledtotherapidburialoforganiccarbon.ThroughoutEarth’shistory,carbon-richbiomasshasbeenamajorconsumerofoxygen.Themorebiomassthatdecays,thefasteroxygenisconsumed.(Forestfiresrepresentanunusuallyrapidenactmentofthisongoingoxygen-depletingphenomenon.)Bythesametoken,thefastercarbon-richbiomassisburied,thefasteroxygenlevelsrise.Buthowcanweknowifbiomasswasburied?Itturnsoutthatlimestone,whichprecipitatescarbon-richminerallayersontheshallowoceanfloor,preservesasubtletelltalerecord.Carbonisotopesinlimestonepointtochangesintheproductionrateofalgae.
Life’sessentialchemicalreactions—theconversionofwaterandcarbondioxidetosugarinphotosynthesis,forexample—alwaysconcentratelightercarbon-12relativetocarbon-13.Consequently,thecarboninbiomass(thatis,algae,deadoralive)isalways“isotopicallylight”comparedwithinorganiccarboninlimestone.Duringnormaltimes,whenmicrobiallifeflourishesandlightcarbonisdepletedfromtheoceans,limestonedisplaysacorrespondinglyheavyisotopicsignature.Andattimesofunusuallyrapidbiomassburial,asevenmoreofthelightercarbonisotopeissystematicallyremovedfromtheoceans,theleftovercarbonthatmakeslimestoneisevenheavieronaverage.Sureenough,limestonedepositedalongtheshoresofRodinia790to740millionyearsagoisunusuallyheavy.Duringthatinterval,algaemusthavespreadandbeenburiedatanunprecedentedrate.ThisprofligatebloomingoflifemayhavehadasignificanteffectonEarth’s
climate.Microbiallifeconsumesthegreenhousegascarbondioxide,whichisconstantlypumpedintotheatmospherebyvolcanoes.Innormaltimes,CO2inputsandoutputsarebalanced,soatmosphericconcentrationsremainrelativelyconstant,butduringtheNeoproterozoicpulsesofrapidalgalgrowth,carbondioxidelevelsmayhavedropped,thusreducinggreenhousewarming.AnotherratherconvolutedfeedbacklooprelatedtoCO2mayhavealso
enhancedEarth’scooling.TheriftingofRodiniaresultedinthousandsofmilesofnewseafloorvolcanoes,whichmanufacturedhot,lower-densityoceancrust.Suchhot,buoyantcrusttendedtosupportshalloweroceansthanbefore,soaveragesealevelsrose.Itfollowsthattheperiodafter750millionyearsagowasprobablyoneofmanyinlandseas.Inlandseasmeanmoreevaporationand
increasedrainfall,whichleadstomorerapidweatheringofexposedrock.Butrockweatheringrapidlyconsumesthegreenhousegascarbondioxide,andlowerCO2levelscaninturnleadtoglobalcooling.Thedistinctivepositionsofcontinentsandoceansjustpriortoandduring
Rodinia’sbreakupmayhaveplayedanadditionalroleinalteringtheglobalclimate.Oceansandlandcontrastsharplyinalbedo—theabilitytoreflectorabsorbsunlight.Thedarkeroceanshaveacorrespondinglylowalbedo;theysoakupmostoftheSun’senergyandgrowwarmerintheprocess.Drybarrenland,bycontrast,ismuchmorereflective.Adesiccated,desolatesupercontinentlikeRodiniawouldhavebouncedmuchoftheincidentsunlightbackintospace.Suchajuxtapositionofpolaroceansandequatorialcontinentswouldhaveexaggeratedanyglobalcoolingevent,sincetheEquatorreceivesmuchmoresolarenergythanthepoles.Detailsofsuchglobal-scaleprocessesandcomplexfeedbackloopsarestill
beingresolved,butit’sclearthattheNeoproterozoicEarth,afteritslongperiodofrelativestability,waspoisedforbigchanges.
SnowballEarth–HothouseEarth
Three-quartersofabillionyearsago,Earthenteredaperiodofclimateinstabilitythelikesofwhichhavenotbeenseenbeforeorsince.Itallbeganwithabrutaliceage.Glaciersleaveanunambiguoussuiteofsedimentaryfeatures.Foremostare
thick,irregularlayersofdiagnosticrockscalledtillites,whichpreservechaoticjumblesofsand,gravel,angularrockfragments,andfinerockflour.Glaciersalsoleavebehindroundedoutcropsofbedrockthathavebeenscratchedandpolishedbytheslowlyadvancingicesheets.Erraticbouldersandmoundlikemorainesaddtotheevidence,asdofinelylayeredvarvedsediments,representingseasonalrunoffintoglaciallakes.Fieldgeologistsaroundtheworldhavediscoveredtheseglacialfeaturesin
rocksbetween740and580millionyearsoldjustabouteverywherethey’velooked.Indeed,evidenceforanabruptanddrasticclimatechangeabout740millionyearsagohadbeenaccumulatingfordecadeswhengeologistPaulHoffmanandthreecolleaguesatHarvardandtheUniversityofMarylandpublishedtheirshort,electrifyingpaper“ANeoproterozoicSnowballEarth”intheAugust28,1998,issueofScience.Hoffmanandhiscoworkersmadetheextraordinaryleapthatatleasttwiceduringthatinterval,Earthhadnotjust
experiencedaniceagebuthadcompletelyfrozenover,frompolestoEquator.TheybasedtheirclaiminpartonmeticulousfieldobservationsofasequenceofrocksfromtheSkeletonCoastofNamibia:thickglacialtillitedeposits,sidebysidewithpaleomagneticsignalsthattheglacierswereclosetotheEquator,atabout12degreeslatitude.Andtheseweren’talpineglaciersfromhighmountainseither;thetilliteswereclearlydepositedinshallowcoastalwaters,atsealevel.TheclimatemusthavebeencorrespondinglyfrigidneartheEquator.Bycontrast,duringEarth’smostrecenticeage,advancingglaciersnevergotfarthersouththanabout45degreeslatitude,andfossilevidencepointstoarelativelywarmtropicalzoneevenduringthemaximumextentofice.TheHarvardteamhadunassailableevidenceforNeoproterozoiciceaccumulationsatsealevel,closetotheEquator.HencethesnowballEarth.FormanyscientistsreadingHoffman’sarticlein1998,carbonisotopes
providedthesmoking-gunevidenceforsuchasuddenandcatastrophicchange.DuringthemillionsofyearspriortothefirstpresumedsnowballEarthepisode—beforeabout740millionyearsago—therapidgrowthofalgalbiomasshadconcentratedisotopicallylightcarbon.ContemporaneouslimestonesdepositedincoastalwatersaroundthefragmentingRodiniasupercontinentarecorrespondinglyheavy.Ontheflipside,ifmicrobialproductivityslowsorstops,thecarbonisotopesinlimestonemustbecomeonaveragemuchlighter.That’sexactlywhatHoffmanandhiscolleaguesfound—ahugedecreaseofmorethan1percentinheavycarbon,justbeforeandjustaftertheappearanceofglacialdepositsaboutsevenhundredmillionyearsago.Themodelthatemergedreliesonnestedpositivefeedbackloops,eachof
whichdroveEarthtoacolderandcolderstate.Onefeedbackdependedoncontinentalweathering—aprocessthatacceleratedinhotandhumidtropicalzonesasitpulledmoreandmorecarbondioxidefromtheair.AnotherfeedbackclickedinasmassivebloomsofphotosyntheticalgaescavengedevenmoreCO2outoftheair.Meanwhile,asEarth’satmosphericgreenhouseweakenedanditsclimatecooled,icecapsbegantoformandgrowatthepoles.Thisfreshwhiteiceandsnowreflectedmoresunlightintospace,apositivefeedbackthatcooledEarthmorerapidlythanbefore.Andevenastheicesheetsspreadtolowerandlowerlatitudes,thestill-warmequatorialcontinentandthefecundalgalecosystemcontinuedtopullmoreandmoreCO2fromtheskies.Earth’sclimate,temporarilyskewedoutofbalance,reachedatippingpointaswhiteicefrombothpolesextendedtowardtheEquatorandeventuallymayhaveencircledtheglobe.Intheextreme“hardsnowball”versionofthisscenario,theonepromotedbyPaulHoffmanandhiscolleagues,averageEarthtemperaturesplungedto–50
degreesFahrenheit,asanicemantleuptoamilethicksurroundedtheglobe.Formanymillionsofyears,Earthwasencasedinice(oratleastslush).The
white“snowball”Earth,unabletosoakupsolarradiation,seemedforeverlockedinitsicycocoon,astemperaturesneverroseabovefreezing.Theglobaliceageshutdownalmosteveryecosystem.Earth’sformerlyabundantmicrobiallifewasallbutwipedout.Afewhardymicrobespersisted,astheyhaveforbillionsofyears,intheperpetualdarknessofocean-floorhydrothermalvents.Othersparsesurvivingpopulationsofphotosyntheticalgaemusthavefoundtheirhomesinsunlitzonesofcrackedthiniceorshallowopenwaternearthewarmflanksofvolcanoes.
HowcouldEarthhavepossiblyrecoveredfromsuchalong,coldglobalwinter?Theanswerliesinourplanet’smuchdeeper,inexorablechurnings.Thewhiteveneeroficeandsnowcouldnotstopplatetectonics;norcoulditslowtheincessantexhalationofvolcanicgasesfromhundredsofblackconesthatpokedthroughtheice.Carbondioxide,thedominantvolcanicgas,onceagainbegantobuildupintheatmosphere.Withthelandjacketedinice,timelyremovalofCO2byrockweatheringandphotosynthesisallbutceased.Carbondioxideconcentrationsgraduallyrosetolevelsnotseeninmorethanabillionyears,eventuallytoperhapsseveralhundredsoftimesmodernlevels,triggeringanewpositivefeedback—arunawaygreenhouseeffect.Sunlightstillscatteredoffthewhitelandscape,butcarbondioxideintheatmospherebouncedthatradiantenergyrightbacktothesurface,inevitablywarmingtheplanet.Astheatmospherewarmed,smallpatchesofequatorialicemeltedforthefirst
timeinmanymillionsofyears.Asdarklandbecameexposed,moresunlightwasabsorbedandthewarmingaccelerated.Theoceans,too,begantoclearoftheirwhitecoveringsaspositivefeedbacksbetweentheSunandthesurfacecausedEarthtobecomewarmerandwarmer.
ACaseofGas
Manyscientistsnowsuspectthatanotherpositivefeedback—amechanismthatisalsoofsignificantconcerntoourmoderntimes—mayhaveexacerbatedtherapidglobalwarming.Methane(CH4),thesimplesthydrocarbonfuelandwhatweburninourhomesas“naturalgas,”isalsoagreenhousegas,butonethatis,moleculeformolecule,muchmoreeffectivethancarbondioxideintrappingtheSun’senergy.Forbillionsofyears,methanehasaccumulatedinocean-floorsediments,probablybytwocontrastingmechanisms.Thefirst,whichismuch
betterdocumentedandthusalotlesscontroversial,involvesmicrobesthatreleasemethaneaspartoftheirnormalmetaboliccycle.Thesemethanogensthriveinanoxicoceansedimentsnearmanyknownmethanereserves,solargenaturalgasdepositsarethoughttohaveformedbythesustainedactionofthesemicroorganisms.Recentexperimentspointtoapossiblesecond,muchdeepersourceof
methane—asourcethatdoesn’trelyonbiologyatall.Somescientistssuggestthatinthedeepcrustanduppermantle,atdepthstomorethanahundredmileswhereextremetemperaturesandpressuresprevail,waterandcarbondioxidecanreactwithcommoniron-bearingmineralstoproducemethane.Experimentsathightemperaturesandpressuresattempttomimicthesesuspecteddeep-Earthreactions.Inanoft-cited2004studyattheGeophysicalLaboratory,postdoctoralfellowHenryScottmixedtwocommoncrustalingredients,calcite(calciumcarbonate—thecommoncarbon-containingmineraloflimestone)andironoxide,withwater.Scottsealedtheseingredientsintoadiamondanvilcellandlaser-heatedthesampletomorethanathousanddegrees—thesameextremeconditionsfoundintheuppermantle.Remember,theneatestthingaboutthediamondanvilcellisthatdiamondsaretransparent,soyoucanseethesampleasitreactsandchanges.HenryScottwatchedastinybubblesofmethaneformedinthesamplechamber.Thehydrogenofwaterhadreactedwiththecarbonofcalcitetoformnaturalgas.OtherexperimentsinRussia,Japan,andCanadahavefoundsimilarsynthesisofhydrocarbonsunderarangeofsupposeddeep-Earthconditions.TheseexperimentsarepotentiallyimportantforunderstandingNeoproterozoic
globalwarming,formethanemayhavecontributedtoaparticularlystrongpositivefeedback.Muchofthemethanestoredneartheoceanflooristrappedinafascinatingcompoundcalledmethaneclathrate—anicelikecrystallinemixtureofwaterandgasthatformsoutcropsonthecontinentalslopes.(Thisfreezing-coldmethaneiceactuallyburnswithabrightflame—checkoutthevideosonYouTube.)Vastquantitiesofmethane—severaltimesallotherknownmethanereservescombined,bysomeestimates—arelockedintothesemethaneices,whichformwhengasrisingfrombelowreactswithcoldseawater.SignificantadditionalmethaneiceislockedinArcticpermafrost—soilsinSiberia,northernCanada,andotherregionsthathavebeenfrozenforthousandsofyears.Anextremepositiveclimatefeedbackmightoccurwhenoceanwaterswarm
evenslightly,whichcausestheshallowestclathratedepositstomeltandreleasecopiousamountsofmethanegasintotheatmosphere.Thismethaneaddssignificantlytothegreenhouseeffect,whichcausestheoceanstowarmeven
more.SomescientistsnowpointtoapossiblecatastrophicNeoproterozoicreleaseofocean-floormethaneasawaytoaccelerateglobalwarming,perhapsflippingEarthfromcoldtohotinamatterofdecades.ThisNeoproterozoicscenariodependsstronglyonthesourcesofmethane.If
microbesproducemostoftheocean’snaturalgas,thenclathrateproductionprobablyslowedduringsnowballepisodes,andmethanereleasemaynothaveplayedsuchamajorroleinwarming.If,ontheotherhand,asignificantamountofmethanerisesfromthehotpressurizedmantle,thenstoresofmethaneclathrateswouldhavebuiltupcontinuouslythroughoutanyglobalcoolingspellindependentofmicrobiallife,triggeringamuchgreaterfeedback.Sowhatprocessproducesthemethane—deeprocks,shallowmicrobes,oracombinationofthetwo?Thequestionofdeepversusshalloworiginsofmethanemayseem
straightforward,butit’saproblemcoloredbyalong-standing,sometimesheatedinternationalcontroversyintheoilandgasbusiness.Petroleumisformedprimarilyofhydrocarbonmolecules,ofwhichmethaneisthesimplestandmostabundant.It’swidelyassumedthatwhatevernaturalprocessesformmethanealsoplayaroleintheformationofoil.OnonesideofthedebateistheRussian-Ukrainianschool,fatheredinthe
mid-nineteenthcenturybythefamedRussianchemistDimitriMendeleev,bestknownforhisubiquitousperiodictableoftheelements.Mendeleevproposedanabiogenicoriginforpetroleumlongbeforetherewereexperimentstobolsterhisclaims.“Thecapitalfacttonote,”hewrote,“isthatpetroleumwasborninthedepthsoftheEarth,anditisonlytherethatwemustseekitsorigin.”Mendeleev’sideasenjoyedresurgenceinRussiaandUkraineinthesecondhalfofthetwentiethcentury,andtheyinformRussia’sthrivingoilandnaturalgasindustry.SomeRussiangeochemistsstilladvocatethatvirtuallyallpetroleumandnaturalgasderivesfromdeepabiogenicsources.Intheirview,someproductiveoilfieldsarerenewableresources,continuouslyfillingfromvastmantlereservoirsbelow.SuchthinkingisscientificheresytomostAmericanpetroleumgeologists,
whocitealitanyofevidenceforanexclusivelybiologicaloriginforpetroleum:oilisonlyfoundinsedimentaryhorizonswherelifeoncethrived;oilisloadedwithdistinctivemolecularbiomarkers;theisotopiccompositionofoilisuniquelylifelike;thetraceelementsalsopointtoalivingsource.FormanyNorthAmericanpetroleumgeologists,thecaseissettled:virtuallyallpetroleumandnaturalgasisbiogenic.Thedebate,polarizedbydecadesofRussian-Americanrivalry,wasrekindled
inNorthAmericabythebrilliant,pugnacious,far-reachingAustrianastrophysicistThomas(Tommy)Gold,whotaughtatCornellUniversitybeforehisuntimelydeathin2004.Gold’schiefclaimtoscientificfame,atleastwithinhischosenspecialtyofastrophysics,washisrealizationthatmetronomicradiopulsesfromdeepspace,theso-calledpulsars,areinfactrapidlyrotatingneutronstars.(Foratime,someastronomersthoughttheseradiosignalsmustoriginatefromdistantalientechnologies,hencetheastronomicaldesignationforallpulsarsisLGM—shortfor“LittleGreenMen.”)ThoughGoldventuredintomanyotherspheresofscience—fromthe
physiologyofhearingtotheconsistencyofthepowderylunarsurface—hismostnotablecontributionoutsideastrophysicswastochampiontheabioticoriginsofpetroleumandnaturalgas.Petroleumlooksbiological,heargued,simplybecauseathrivingcommunityofmicrobes—the“deephotbiosphere”—usestheabiotichydrocarbonsasfood.Microbesthusoverprintabiotichydrocarbonswiththeirdistinctivebiochemicalmarkers—hopanes,lipids,andmore.Basedonthishypothesis,Goldadvocatedhydrocarbonexplorationinunconventionalplaceslikeigneousandmetamorphicrocks.HeevenpersuadedaSwedishcompanytodrillanexploratorywellintosuchhardrocks—aprojectthatyieldedintriguing,ifambiguousresults(andlostalotofunhappyinvestorsalotofmoney).Ifyoulistencloselytobothsidesofthisargument,it’sclearthattheanswerto
thequestionofhydrocarbonoriginsisnotyetresolved.TommyGoldwasendlesslyinquisitiveandeagerforanswers.Shortlybeforehisunexpecteddeath,hecametomylabtolectureonthedeephotbiosphereandtodiscussapossiblecollaboration—experimentsthatmighthavehelpedtoresolvethematter.Thecriticalquestionofmethane’soriginsremainsunanswered,butitisnotunanswerable.Whatweneedisanew,internationalefforttounderstanddeepcarbon.
TheDeepCarbonObservatory
CarbonisarguablyEarth’smostimportantelement.CarbonisthekeytounderstandingEarth’svariableclimateandenvironment.Carbonhaslongbeen,andcontinuestobe,thecentralelementinourquestforenergy.Carbonisalsothecrucialelementoflifeand,byextension,thecoreelementinthedesignofnewdrugsandmyriadotherproducts.Weneedtounderstandcarbon,notjustinitswell-studiedsurfacecyclesofoceans,atmosphere,rocks,andlife,butfromcrusttocore.Soitwasthatinthesummerof2009,theAlfredP.SloanFoundationandthe
GeophysicalLaboratorylaunchedtheDeepCarbonObservatory(DCO),anambitiousten-yearprogramtostudycarboninourplanet,especiallyitschemicalandbiologicalrolesinEarth’sdeepinterior.Whereisthecarbon?Howmuchisdownthere?Howdoesitmove,especiallytoandfromthesurface?Howextensiveisthedeepbiosphere?Thisinterdisciplinary,internationalefforthasalreadyattractedhundredsofresearchersfromdozensofcountries.Wehavemanyobjectives,fromcompletingaglobalcensusofdeepmicrobiallifetomonitoringthecarbondioxideemissionsfromeveryactivevolcanoonEarth.ButdiscoveringtheoriginsofEarth’shydrocarbons,frommethanetopetroleum,isacenterpieceoftheDCO’smission.GeochemistEdYoungandhiscolleagueEdwinSchauble,bothattheUniversityofCaliforniainLosAngeles,thinkisotopeswillbethekeytodeterminingwhetheraseepofmethaneontheoceanfloorwasproducedbyarockoramicrobe.Buttheirtheoreticalcalculationscan’tbetestedwithanyordinarymeasurementofheavyversuslightisotopes.EdYoungwantstomeasure“isotopologs.”Isotopologsarechemicallyidenticalmoleculesthatdifferinthearrangement
oftheirisotopes.Methane,withonecarbonatomandfourhydrogenatoms,comesinavarietyofisotopologs.About99.8percentofallcarbonatomsarethelightercarbon-12variety,whileoneineveryfivehundredatomsistheheaviercarbon-13isotope.Bythesametoken,hydrogencomesinalighterversion(technically“hydrogen-1,”butalwaysreferredtosimplyashydrogen)aswellastheheavierhydrogen-2isotope,whichisalwayscalleddeuterium.OnEarth,thetypicalhydrogen-to-deuteriumratioisaboutathousandtoone.Theseratiosmeanthataboutoneineveryfivehundredmethanemoleculesholdsacarbon-13isotope,whileaboutfourineverythousandmethanemoleculescontainadeuterium.Traceamountsofeitherofthesetwoheavyisotopesarehardenoughto
measure,butthat’snotwhatEdYoungandhiscolleaguesareafter.Theywanttomeasuretheelusivedoublysubstitutedmethaneisotopologs—theroughlyone-in-a-millionmoleculeofmethanethatholdsbothacarbon-13andadeuterium(denoted13CH3D)orelsetwodeuteriums(12CH2D2).AccordingtoEdwinSchauble’scalculations,theratioofthosetworareisotopologsinanygivensampleofmethaneshouldprovideasensitiveindicatorofthetemperatureatwhichthemethaneformed.Temperatureisthekey:ifabatchofmethaneformedattemperaturesbelow200degrees,thenitmustbemicrobial;ifitformedattemperaturesabove1,000degrees,thenitismostlikelyabiotic.Theidealooksgreatonpaper.Thetroubleis,there’snotaninstrumentinthe
worldthatcanteaseouttheratioof13CH3Dto12CH2D2.Conventionalisotope
analysisisbasedonmassspectrometry,theprocessofseparatingmoleculesaccordingtotheirmasses.Thesetwoisotopologsdifferbylessthanahundredthof1percentinmass,posingsignificantproblemsinresolvingonetypefromtheother.What’smore,theisotopologsarepresentatextremelylowconcentrationsthatchallengeconventionalanalysis.EdYoungandhiscolleaguesneedanewinstrumentthatenhancesbothmassresolutionandmolecularsensitivity.That’swhyoneofthefirstactionsoftheDeepCarbonObservatorywastohelpfunda$2millionprototypeinstrumentdesignedspecificallytomeasuretheisotopologratiosofmethane.(Inasatisfyingdisplayofcooperation,theU.S.NationalScienceFoundation,theU.S.DepartmentofEnergy,ShellOilCorporation,andtheCarnegieInstitutionofWashingtonarealsosupportingtheeffort.)It’sariskyendeavor.Itwilltakeyearstobuildtheinstrument,yearsmorebeforeweknowifitworks.Butadefinitiveanswertothequestionofthesourcesofdeepmethane,andtheresultinginsightsintoamethane-drivenfeedbackloopthatmaydrasticallyalterEarth’sclimate,iswellworththechance.
CyclesofChange
BackonNeoproterozoicplanetEarth,atthetailendofthefirstsnowballepisodesevenhundredmillionyearsago,thetippingpointofclimatechangehadbeenreached.Theinevitableriseincarbondioxideplayedabigrole;thesuddenreleaseofmethanefromclathratesmayhavecontributedaswell.Inageologicalblinkofaneye—perhapsmuchlessthanathousandyears—theclimatelurched.ThesnowballEarthtransformedtothehothouseEarthastemperaturessoaredtorecordlevels.Foralongtime,perhapsthirtymillionyears,awarmclimateprevailed,but
thehothouseensureditsowndemise.Theelevatedatmosphericconcentrationsofcarbondioxidegraduallyfellfromtheirextremes.Someofthegreenhousegaswasremovedbyreactionswithrocks.Thedenudedland,exposedtorainfalllacedwithcorrosivecarbonicacid(aconsequenceofhighatmosphericCO2),weatheredrapidly.Theinfluxofmineralnutrients,coupledwiththeresurgenceofsunshine,ledtoexplosivealgaebloomsthatdevouredthegreenhousegas.Alloftheseeventsaredulypreservedinthecarbonisotoperecord.Andsoforthenext150millionyears,Earthcycledbetweentheseextremes.
Notonce,nottwice,butatleastthreetimestheicegatheredandretreated,theglobalclimateswingingdrunkenlyfromarctictotropicalandbackagain.Thefirstepisode,calledtheSturtianglaciation,reachedamaximumabout720millionyearsago.TheMarinoanglaciationfollowedat650millionyears,and
thelesssevereGaskiersglaciationoccurredat580millionyears.Thickaccumulationsofrockinadozencountriesrevealdetailsofthisdramaticcycle.Asiceretreated,theglaciersleftbehindhugepilesofplucked-upbouldersandground-uprock,lumpytillitesandpolishedroundedbedrock.Soonthereafterthickcrystallinedepositsofcarbonatemineralscoveredthetillitelayers—anothertelltalesignofwarmingoceans.ThecarbonatesformedsorapidlyintheCO2-supersaturatedseasthatgiantcrystalsseveralfeetlongblanketedtheshallowoceanfloor.ThesehastydroppingsspeakofatimewhenEarth’storturedsurfacehadlostitschemicalequilibrium—foreverabandoningitsboringbillionstasis.ForatimefollowingPaulHoffman’s1998publicationaboutsnowballEarth,
geologistsembracedthefrozen-planetscenario,butthebloomisnowfallingofftherose.ClimatemodelershavefounditdifficulttoencaseallofEarthinice,fortheircalculationssuggestthatevenattimesofsignificantcooling,theEquatorshouldremaintemperate.Fieldgeologistsnowfindevidenceofmovingice,surfacewaves,andoceancurrentsduringthemaximumfreeze—signsofatleastsomeopenwater.Formostgeologists,thehardsnowballhasbeenreplacedbyamorebenign“slushball”scenario,thenewmodeltobeat.Hoffmancountersthattheslushcouldrepresentconditionsjustbeforeorjustaftertheglacialmaximum.Howmightwetellthedifference?Oneintriguinglineofevidencesupporting
ahardsnowballisastriking,short-livedpulseofbandedironformationslaiddownaboutthesametimeasiceisthoughttohavecoveredtheglobe.It’shardtoexplainsuchdeposits,fortheoceanshadbeenstrippedoftheirironmorethanabillionyearsearlier,beforethestartoftheboringbillion.How,then,couldtheoceansbecomerechargedbyiron?Onemodelsuggeststhatthesnowballepisodesealedtheocean,cuttingoffalloxygentotheoceanwatercolumn.Meanwhileseafloorhydrothermalventscontinuedtopumpfreshironfromthemantleintothedeepocean.Graduallyironconcentrationsrose,onlytoberapidlydepositedasnewbandedironformationswhentheglacialepisodesended.Snowballversusslushball:suchcontroversiesarenothingnewinscience,and
thisonehasremainedlow-key,friendlierthanmost.PaulHoffmanhasretired,andanewgenerationhastakenupthechallenge,fortheanswersstillliehiddenintherocks.
TheMysteryoftheIce
Agreatermysteryremains.Thesnowball/slushballEarthepisodeswerebynomeansthefirstperiodsofglaciationonEarth,norwouldtheybethelast,butthethreemajorNeoproterozoicintervalsstandoutagainsttherest.Tothebestofourknowledge,neverbeforeandneversincehassuchanextremecoldsnapoccurredonEarth.Whyshouldthatbe?HowcanonebriefperiodofEarthhistoryhavebeensodifferentfromanyother?Twoearlierperiodsofglaciation,bothwellpreservedintherockrecord,were
evidentlyalotlesssevere.Theearliestknowniceadvance,arelativelybriefeventrevealedbytillitedepositsonancientSouthAfricancratons,occurredabout2.9billionyearsago,inthemiddleoftheArcheanEon.ThatitshouldhavetakensolongforEarth’sicecapstoexpandfromthepolesisinitselfsomethingofamystery.EarlierinEarthhistorytheSunwasmuchfainter—only70percentofitspresentradianceinthefirstfewhundredmillionyears,andnotmorethanabout80percentduringthemid-Archeanglaciation.WithsomuchlessenergycomingfromtheSun,otherwarmingmechanismsmusthavebeenatplay.Manyscientistspointtomuchhigherlevelsofgreenhousegases—carbondioxide,methane,andanorangehydrocarbonhaze—aschiefamongthesuspectedmoderatinginfluences.HigherheatflowsfromEarth’sturbulentdeepinterior,andgreatervolcanicoutputsalsomusthaveplayedaroleinmoderatingclimate.Ironically,Earth’sfirstglacialepisodemayhaveresultedinpartfromtoo
muchgreenhousegas.Iftheatmosphere’smethanecontentrose,thenreactionshighinthestratospherewouldhaveproducedmoreandmoreofthebighydrocarbonmoleculesthatmayhavegivenearlyEarthahazyorangesky.Ifthathazebecametoothick,thensomeoftheSun’senergywouldhavebeenblockedandEarthwouldhavecooled.Asecond,longercoolingepisode,markedbyextensiveglacialdeposits
betweenabout2.4and2.2billionyearsago,followedthebreakupoftheequatorialKenorlandsupercontinent.Atmosphericmodelingsuggeststhatincreasedweatheringandsedimentdepositionalongnewlyformedcoastlinesgobbledupmuchofthecarbondioxidepresentatthetime.Contemporaneously,theriseofoxygenspelledthedemiseofatmosphericmethane,theotherimportantgreenhousegas.ThefaintSun(perhaps85percentofmodernlevels)wasinsufficienttomaintainaseffectiveagreenhouse,soaprotractedcoldperiodensued.Forthenext1.4billionyears—almostathirdofEarthhistory,includingthe
boringbillion—notraceofaniceagehasbeenfound.Earth’sclimateseemstohaveremainedinremarkablebalance,nottoohotandnottoocold.Toexplain
suchalongtimewhenchangesweresorestrained,wecaninvokealitanyofpossiblenegativefeedbacks,allofwhichmayhavecontributedtostasis,butit’shardtopinpointthecausewhenthere’snoobviouseffect.WhatwecansayforsureisthatEarthreachedatippingpointroughly740millionyearsago,andthesnowball-hothousecycleensued.
TheSecondGreatOxidationEvent
Thelivingworldwasnotinsensitivetosuchextremeglobalchanges;foratleastthelast3.5billionyears,changesinthegeospherehaveprofoundlyaffectedthebiosphere.AsEarthvacillatedbetweenitshotandcoldextremes,exposedandweatheredcontinentalshorescontributedpulsesofessentialnutrientstocoastalecosystems.Manganese,requiredforphotosynthesis,wasonesuchvitalmineral.Molybdenum(usedforprocessingnitrogen)andiron(employedinvariedmetabolicroles)werealsosuppliedinabundance.Butofallthechemicalelements,phosphorusmayhavebeenthemostimportantintheNeoproterozoicseas.Phosphorusisessentialforalllife.IthelpstoformthebackboneofthegeneticmoleculesDNAandRNA,itstabilizesmanycellmembranes,anditplaysakeyroleinstoringandtransferringchemicalenergyineverycell.ThestoryofphosphorusfascinatesDominicPapineau,mycolleaguewhodid
hispostdocworkattheGeophysicalLab.Papineau’sFrenchCanadianrootsarequicklyrevealedbyhisgentleaccent;hispassionforEarth’soldestformationsisevidentineverycornerofhisrock-strewnofficeatBostonCollege.Polishedchunksofstromatolitesandbandedironformationstestifytohismanyfieldareasinremotelands.Papineaurealizedthatinsomeecosystems,theextentofmicrobialgrowthis
directlytiedtotheamountofavailablephosphorus.HeenvisionsatimewhentherewasanunprecedentedamountofthenutrientflowingintotheshallowcoastalseasoftheNeoproterozoic.Someoftheworld’slargestphosphoritedeposits—sedimentslaiddownasphosphorus-richcellsthatdiedandsettledtothebottom—areconcentratedinthesametimeintervalsasthesnowball-hothousecycles.Hehastraveledtheglobeinsearchoftheseancientphosphoritestrata—northernCanada,Finland,Africa,andIndia—tostudytheirdistinctivegeologicalsettingsandfascinatingchemistries.Thephosphorus-drivenalgalbloomspumpedatmosphericoxygentonew
levels,perhapsbreathableconcentrationsof15percent.Butparadoxically,rottingclumpsofalgaesettlingtotheoceanfloorwouldhavereactedrapidlywithoxygeninthewatercolumn,returningthedeepoceanstoadeadlyanoxic
state.ThustheresurgenceoflifefollowingthesnowballEarthmaywellhaveledtoastratifiedoceanwithanoxygen-richlayernearthesurface,anoxicwatersbelow.DominicPapineaualsoseesstrongparallelstotoday’scoastalareas,wherelargefluxesofphosphatefromfertilizerrunoffmaystimulatesimilaralgalbloomsanddeep-wateranoxicdeadzones.Whichreturnsustooneofthecentraltenetsofmineralevolution:the
coevolutionofthegeosphereandbiosphere.Mineralschangelife,evenaslifechangesminerals.WhenIbeganmygraduatestudiesinEarthsciencefourdecadesago,biologyseemedallbutirrelevanttogeology.Thegrandrockcyclewasviewedasseparatefromthecyclesoflife.WhenIaskedmythesisadviserwhetherIshouldtakeabiologycourseasmyfinalelective,hepersuadedmetotakequantummechanicsinstead.“You’llneverusebiology,”heassuredme.Dubiousadvice,consideringthatateveryphaseofEarth’sevolution,fromthe
originsoflifeonward,lifehasinfluencedgeologyandgeologyhasinfluencedlife.In2006geochemistMartinKennedyoftheUniversityofCalifornia’sRiversidecampusandfourcoauthorsproposedaparticularlynovel,ifspeculative,exampleofthiscodependence.Theirarticle,“TheInceptionoftheClayMineralFactory,”appearedintheMarch10issueofScience.Accordingtotheircleverthesis,theriseofatmosphericoxygenfromafewpercenttoitspresentlevelwasacceleratedbypositivefeedbacksbetweenmicrobesandclayminerals.Clayconsistsprimarilyofultra-fine-grainedmicroscopicmineralbitsthat
soakupwaterandformsticky,gooeymasses.Ifyou’veevergottenyourfootoryourcarstuckindeep,wetclay,youwon’tsoonforget.Aprincipalmodeofclaymineralformationisweathering,especiallyweatheringbychemicalalterationunderthewet,acidicconditionsofthelateNeoproterozoic.Kennedyandhiscoworkerssuggestthattherapidpostglacialweatheringofcontinentsproducedsignificantlymoreclaymineralsthanbeforethethreegreatsnowball-hothousecycles.What’smore,thereisgrowingevidencethatmicrobialcoloniesbegantocolonizethecoastallandscapeaboutthistime,andmicrobescanbeespeciallyefficientatturninghardrockintosoftclay.Oneofthemoststrikingpropertiesofclaymineralsistheirabilitytobindto
organicbiomolecules.Anincreasedproductionofclaymineralswouldhavesequesteredcarbon-richbiomass,andastheclaymineralswashedintotheoceans,theywouldhavesequesteredthatcarboninthickpilesoffine-grainedsediments.AccordingtotheKennedyscenario,burialofcarbonledtotheriseofoxygen,whichfurtheracceleratedthechemicalproductionofclaymineralsonland,whichledtoevenmorecarbonburial.Hence,the“claymineralfactory”
mayhavecontributeddirectlytotheriseofatmosphericoxygenandtheevolutionofthemodernlivingworld.
TheInventionofAnimals
Hothousealgalblooms,aidedbyphosphorusandothernutrients,undoubtedlycontributedtothesharpspikesinatmosphericoxygen.Theclaymineralfactorymayhaveamplifiedtheeffect.Andsobyabout650millionyearsago,atmosphericoxygenhadrisentonear-modernlevels.Elevatedoxygen,inturn,hasbeenimplicatedintheriseofcomplexmulticellularlife,foronlywithsuchhighlevelsofoxygencouldorganismsadopttheactive,energy-demandinglifestylesofjellyfishandworms.Indeed,theearliestknownmulticellularorganismsappearinthefossilrecordabout630millionyearsago,justafterthesecondglobalsnowballglaciation.TounderstandtheriseofanimallifeintheNeoproterozoicEra,wemustfirst
lookfurtherback,morethanabillionyearsback,tojustbeforetheboringbillion.Thesparsefossilevidencepointstotheriseofawhollynewkindofsingle-celledlifeabouttwobillionyearsago.Priortothattime,allcellsseemtohaveledphysicallyseparate,ifcodependent,lives.Butabouttwobillionyearsago,accordingtoarevolutionaryideafirstexpoundedbybiologistLynnMargulisattheAmherstcampusoftheUniversityofMassachusetts,onecellswallowedanotherwhole.Ratherthandigesttheengorgedcell,thebiggercellco-optedthesmalleroneinasymbioticrelationshipthatforevertransformedlifeonEarth.Margulisisacreativepowerhouseandanintellectualomnivore.Herscientific
careerhasbeendevotedtounderstandinghowgroupsoforganismsinteractandcoevolve;sheseessymbioticrelationshipsandthesharingofbiologicalinventionsasapervasivethemeinlife’shistory.Herideashaveruffledmorethanafewfeathers,inpartbecausetheydeviatefromthemoreorthodoxDarwinianviewofevolutionprimarilybymutationandselection.Inspiteofthecontroversies,Margulis’stheoryofendosymbiosisiscompellingandalmostuniversallyacceptedtoday.Modernplants,animals,andfungiconsistofcellswithmanyinternalstructures—mitochondriathatactliketinypowerplants,chloroplaststhatharnesstheSun’senergyinphotosyntheticorganisms,thecellnucleusthatholdsthegeneticmoleculeDNA.Theseandother“organelles”incomplexcellshavetheirowncellmembranesand,insomecases,theirownDNAaswell.Margulisproposedthateachoftheseorganellesevolvedfromearlier,simplercellsthatwereengulfedandultimatelyco-optedtoperform
specificbiochemicaltasks.Accordingtoourbestguess,thattransitionbegantooccurabouttwobillionyearsagoandsetthestageformuchmorecomplex,multicellularlife.Marguliscontinuestoseelife’sevolutionasdrivenbysymbiosisandthe
sharingoftraitsamongdisparateorganisms—aviewthatshehastakenwellbeyondendosymbiosis(andthatattimesplacesheroutsidethemainstream).Oneofherrecentcrusades,beautifullyoutlinedinalectureatameetingofgeologistsinDenver,Colorado,isinsupportofacontroversialideabyBritishbiologistDonaldWilliamson.In2009Williamsonproposedthatbutterfliesrepresentthemergingofthegeneticmaterialoftwoverydifferentanimals—thewormlikecaterpillarandthewingedbutterfly.ThecontroversyintensifiedwhenMargulisusedherprivilegeasamemberoftheNationalAcademyofSciencestoshortcutthepeer-reviewprocessandsponsorWilliamson’spublicationintheAcademy’sprestigiousjournal,Proceedings.Somememberswereincensed,callingthehypothesis“nonsense,”moresuitedtotheNationalEnquirerthanascientificperiodical.MarguliscounteredthatWilliamson’sarticleisworthyofseriousscrutinyanddebate.“Wedon’taskanyonetoacceptWilliamson’sideas,”shesaid,“onlytoevaluatethemonthebasisofscienceandscholarship,notknee-jerkprejudice.”Whatevertheeventualoutcomeofthatdebate,Margulis’stheoryof
endosymbiosisisnowconventionalthought.BytheNeoproterozoicEra,complexcellswithnucleiandotherinternalstructureswerewellestablishedandwerepoisedtocrossanewsymbioticthreshold.Morethansixhundredmillionyearsago,single-celledorganismslearnedhowtocooperate,tocongregate,tospecialize,andtogrowandmoveinacollective.Theylearnedtobecomeanimals.Theearliestfossilevidenceforananimal-dominatedecosystemcomesfrom
theso-calledEdiacaranPeriod,whichbeganabout635millionyearsago,shortlyafterthesecondofthethreegreatsnowballEarthevents.Thefirstdistinctlypatternedfossilswererecognizedfrom580-million-year-oldrocksfromEdiacarainsouthernAustralia(hencethename).Thesesoft-bodiedanimals,possiblerelativesofjellyfishandworms,leftpleasinglysymmetricalimpressions,likeornatelylinedpancakesorfancifullystriatedleavesuptotwofeetacross.Similarfossilshavesubsequentlybeenfoundallovertheworldinrocksbetweenabout610and545millionyearsold.Mostremarkably,the633-million-year-oldphosphate-richDoushantuoformationofsouthernChinaholdsclumpsofmicroscopiccellsinterpretedasanimaleggsandembryos.Thesestructures,whichgrewinshallowseasjustaftertheMarinoanglaciation,appear
identicalineveryrespecttomodernanimalembryos.
Soitappearsthattheseveresnowball-hothousecycleultimatelyplayedacentralroleintheevolutionofthemodernworld.ItwouldevenbefairtosaythatwemulticelledorganismsoweourexistencetothatmomenteighthundredmillionyearsagowhenEarthreachedaclimatictippingpointaftermorethanabillionyearsinwhichsteadysunlightandheat-trappingCO2hadkeptitwarm.Whenthatcarbondioxidewasrapidlyconsumedbytheweatheringofnewequatorialcontinents,andreflectiveicespreadfrombothpolestotheEquator,Earth’stemperaturesplungedformillionsofyears—untilsteadyCO2buildup,perhapsamplifiedbytherapidreleaseofmethanefromtheoceanfloors,triggeredanequallyrapidrunawaygreenhouseeffect.PerhapsmorethananyothereventsinEarth’shistory,thesewildlyerratic
snowball-hothousecyclesrevealaplanetknockedoutofkilter.Theflip-floppingNeoproterozoicclimateleddirectlytoanunprecedentedriseinatmosphericoxygen,atransitionthatpavedthewayforthefirstanimalsandplantsandthecolonizationofthecontinents.Withsuchbiologicalinnovation,evolvingEarthsoonbecameinfestedwithnovelties—swimming,burrowing,crawling,andflyingcreaturesboastingevermoreextremehabitatsandbehaviors.Indeed,withtheadventofanoxygen-richatmosphere650millionyearsago,fortheveryfirsttimeinEarth’slonghistoryyouthetimetravelercouldhavestoodontheancientalienlandscapeandbreatheddeeplywithoutdyinginagony.Forthefirsttime,youmighthavegatheredameagermealofgreenslime,whileavoidingafataldoseofultravioletradiation.Todayweareonceagainenteringaperiodofdramaticclimatechange,and
positivefeedbacksappeartobetakinghold.Reflectiveglacialiceismeltingatanacceleratingrate,exposingmoreandmoreoceanandlandtoabsorbmoreoftheSun’senergy.Treesarebeingcutandburned,thuspumpingmorecarbondioxideintotheatmospherewhiledecreasingthesizeofthecriticalCO2-consumingforest.Andperhapsmostcritically,theacceleratingreleaseofmethanefrompermafrostanddeepoceanicesmayraiseglobaltemperaturesevenmore,triggeringthereleaseofevenmoremethane,tippingthebalance.IfEarth’spastholdsanylessonsforourtime,theNeoproterozoic’sstoryofsuddenclimatechangeshouldappearatthetopofthelist.Forevenasitssnowball-hothouseshiftsopenedupnewopportunitiesforevolvinglife,witheachepisodeofclimatereversal,almosteverylivingthingdied.
Chapter10
GreenEarth
TheRiseoftheTerrestrialBiosphereEarth’sAge:4.0to4.5billionyears(thelast542millionyears)
PlatetectonicssavedEarthfromitself.Slowly,relentlessly,Earth’sconvectinginteriorpropelledthebreakupofthesprawlingequatorialRodiniansupercontinentintomoremanageablechunks.Continentalmassesshiftedpoleward,liberatingtheEquatorfromice-accumulatinglands,moderatingtheextremesnowball-hothousecycle.Abundantnewphotosyntheticalgallifealsohelpedbufferthewildfluctuationsofcarbondioxide,whileraisingoxygenconcentrationsclosetomodernlevels.EarthhasnotsinceenduredsuchexcessesofglobaltemperatureasthosethatprecededthePhanerozoicEon.AtleastfivekindsofchangeshavebeenatworkonEarthduringtheselast
542millionyears.Continentshavecontinuedtoshift,firstclosingoneoceantoformyetanothergreatsupercontinent,thenbreakinguptoformthestill-wideningAtlanticOcean.Climatehasfluctuatedfromhottocoldandbackagainmanytimes,thoughnottothesnowball-hothouseextremesoftheNeoproterozoic.Oxygenhasenjoyedathirdgreatenrichmentevent,onlytoseeatmosphericconcentrationsdropinhalfandreboundagain.Sealevelshavealsochangedrepeatedly,dramaticallyreshapingEarth’scoastlines;therockrecordrevealscountlessrisesandfalls,oftenbyseveralhundredfeet.Butmostspectacularofall,lifehaschangedandevolvedradicallyandirreversibly.Andthroughoutallthesetransformations,lifeandrockshavecoevolved.Earthhasalwaysbeenaplanetofchange,butthestoryofthePhanerozoic
Eonismuchmoresharplyinfocusandseemscorrespondinglymoreelaborateandnuancedinitsvariations,thankstoamoreextensiveandlessalteredrockrecord.Thekeytothisrichstoryisawealthofexquisitelypreservedfossils,theconsequenceoflife’snewfoundabilitytomakedurablehardparts:teeth,shells,bones,andwood.AnimalsandplantsturnouttobeparticularlysensitivetochangesinEarth’snear-surfaceenvironment,andsotheirfossilizedremainsrecordepisodeafterepisodeofadaptation.Microbescanweatheralmostanystorm;thatresilience,coupledwiththeirunhelpfulsimpleshapesandsparsenessinthefossilrecord,meansthatnoobviousmassextinctioncanberecognizedin
rocksofthePrecambrian,whenmicrobesruled.ButthePhanerozoicEonisanaltogetherdifferentstory.Thus,overthelast542millionyears,weseeEarthinanewlight.Notaplanet
changingleisurelyovertensorhundredsofmillionsofyears,butarapidlyevolvingworld—everyhundredthousandyearswasvisiblydifferentfromthelast.Partofthereasonisthatwehaveamoredetailedrecord,butit’salsothenatureoflife.Animalsandplants,especiallycreaturesthatcolonizethelands,respondtoEarth’scyclesquickly—theyevolvefastortheydie.Asoldspeciesdieout,newspeciestaketheirplaces.
AlltheWorld’saStage
Shiftingcontinentsofthepast550millionyearscontinuedtoprovideachangeablestagefortheevolutionofEarthanditsincreasinglydiversebiota.Thestoryiswellunderstoodinitsbasicoutlines—arathersimpleplayinthreeacts.ActOne:ThebeginningoftheCambrianPeriod,542millionyearsago,found
theProterozoicsupercontinentofRodiniabrokenintoseverallargeandscatteredpieces.Thelargestexpanse,stretchingfromthesouthpoletobeyondtheEquator,wasthesprawlingcontinentofGondwana,namedforageologicallyrevealingregionofIndia.Alloftoday’ssoutherncontinentsplusabigswathofAsiawerejumbledtogetherinthisonegiantlandmassmeasuringmorethaneightthousandmilesfromnorthtosouth.Otherpost-Rodiniacontinents,allofwhichwerelocatedinthesouthernhemisphere,includedthecoreofLaurentia(whatistodayNorthAmericaandGreenland)andseveralotherbigislands(includingmuchofEurope).Aglobalocean,allbutdevoidofland,dominatedthenorthernhemisphere.Overthenext250millionyears,platesmovedallthecontinentsnorthward.Laurentiamorethandoubleditssize,bymergingfirstwithwhatwouldbecomeEurope,thenwithasignificantpartofSiberia.ActTwo:Aboutthreehundredmillionyearsago,northward-tracking
GondwanacollidedwithLaurentiatoformthemostrecentsupercontinent,Pangaea.OneofthemostspectaculargeologicalconsequencesofthismergingofGondwanaandLaurentiawastheclosingoftheancientseabetweenNorthAmericaandAfrica,acollisionthatbirthedtheAppalachianMountains.TodaytheAppalachians,stretchingfromMainetoGeorgia,seemarelativelybenign,well-roundedsortofmountainrange.Suchgentlyrollingtopographyspeakstothepoweroferosion,forthreehundredmillionyearsagotheirjaggedstill-risingpeakssoaredsixorsevenmileshigh,rivalingtoday’sHimalayasassomeofthe
mightiestmountainsinthehistoryoftheworld.LopsidedPangaeaconcentratedalmostallofEarth’sdrylandononesideoftheplanet,three-quartersofitlocatedinthesouthernhemisphere.ForonehundredmillionyearstheappropriatelynamedsuperoceanPanthalassa(Greekfor“allsea”)surroundedPangaea.ActThree:TheopeningoftheAtlanticOceancommenced175millionyears
ago,whenthegreatPangaeanlandmassbegantofragmentintosevenmajorpieces.FirstLaurentiaandGondwanafractured,formingtheincipientNorthAtlantic,whilethegrowingcontinentalsplitstretchedeverfarthertothenorthwestandsoutheast.AntarcticaandAustraliasplitoffGondwanaandmovedsouth,formingtheirownislandcontinents.AriftbetweenSouthAmericaandthewestcoastofAfricaopenedtheSouthAtlantic,whileIndiabrokeofftheeastcoastofAfricaandbeganits50-million-yearjourneynorthward,ultimatelytosmashintoAsiaandcrumpleuptheHimalayanMountains.Throughoutthisprotractedhistory,eachofthevariouscontinentalplayers
scurriedhereandthere,formingpartnershipsandthenbreakingup,notunlikeahumandrama.Ithelpstoseethisglobalplayunfold:justGoogle“Pangaeaanimations.”Asyouwatch,rememberthattheshiftingcontinentsimposedotherchangesonEarth.Greaterstretchesofcoastlinepromotedmorelifeinshallowwaters.Polarlandmassespromotedthickicesheets,whichinturnloweredsealevel.Lifeevolvedinharshcompetitiononlargerlandmasses,butevolutionproceededindependentlyonisolatedcontinentsorinwidelyseparatedseas.Thelocationsofmountainrangesandoceansalteredclimate.Throughouthistory,astoday,eachofEarth’sgreatcycleshasaffectedeveryotherone.
TheAnimalExplosion!
Forbillionsofyears,theextentofEarth’smicrobiallifehadebbedandflowedinresponsetoclimate,nutrients,sunlight,andmore.Newevidencefromshallow-watersedimentssuggeststhatthegreatalgalbloomsattheendoftheNeoproterozoicweremorethanjustafewtemporaryblips.Forthefirsttime,greenphotosyntheticalgaeevolvednewstrategiestoachieveafirmfootingonswampyland—thecontinentswerefinallylookinggreenattheedges,ratherthanMartianorangeagainstablueocean.Asatmosphericoxygensoaredinconcentration,sotoodidthestratosphericozonelayer—theradiationbarrierthateffectivelyshieldsEarth’ssolidsurfacefromtheSun’slethalultravioletrays.Suchaprotectiveblanketwasanessentialpreludetotheriseofaviableterrestrialbiosphereoffirmlyrootedplantsandfreelyroaminganimals.
Strangely,ittookanimallifeanotherhundredmillionyearstocrawlfullyontoland.Foraverylongtime,mostbiologicalinnovationtookplaceintheshallowsunlitseas.Forfortymillionyears,multicellularjellyfishandwormsappeartohavedominatedthepostglacialoceans.Myriadsoft-bodiedanimals,rarelypreservedinthefossilrecord,fedonseafloordetritusandhidintherecessesofmineralslaiddownbytheirmicrobialforebears.Fortensofmillionsofyears,anecologicalstatusquoseemstohaveprevailed.Thatstatusquowaspermanentlydisruptedroughly530millionyearsagobya
strikingevolutionarytrick:manytypesofanimalslearnedtobuildtheirownprotectiveshellsoutofhardminerals.Nooneisquitesurehowthisevolutionarydevelopmentoccurred,thoughlifehadalreadybeendepositingminerallayersinreeflikestromatolitesforbillionsofyears.Somehow,somewhere,followingtheGaskiersglaciation580millionyearsago,anunknownanimalevolvedtheexquisitetrickofgrowingitsownprotectivehardpartsoutofcommonplaceminerals—mostoftencalciumcarbonateorsilica.Suchaninnovationmeantalotinthestruggleforsurvival,forpredatorswouldrathereatasoft-bodiedmorselthanwasteenergybreakingatoughmineralizedexoskeleton.Soonitwasmakeyourownshellordie.Theresultingfossilrecordisamazinglyrich,markingatimewhensedimentarylayersbecamepackedwithlifelikeremains—atimethathasbeencalledtheCambrian“explosion.”Explosionisamisleadingmoniker.Thiswasnosuddentransformation;ittook
manymillionsofyearsfor“biomineralization”tocatchon.Afewspongeswithhardenedspines,preservedinthefossil-richDoushantuoformationofsouthernChina’sGuizhouprovince,mayhavelearnedthetrickasfarbackas580millionyears.Byabout550millionyearsago,atthetailendoftheEdiacaranPeriod,avarietyofwormlikecreatureshadlearnedtocraftcarbonatemineralsintotube-shapedprotectivehomesontheoceanfloor.Thefirstrecognizableshellyfauna,thoughsmallandfragile,appearedaround
theworldinrocksabout535millionyearsold.(IrecallaspecialundergraduatefieldtriptoNahant,ontheMassachusettscoastjustnorthofBoston,tocollecttheserarefossils.Thebriskseaair,thewavesbreakingonthepicturesquerockyshore,thegorgeouswhitepuffyclouds,andtheblueoceanwereallmemorable—thescrappyweatheredfossils,barelyvisibletothenakedeye,notsomuch.)Thereal“explosion”occurredafewmillionyearslater,roughly530million
yearsago,whenallmannerofshelledanimalsrathersuddenlycameonthescene.Anevolutionaryarmsraceensued.Armoredpredatorsandarmoredpreyassumedlargerandlargerdimensions.Teethandclawsarose,asdidbonyprotectiveplatingandsharpdefensivespines.Eyesbecamemandatoryinthe
cutthroatworldoftheteemingPaleozoicoceans.Ascountlessgenerationsofshelledcreatureslivedanddied,theircarbonatebioskeletonscontributedtomassive,resistantlimestonelayers,whichnoblydecoratethelasthalf-billionyearsofEarthhistory.Stupendousfossil-packedcarbonatecliffsandridgesdottheglobe,dominatingthelandscapeindozensofcountries,formingthetallestpeaksoftheCanadianRockiesandthechina-whitecliffsofDover,evencappingthesummitofMountEverest.OfalltheCambrianevolutionaryinnovations,wide-eyedseacreaturescalled
trilobitesarethemostprized,themostphotogenic.Atthispoint,adisclaimerisrequired.Ilovetrilobites.IunearthedmyfirstnearlycompletespecimennotfarfrommyboyhoodhomeinCleveland,Ohio,attheageofsevenoreight,andI’vebeencollectingthemeversince.Mycachenowholdsmorethantwothousandpieces,allofwhicharebeingdonatedtotheSmithsonian.(YoucanseesomeofthebestspecimensintheSantOceanHallattheNationalMuseumofNaturalHistory.)SoI’mbiased.Thoughtheinitialriseofbiomineralizationwasgradual,at530millionyears
agolifewithhardpartsseemssuddenlytohavebeeneverywhere.Allmannerofleggytrilobitesandstriatedclams,nutlikebrachiopodshellsanddelicatefanlikebryozoa,porousspongesandhorn-shapedcoralsarepreservedinlayerafterlayerofsedimentaryrocksacrosstheglobe.InrocksequencesfromMontanatoMorocco,youcanlayyourhandontheexactlayer—theexactsliverofhistory—whenthisstartlinginventionofbioarmorreallytookoff.Oneofthemostimpressivespotstostudytheabruptshiftfromsoft-bodiedto
shelledanimalsisnearthehistoricoasisvillageofTiout,nestledinthescenicfoothillsoftheAnti-AtlasMountainsinwesternMorocco.Manythousandsoffeetofcarbonatesediments,standingalmostverticallyonendandexposedinthesteep-walledvalleyoftheSoussRiver,provideacontinuousrecordoftheendoftheEdiacaranandthebeginningoftheCambrian.Layeruponlayerofthethin,reddish-brownlimestoneisutterlydevoidoffamiliarfossils.Youcanwalkamilealongtheriver’sgravel-strewnbed,whichisdrymostoftheyear,andseelittlemorethantheoccasionalsuggestionofawormburrow.Thensuddenly,inalayeroflimestoneonahillsideabovethevillage—a
horizonthat,whenviewedfromadistance,seemsnodifferentfromthoseaboveorbelow—thefossilsappear.TheancientEofallotaspis,perhapstheearliestofalltrilobites,markstheverybeginningoftheCambrianexplosion.Inlayersashortdistanceabove(thatis,youngerthan)thosehistoricstrata,newspeciesarefound:thedistinctive,ellipticaltwo-inch-longformsofChoubertellaandDaguinaspis.Thelatterspeciesisbyfarthemostcommon,butoneofthemost
productiveandaccessibleoutcropsliessmackinthemiddleofasaint’sgravesite,aholyMuslimshrine.Thesmall,round-domed,whitestructureissurroundedbyalowrockwallthat’sfulloftrilobites.Itwouldn’tdoforvisitinggeologiststobreakouthammerandchisel,todisturbthequietplace.Localchildrenseemexempt,however,andtheysellthe“Tioutbugs”totourists,tappingonyourcarwindow,holdinguptheirfreshlyexhumedwares.“Heymister,onehundreddirhams!”Abouttwelvedollars.Idon’thaggle.Ibuythemall.
FaciesChange
Formanyyears,myfossilcollectingwasfocusedonthebugs.It’shardtooverstatethethrillofcrackingopenarockandfindingacompletetrilobiteinside.Fishermenmustgetasimilarbuzzwhentheyhookabigfish,andpokerplayerswhentheydrawafullhouse;forme,it’sfindinganexquisiteanimalthat’sbeenhiddenforfivehundredmillionyears.Foryears,thehuntwasenough.Thenasaseniorundergraduateinthespring
of1970,ItookmyfirstrealpaleontologycoursefromthevenerableRobertShrock.Foralmostfourdecades,BobShrocktaughtatMIT,andforalmosttwentyyearsfollowingWorldWarII,hechairedtheMITdepartmentofgeologyandgeophysics.Hewasagiantinthefield,withnumerousclassicpublications,perhapsmostnotablyIndexFossilsofNorthAmerica,amassivephotographiccompendiumofcharacteristicspeciesfromeverygeologicaltimesincetheCambrianexplosion.RobertShrockwasagiftedteacherwithagentlesmile,anaturalwhoimbued
hisclasseswithhumorandanunabashedpassionforhisprofession.Hetaughtinanavuncularstylebytellingvividstoriesofpasttimes.HetoldofthechancehorsebackdiscoveryacenturyagooftheBurgessShaleinBritishColumbia,a505-million-year-oldsitewhoseunparalleledsoft-bodiedfossilsweremadefamousinStephenJayGould’sWonderfulLife.Hedescribedhowcutefossilsoflittlefrogswerepreservedinfine-grainedsiltin300-million-year-oldtreestumpsatJoggins,onNovaScotia’swesterncoast(thelittlefrogsjumpedintothehollowed-outstumpsbutcouldn’tjumpout).Hepaintedvividpicturesof90millionyearsago,whenavastinlandseacoveredwhatisnowtheGreatPlainsoftheAmericanMidwest—aseawheremonsterreptilesandsquidlikeammonitesviedforsupremacy.Byastrangetwist,mywife,Margee(thenasenioratWellesleyCollege),and
IwoundupbeingBobShrock’slasttwostudents.Inthespringof1970,studentprotestsagainsttheVietnamWarturnedangry;classesweredisruptedandpropertydestroyed.Giventhepervasivedistractions,MIT’sadministrationgavestudentstheoptiontotakecoursespass-failandthusskipfinalexams.MargeeandIweretheonlytwostudentstooptforagradeinpaleontology.Ourexhaustingfinalexam,administeredpiecemealoverthespaceofaweek,wastoidentifyeveryunknownspecimeninatrayofahundredfossilsandthendrawthespecimenbyhand.Drawingfromnatureisadmittedlyagreatwaytohoneobservationalskills,butIwasnoartist.Eachpencilsketchwasamini-nightmare;ittookforeverandconsumedmoreerasersthanIcanrecall.ThatwasBobShrock’slastpaleontologyclass.Thearrivaloftheeminent
seismologistFrankPressasthenewdepartmentchairmanin1965hadmarkedachangingoftheguardandaswiftshifttoamorequantitativeandphysics-basedapproachtoEarthscience.Hand-drawnfossilshadnoplaceinthatmodernworld,whereplatetectonicsshiftedcurriculaassurelyasitdidcontinents.Inspiredbythatfinalclass,MargeeandIspentmanyweekendscampingat
nearbyfossil-richlocalities.Overthenextfewyears,wecollectedfossilfernsinsouthernMassachusetts,coralsinnortheasternPennsylvania,brachiopodsineasternNewYork,andtrilobitesinnorthwesternVermont.Shrock’scoursehadtaughtustoseethesefossilsinanewcontext.Eachtypeofrockandeachsuiteoffossilstoldastoryofdiverseancientecosystems.Welearnedthatatanygiventime,severaldifferenttypesofrocks—different
facies—arebeingformed,eachatadifferentplaceanddepthofwater.Sandstonesformnearestthebeachinrough,shallowtidalzones.Theyfeaturepopulationsofrobustfossilclamsandsnailswiththickshellsthatcouldwithstandthebatteringsurf.Limestone,bycontrast,representsancientcoralreefsandthushostsaricharrayofanimallife—stalkedcrinoids,starfish,snails,brachiopods,andothergroupsthatthriveinaprotectedsunlitlagoon.Themanyeleganttrilobitesfromreefecosystemstendtohavelargeeyesthatcouldscanthefull360degreesoftheirenvironment.Fartheroffshore,blackshalesaccumulateslowlyindeepdarkwater;theirfaunacommonlyincludefilterfeedersandblindtrilobites—animalsquitedifferentfromthoseofshallowerphoticzones.Ifeachoutcroppaintsapictureofatimeandplace,thenasequenceofrock
layerspiledoneatopthenexttellsarichstoryofchange.Particularlydramaticsequencesoflayeredrocktypesoftenoccurinassociationwitheconomicallyvaluable(andcorrespondinglywell-studied)coaldeposits.Coal,whichformedabundantlyinswampycoastalzonesthreehundredmillionyearsago,commonly
occurssandwichedbetweenlayersofsandstone,whichareinturnboundedbyshale.Suchasequence—shale,sandstone,coal,sandstone,shalerepeatedoverandover—impliessignificantshiftsinsealevelasitdrops,thenrises,thendropsagain,perhapsinresponsetotheretreatandadvanceofpolariceandglaciers.Aninescapableconclusionisthatforhundredsofmillionsofyears,oceandepthshaverepeatedlyvariedbyhundredsoffeet.Tomodernhumans,withourimmensecoastalcitiesandvastseaside
infrastructure,theheightoftheoceans(atleastwithintheebbandflowoftides)seemstobeafixedaspectoftheglobe.It’shardtoimagineachangeofeven10feet,muchlesshundredsoffeet.Buttherecentsedimentaryrecordisunambiguousonthispoint.Withinthelastfewtensofthousandsofyears,theoceanshavebeenatlevelsmorethan150feethigherandmorethan300feetlowerthantoday.Withoutanypossibledoubt,suchchangeswillcomeagaintoradicallyaltertheshapesofcontinentalcoastlines.Suchisthestorytoldbytherocksandtheirfossilecosystems.
LifeonLand
ThemostdramaticterrestrialtransformationinEarthhistoryhadtoawaittheriseoflandplants—aninnovationrecordedasdistinctive,sturdymicroscopicfossilsporesinrocksasoldas475millionyears.Thoughnobodyfossilsofthedelicate,easilydecayedvegetationfromthattimehaveyetbeenfound,thosefirsttrueplantswereprobablynotunlikemodernliverworts—rootless,ground-huggingdescendantsofgreenalgaethatcouldsurviveonlyinlow,wetplaces.Foraspanofmorethan40millionyears,interrestrialrockformationsaroundtheworld,decay-resistantsporesaretheonlyphysicalevidenceforlandplants.Evolutionofthesehardygreenpioneersseemstohavebeensteadybutslow.About430millionyearsago,asignificantchangeintheglobaldiversityof
sporefossilspointstoamarkedshiftinthedistributionoflandplants.Overthenext30millionyears,theliverwortlikesporesbecamelessabundant,whilethosesimilartomodernmossesandsimplevascularplantsassumeddominance.RocksofthisintervalfromScotland,Bolivia,China,andAustraliaalsoholdtheoldestknownunambiguousfossilsoftheplantsthemselves—fragmentaryremainsofclubmossesandothersuspectedrelativesofmodernvascularplants(thosewithaninternalplumbingsystemofwater-filledtubes).Lackingextensiverootsystems,theseshort,stubbyplantswouldhavebeenrestrictedtolowlying,wetareas.Thefossilrecordimproveswithtime,asearlyplantsbecamemorewidespread
androbust.Byfourhundredmillionyearsago,primitivevascularplantshadbeguntocolonizeonce-barrenlandsacrosstheglobe.Theyappearedasspindly,leafless,miniatureshrubswithgreenish,sun-seekingphotosyntheticstemsandbranchesthatrosejustafewinchesabovetheground.Theirrootsefficientlypenetratedtherockygroundandprovidedasturdyanchor,whilecapillaryactiondistributedwaterupward.Inspiteoftheirobviousimportancetolife’scolonizationoftheland,
fossilizedplantshavelongplayedsecondfiddletotrilobitesanddinosaurs.Animalshavemoredynamiclifestylesaspredatorsandprey,andtheyseemmorevariedinformandbehavior—morelikeus.What’smore,fossilplantstendtobefragmentary—typicallyanisolatedleaforstem,apieceofbarkordiagnosticallypatternedwood.PlantslackwhatUniversityofChicagopaleobotanistKevinBoycecalls“thesatisfyingcompletenessofclams,”buttheytellamazingstoriesnonetheless.IfirstmetKevinin2000,whenhewasanenthusiasticandcreativegraduate
studentatHarvard,workingwithAndyKnollandthinkingaboutnewwaystoteaseoutthosestoriesfromsomeofEarth’soldestfossilplants.Avoraciousreaderandgiftedwriter,Kevinhasaknackfortellingstories—he’sthekindofscientistwhocanmakethehistoryofplantscaptivating.ButtotellnewstoriesaboutEarth’soldestplants,Kevinneedednewkindsofdataaboutplantfossils.AndysentKevintotheGeophysicalLaboratorytolearnaboutmicroanalyticaltechniquesforelements,isotopes,andmolecules—techniquesneverbeforeappliedsystematicallytofossilizedplants.Ourfirstjointresearchventurefocusedonremarkablypreservedplantfossils
fromthefour-hundred-million-year-oldRhyniechertofAberdeenshire,Scotland.ThefortunateRhynievegetationwassavedfromrottingwhenhotspringssurroundedtheirtissueswithmineral-richwaters,hermeticallysealingandpartiallyreplacingburiedplantmatterwithfine-grainedsilica.AcenturyagogeologistsdiscoveredbouldersofthechertinastonefenceinthesmallvillageofRhynie.Onlyafterconsiderableexcavationwasasmalltractofactualbedrockfoundandquarried.Rhyniechertspecimensremainpreciousandquitedifficulttocomeby,butKevinBoycehadaccesstoanoldHarvardcollectionoffist-sizespecimensandthin,glass-mounted,two-by-three-inchtransparentpolishedrocksectionsinwhichplantanatomydowntocellulardetailscanbestudiedinamicroscope.Thesefossilsrevealfragmentsofabizarrelandscape,atoncefamiliarandprofoundlyalien,coveredinstalklikebranchingplants,eachwithgreenphotosyntheticstemsbutnoleaves.DecadesagotheefforttoextractinformationfromtheRhynieplantfossils
wasnothinglessthanheroic.Itrequiredthepreparationofhundredsofthinslices,eachprovidingatwo-dimensionalviewofacomplexthree-dimensionalobject.Imaginetakingyourfavoriteflower,embeddingitinhardopaqueepoxy,andthentryingtoreproducetheflower’sshapebycuttingtheepoxyintoflatslicesandreassemblingthewhole.That’swhatthepioneeringRhyniepaleobotanistshadtodo.Whattheyfoundwasaminiaturesuiteofodd,spindlyleaflessplants—theancestorsofourgreenworld.KevinBoycedecidedtorevisittheRhyniechert,toteaseoutnewinformation
aboutEarth’sancientflora.HisstrategywastoanalyzenewlycutandpolishedsectionsofRhyniefossils,roughlythesizeandshapeofaquarter.Weemployedanelectronmicroprobe,amachinethatmapsoutthedistributionofchemicalelementsacrossapolishedrocksurfacelikeourglass-mountedsectionsofchert—amachinefamiliartomineralogistsbutrarelyusedbypaleontologists.Wehopedtoseeifanyoftheoriginalplantmaterialhadbeenpreserved.Thetrickwastotunethemicroprobetocarbon,anelementmorecommoninlifethaninhardrocks.WeweredelightedtofindthatRhyniechertfossilsareloadedwithcarbon—andisotopicallylightcarbon,toboot,acompellingsignofitsbiologicalorigin.Thecarbondistributionbeautifullyhighlightsthedistinctivetubularstructuresoftheseearlyvascularplants.Ourfirstpaperoncellular-scalemappingofavarietyofancientplantfossils,includingoddstemsandplantsporesfromRhynie,appearedintheProceedingsoftheNationalAcademyofSciencesin2001.Kevin’snextstepwastoseeifhecouldpullanybiomolecularinformation
fromthefossils.Couldwefindactualmolecularbitsfromtheoriginalplanttissues?KevinBoycefocusedonamysterioustwenty-five-foot-talltreelikeorganismcalledPrototaxites,whichhadbeenthelargestknownlivingthingonlandfourhundredmillionyearsago.Thefossilsofthisorganismareenigmaticbecausetheyseemtolackthesamecellularstructuresofmuchsmallercoexistingplants.Rather,their“trunks”appeartobecomposedofintricatelyinterwoventubelikestructures.WorkingwithmyGeophysicalLabcolleaguesMarilynFogelandGeorgeCody,BoycewasabletoextractandanalyzeunambiguousmolecularfragmentsfromseveralPrototaxitesspecimens—fragmentsquitedifferentfromthoseoftheadjacentplantfossils.Hisremarkableconclusion:Prototaxiteswasagiantfungus,perhapsthelargesttoadstoolinEarth’shistory.KevinBoyce’sresearchreinforcestheconclusionsofthepaleobotany
community.Earth’slandscapefourhundredmillionyearsagowasatlastgreen,butinanutterlyalienway.Scrubby,stalklikeplantssharedthelandwith
toweringtreelikefungiandafewsmallinsectsandspiderlikeanimals.
InventingLeaves
YoucouldhavesurvivedquitecomfortablyonEarthfourhundredmillionyearsago.Therewasplentyofoxygenandwater.Therewasfoodintheformofplantsandbugs.TherewasshelterunderthegiantstandsofPrototaxites.Butthelandscapewouldhaveseemedsostrange.Thereweregreenstemsandgreenbranchesbutabsolutelynoleaves.Indeed,theinventionofthefirsttinyenergy-trappingleavestooktensof
millionsofyearsmore—atransformativedevelopmentthatraisedthestakesintheplantkingdom’sevolutionarystruggleforsunlight.Thetallestplantwiththebiggestleavesenjoyedanadvantageandsofollowedtheevolutionoffanlikeferns,branchinglimbs,andsturdywoodytrunks.By360millionyearsago,forestshademergedasanutterlynewterrestrialecosystem.Forthefirsttimeinhistory,Earth’slandwasemeraldgreen.Inathemethathasbeenrepeatedoverandoveragain,rockscoevolvedwith
thisnewverdantlife.Theriseoffast-spreadinglandplants,someofwhichachievedgianttreelikestature,hadprofoundmineralogicalconsequences.Weatheringratesofmanysurfacerocks,includingbasalt,granite,andlimestone,increasedbyanorderofmagnitudeasaconsequenceofrootsandtheirrapidmodesofbiochemicalbreakdown.Theresultantsoils,richinclayminerals,organicmatter,andahostofmicroorganisms,becamedeeperandmorewidespreadandprovidedanever-expandinghabitatformoreandlargerplantsandfungi.Rootsystems,thoughhiddenfromview,evolvedinremarkablewaysaswell.
Mostimportantwerenewsymbioticrelationshipsbetweenplantrootsandvastnetworksoffungalfilamentscalledmycorrhiza.Thisastonishingevolutionarystrategyaffectsthegreatmajorityofplantsyouseetoday;indeed,manyplantstendtogrowpoorlyinsoilslackingfungalspores.Themycorrhizalfungiefficientlyextractphosphateandothernutrientsfromthesoilandpassthemtotheplant,whichinturnprovidesthefungiwithasteadydietofenergy-richglucoseandothercarbohydrates.It’shardtoimaginethatsubterraneangeometry,buttheextensivenetworkofatree’srootsandfungalfilamentsbelowgroundisquiteoftenfarlargerthanthetreeweseeaboveground.Animals,too,experiencedprofoundevolutionaryadvancesasedibleplants
expandedacrossthelandscape.Invertebrates—insects,spiders,worms,andothersmallcreatures—werethefirstterrestrialexplorers.Vertebrates,whichinitially
appearedintheguiseofprimitivejawlessfishabout500millionyearsago,underwentmorethan100millionyearsofevolutionintheoceansbeforetheirfirsthaltingattemptstocolonizedryland.Vicious,alien-lookingarmoredfishwithbony-platedjawsarose420millionyearsago;muchmorefamiliarcartilaginoussharksandbonyfishappearedanddiversifiedoverthenext20millionyears.Butdrylandwasutterlydevoidofvertebrates.Therecentdiscoveryof395-million-year-oldfossilfishbonesinChina
providedtheearliestsignsoftheevolutionarytransitionfromfinstofour-footedlandanimals.Foratleast20millionyears,fishflirtedwiththeshallow,sometimesdrycoastalenvironment.Afewfishdevelopedprimitivelungsandventuredontolandforlongerandlongerstays,butmanymillionsofyearspassedbeforethefirstbonyanimalfeltfullyathomebreathingair.Theoldestknownfossilbonesofafour-leggedlandanimal,awalkingfishwithfinlikefeet,comefromrocksabout375millionyearsold.Thegradualtransitionfromfishtoamphibianhasincreasinglycomeinto
focusoverthepasttwodecades—ablossomingperiodofspectacularpaleontologicaldiscoveriesfromChinatoPennsylvania.Newfossilfindspointtoa30-million-yearintervalofintermediateforms,progressivelymoresuitedtolandbutstillretainingdistinctivelyfishlikeanatomicalfeatures.Thefirsttrueamphibiansappearedabout340millionyearsagointhemiddleoftheso-calledCarboniferousPeriod,atimewhenswampyforestswerethrivinginlowlyingareasaroundtheworld.Theseprimitivelandanimals,characterizedbybroad,flatskullsandequippedwithsplayedlegs,five-toedfeet,earssuitedforlisteninginair,andotherterrestrialadaptations,wereclearlydifferentfromtheirfishancestors.BytheCarboniferousPeriod,Earth’ssolidsurfacehadforthefirsttimeevolvedtoastrikinglymodernappearance,withdensegreenforestsoftallfernliketrees,dankswamps,andlushmeadowspopulatedwithanever-wideningcastofinsects,amphibians,andothercreatures.Andthankstotheprofoundinfluenceoflife,Earth’snear-surfacerocksandmineralshadalsoachievedsomethingakintotheirmodernstateofdiversityanddistribution.NotthatEarthhadachievedanythingclosetostasis,mindyou.Climates
waxedandwaned,droughtsandfloodsstressedtheland,andtheoddasteroidimpactandsupervolcanoeruptioncausedtraumastolifethelikesofwhichwemayhopenevertosee.ButEarthanditsbiotahaveprovenunfailinglyresilienttosuchinsults.Lifealwaysfindsawaytoadapttotherealityofnow—whenevernowis.
TheThirdGreatOxidationEvent
By300millionyearsago,Earth’sforestswereflourishing.Indeed,somuchleafybiomasswasbeingproducedandburiedthatanewrocktype,carbon-richblackcoal(hencetheCarboniferousPeriod),begantoformbythepressure-cookingofthickmassesofdeadplants.Oneconsequenceofthisorganiccarbonsequestrationwasanewpulseofoxygenintotheatmosphere,justasinthepreviousNeoproterozoicoxidationevent.Theriseinoxygenwasgradual,fromroughly18percentoftheatmosphere380millionyearsago,to25percentabout350millionyearsago,toaremarkable30percentormore300millionyearsago.Indeed,bysomeestimatestheatmosphere’soxygencontentbrieflysoaredtomorethan35percent,wellabovemodernlevels.Andtheseextremefiguresaren’tsimplyguesses:beautifulspecimensofCarboniferous-ageamber,fossilizedtreesap,preserveancientatmosphericbubblesthatstillhold30percentormoreoxygen.Theriseinoxygenhadbeneficialconsequencesforanimallife.Moreoxygen
meantmoreenergyandincreasedratesofanimalmetabolism.Somecreaturestookadvantageoftheextraoomphbygrowinglarger—muchlarger.Mostdramaticwerethegiantinsects,exemplifiedbymonsterdragonflieswithtwo-footwingspans.Theincreasedoxygenalsoenhancedatmosphericdensityandmadeflyingandglidingthatmucheasier.Otheranimalsundoubtedlymigratedtopreviouslyuninhabitablehigherelevationswiththickeningairtheycouldnowbreathein.Foraspanoftensofmillionsofyears,lifeonthePangaeansupercontinent
flourished.Theclimatewasbenign,resourceswereplentiful,andlifeevolvedwithabandon.Butthenrathersuddenly,mysteriously,251millionyearsago,lifecollapsed,inthemostcalamitousextinctioneventinEarthhistory.
TheGreatDyingandOtherMassExtinctions
Forthepast540millionyears,thefossilrecordhaspiledup.Itspeaksofprofligatebiologicalinvention—hundredsofthousandsofknownfossilspeciesofcoralsandcrinoids,brachiopodsandbryozoa,clamsandsnails,nottomentionthevastnumberofdifferentmicroscopicanimals.Specialistsestimatediversityinexcessoftwentythousandknownspeciesoftrilobites,withdozensmoredescribedeveryyear.GiventhattrilobitesinhabitedEarthforonlyabout180millionyears(betweenabout430and250millionyearsago),that’sanaverageofanewtrilobitespecieseveryfewthousandyears.Takingalltherich
diversityoffossillifeintoaccount,severalnewspeciesmusthaveappearedonaverageeverycenturyformorethan500millionyears.What’snotsoimmediatelyobviousfromthefossilrecordareafewstark
episodesofmassdeath,thesuddenextinctionofmillionsofspecies.It’srelativelyeasytospotsomethingnew,andpaleontologistsarenotimmunetothetemptationofdescribing“thefirst”or“theearliest”appearanceofasignificanttaxonortrait.Thefirstplant,thefirstamphibian,thefirstcockroach,andthefirstsnake(albeitwithtinyvestigialhindlegs)allhavemadethefossilnews.OnerecentpapereventrumpetedthediscoveryofEarth’soldestknownfossilpenis(fromafour-hundred-million-year-oldspider)—yetanotherremarkablefindfromtheRhyniechert.Lossishardertorecognizeinthefossilrecord.Extinctionsrequiremeticulous
teasingoutoffossildiversitylayerbylayer,timeintervalbytimeinterval,acrosstheglobe.Decadesofefforthavepaidoffinthedocumentationoffivegreatmassextinctions—fivehellishtimesoverthepast540millionyearswhenEarthhassufferedthelossofmorethanhalfitsspecies.Asmoredataaccumulate,itseemstheremayhavebeenasmanyasfifteenotherlessseveremassextinctionepisodesaswell.It’snoteasytodocumentthesuddenlossofspeciesfromthefossilrecord.
Giventhemanyadvancesandretreatsoftheoceans,theopeningandclosingofshallowseas,theslowingofsedimentationduringcoolperiods,andtheirreversiblelossesowingtoerosion,therockrecordisspottyandincomplete,likeanencyclopediawithmanyofitspagesrandomlyrippedout,withafewentirevolumeslost.It’salsooftendifficulttoobtainexactagesofstrataandtomatchupthetimingsofformationsonoppositesidesoftheglobe.Sothedisappearanceofanygroupofanimalsmightsimplyreflectalongishgapintherecord.Nevertheless,asfossildatabasesgrowandpaleontologistsaroundtheworldcomparenotes,thelargestextinctioneventstendtostandoutagainstthemorenormalbackgroundoflifeanddeath.TheendofthePaleozoicEra,251millionyearsago,witnessedthegreatest
massextinctionofall.Anestimated70percentoflandspeciesandawhopping96percentofmarinespeciesvanished—adisastrousglobaleventcalledtheGreatDying.NeverbeforeorsinceinEarthhistoryhavesomanycreatures(includingallthetrilobites)disappearedforever.Scientistsaren’tyetagreedonwhatcausedtheGreatDying.Itcertainly
wasn’tasimple,singlecauselikeagiantasteroidimpact;nordiditoccurallatonce.Indeed,multiplereinforcingstressfactorsmighthavecomeintoplay.Foronething,oxygenlevelshadbeguntodroprapidlyfromtheirCarboniferous
highsof35percent;by251millionyearsago,theywerebacktoroughly20percent.That’senoughoxygentosupportcomplexanimallife,butthedropperhapsaddedstresstoanimalsthathadadaptedtomoreprofligate,demandinghigh-oxygenmetabolisms.TheendofthePaleozoicalsosawanepisodeofglobalcoolingandamodesticeage,withthickicecoveringthesouthpolarportionsofPangaea.Aconsequentlargedropinoceanlevelswouldhaveprovidedadditionalstressesbyexposingmostoftheworld’scontinentalshelves.Continentalshelvesaretheocean’smostproductivebiosphere,sothelossofalargefractionofthoseshallowcoastalzoneswouldhaverestrictedthegrowthofcoralreefsandotherdiverseshallowwaterecosystems,constrictingtheentireoceanfoodweb.Large-scalevolcanismattheendofthePaleozoicEra,almostexactly
coincidentwiththemassextinction251millionyearsago,representsyetanothermajordisruptionofEarth’sbiosphere—anotherinfluenceofthegeosphereonthebiosphere.ThatprotractedmegaeruptionofasmuchasamillioncubicmilesofbasaltinSiberia,oneofthelargestvolcaniceventsinEarthhistory,musthaveseverelycompromisedEarth’senvironment.Forhundredsofthousandsofyears,pulsesofvolcanicashanddustwouldhavereducedtheSun’sinputandexacerbatedanyiceage.Thereleaseofhugequantitiesoftoxicsulfurcompoundswouldhaveledtoacidrainandfurtherenvironmentaldeterioration.Asifalltheseenvironmentalinsultsweren’tenough,somescientistspointto
thecollapseoftheozonelayerasyetanotherpossiblestressfactorinEarth’sgreatestmassextinction.Mutantfossilsporesfromend-Paleozoicrocksaroundtheworld,fromAntarcticatoGreenland,provideintriguingevidence,ifnotasmokinggun.PerhapsvolcanicemissionsfromSiberiatriggeredchemicalreactionshighintheatmospherethatdepletedtheozonelayer,openingthewindowformutagenicultravioletradiation.Whateverthecauses,theGreatDyingleftastaggeringholeinEarth’s
biodiversity.Ittookthirtymillionyearstorecover,butrecoveritdid.And,inathemerepeatedaftereveryextinctionevent,lossledtoopportunity.Anewera,theMesozoic,sawnewfaunaandfloraevolvetofillthevacantniches.
Dinosaurs!
AsuccessfulpublisheronceadvisedmethatifIwantedtoselllotsofsciencebooks,Ishouldwriteaboutoneoftwopopulartopics:blackholesordinosaurs.(Thepublisherevenwentsofarastoinclude“blackholes”inthetitleofoneofmybooksthathadabsolutelynothingtodowithblackholes.)
Soheregoes.Dinosaurscameonthesceneabout230millionyearsagoasbeneficiariesoftheend-Paleozoicmassextinction.Thesefascinatingreptilesstartedslowandsmallbutdiversifiedandradiatedintoeveryecologicalnicheoveraspanofmorethan160millionyears.ForatimeaftertheGreatDying,dinosaurscompetedsidebysidewithlargeamphibians,butanothersignificantextinctionevent205millionyearsago,coincidentwithanothermegavolcanoepisode,wipedoutmostnondinosaurvertebrates.Adinosaurexplosionfollowed.DinosaursareonlythemostarrestingandcharismaticoftheMesozoicEra
fauna.Byfarthecommonestfossilsfromthetimearetheelegantlycoiledmarinecephalopodscalledammonites.IfIhadn’tgrownupinthevicinityoftrilobite-richPaleozoicrocks,ifIhadbeenraisedinsteadintheMesozoiclandsofSouthDakota,Iprobablywouldhavecollectedammonites.Theirshellsarestunninglybeautiful,withtheirspiralsymmetryandiridescentsurfaces.Thesesegmentedcephalopods,distantancestorsofthechamberednautilus,featureexquisiteshellornamentationscalledsuturesthatonceseparatedeachinteriorchamberfromthenext.Unliketrilobites,ammoniteshellscan’tprovidearealisticpictureofthecompleteanimals.Thebigprotrudinghead,withitslargeeyesandtensuckeredtentacles,haslongsincedecayed.Whatremainsisjusttheprotectivearmoredhomeofamuchmoreinterestingcreature.For160millionyears,ammonitesevolvedanddiversifiedintheMesozoicseas.TheMesozoicErasawmanyotherimportantbiologicaldevelopments.The
floweringplantsfirstappearedthen.Sodidthefirsttruemammals.AndaswitheveryothersignificantchunkofEarthhistory,thereweremanychangesingeographyandtopographytoaccompanythesedevelopmentsinthelivingworld.Pangaeabegantobreakup,andtheAtlanticOceanwasborn.Atmosphericoxygenlevelscontinueddroppingtoadangerouslylow15percent,onlytoreboundtoroughlythepresentvalueof21percent.Sealevelsfellandroseoverandoveragain,thoughthere’snoevidenceforanysignificantglaciationduringtheMesozoic—nothingtorivaleventheiceagethatendedthePaleozoic.Fast-forwardto65millionyearsagoandoneoftheworstdaysinEarth
history.AnasteroidestimatedtohavebeenaboutsixmilesindiametercollidedwithEarthnearwhatisnowtheYucatánPeninsula.Anepicmega-tsunamiswepttheglobe,followedbymassivefiresthatburnedacrossentirecontinents.Immensecloudsofvaporizedrockdarkenedtheskiesandallbutshutdownphotosynthesis.Thiscosmictraumaappearstohavedescendedonaworldalreadyatrisk.Inanechooftheend-Paleozoicextinctionevent,agreatseriesof
volcaniceruptionsinIndiamayhavealreadybeenalteringEarth’satmosphereandweakeningitsecosystemsforhundredsofthousandsofyears.Inanotherecho,asignificantsealeveldropappearstohaveexposedmuchofthecontinentalshelfataboutthattime,upsettingtheocean’sfoodwebandkillingoffallbuteightknownammonitespeciesoutofthousands.Reasonsforsuchasealevelchangearenotatallobviousfortherewasnoiceageatthetime.Somescientistsspeculatethatmidoceanridgesbecamelessactive,causingacooling,contraction,andconsequentsinkingoftheentireoceanfloor.Whateverthecauses,individuallyorinconcert,allthedinosaursexceptfor
oneminorlineage—thebirds—wentextinct.Thelastoftheammonitesalsodiedout.Thewayforevolvingmammalswaspaved.Thesesmall,rodentlikevertebrateshadbecomewellestablishedinthecompanyoftheirlarger(andthereforedoomed)dinosaurbrethren,andtheirsurvivaloftheend-Mesozoicextinctiongavethemfootinginalmosteveryecologicalniche.WithintenmillionyearsoftheIndianmegavolcanoandcoincidentasteroidimpact,mammalshaddiversified;withinfifteenmillion,earlyancestorsofwhales,bats,horses,andelephantshadevolved.Soitwasthatmassextinctionsrepeatedlychallengedandwinnowedlifeon
Earth.Thelast540millionyearshaveseenthisebbandflowagainandagain.Butwhatofearliertimes?Weretherenomassextinctionspriorto540millionyearsago?Herepaleontologistsarestumped.PriortotheCambrianexplosion,therearealmostnodiagnosticfossilstorecord.Thestatisticsofextinctionrequiresignificantnumbersofdistinctiveorganismslikedinosaursandtrilobites;before540millionyearsago,theysimplydon’texist.Wecanbealmostpositivethatmicrobiallifewentthroughperiodsoftraumaandspeciesloss.TheremusthavebeengiantasteroidimpactsandepisodesofdestructivevolcanismthatsterilizedsignificantfractionsofEarth’ssurface.CertainlylifewasseverelychallengedduringthesnowballEarthepisodes,perhapsduringearlierglaciationsaswell.Therecouldhavebeenhundredsofmassextinctionsstretchingbacktotheverydawnoflife.Butwemayneverknowfromthespotty,microscopicPrecambrianfossilrecord.
TheHumanAge
Formorethan99.9percentofEarth’sexistence,therewerenohumans.Wearebutaneyeblinkinourplanet’shistory.TherecentriseofHomosapiensmaybetracedbacktotherodentlike
survivorsofthatrogueManhattan-sizeasteroidof65millionyearsago.Withina
fewmillionyearsofthedinosaurs’demise,mammalshadradiatedintovacantecologicalniches,tofieldsandjungles,mountainsanddeserts,airandoceans.Evenso,thelast65millionyearshavenotbeeneasy.Manyofthesestrangeandwonderfulnewmammalsdiedinothermassextinctions56,37,and34millionyearsago,fromcausesasyetuncertain.Humansultimatelydescendedfromthesurvivorsofthelastofthose
catastrophes.Monkeys,thegreatapes,andus—weallpointtoacommonprimateancestorabout30millionyearsago.Thefirsthominids,theevolutionaryfamilythatincludesprimateswhowalkerect,aroseperhaps8millionyearsagoincentralAfrica.Meanwhile,aresurgenceofglaciationthatbeganaroundtwentymillionyears
agohasincreasedinintensityandfrequency.Perhapseightseparatetimesinthepastthreemillionyears,icehasspreadfromthepolestocovergreatswathsofthehighlatitudes,reachingasfarsouthastheupperMidwest.ThoughnotasextremeastheearliersnowballEarthepisodes,theserepeatediceageswereeachaccompaniedbydrasticdropsinsealevelbyhundredsoffeet.IcebridgeslinkedAsiaandNorthAmerica,allowingmigrationsofallmannerofmammals,includingmammoths,mastodons,andeventuallyhumanstotheNewWorld.Theseiceagesmayhaveledtoanothersurprisingevolutionarytwist.
Accordingtooneintriguingtheory,coldtemperaturesfavorthesurvivalofinfantswhostayclosetotheirmothersforlongerperiods,aswellasinfantswithbiggerheads(thelargerthehead,thelowertheheatloss).Bigheadsmeanbigbrains,whilemoretimewithmothermeansmoretimetolearn.Perhapsitisnotacoincidencethatthefirsthuman,Homohabilis,or“manthetoolmaker,”appearedshortlyafteroneofthesegreatglaciations,2.5millionyearsago.Throughouttheinterveningmillennia,ithasbeenthehumanlottoendureand
adapttorepeatedchange.Frigidiceadvancesfollowedbyunusuallywarm“interglacial”periods;droughtsfollowedbyfloods;greatretreatsoftheseasfollowedbyequallygreatadvances:suchcycleswereforthemostpartmercifullygradual,spanningmanygenerations,andnomadichumanshadplentyoftimetomoveandsurvive.SuchadaptationsareonlyamongthemostrecentexamplesofliferespondingtothechangeableEarth.Indeed,thelasthalf-billionyearsofEarthhistoryhaveseenthemost
astonishinginterplaybetweenlifeandrocks—acoevolutionthatcontinueswithavengeanceintheageoftechnologicalman.Aeonsagorocks,water,andairmadelife.Life,inturn,madetheatmospheresafetobreatheandmadethelandgreenandsafetoroam.Lifeturnedtherocksintosoilsthathave,inturn,nurturedmorelifeandbecomehometoanever-wideningarrayoffloraand
fauna.
ThroughoutEarthhistory,theair,theseas,theland,andlifehavebeenshapedbyEarth’stransformativepowers:thepowerofsunlightandEarth’sinnerheat,themagicofwater,thechemicalpowerofcarbonandoxygen,andtheceaselessconvectionofthedeepinteriorandconsequentdisruptionsofthecrustthroughearthquakes,volcanoes,andtheincessantlyshiftingcontinentalplates.Inthemidstoftheseforces,ourspecieshasproventoberesilient,clever,andadaptable.Wehavelearnedtechnologicaltrickstoshapeourworldtoourwill:wemineandrefineitsmetals,fertilizeandcultivateitssoils,divertandexploititsrivers,extractandburnitsfossilfuels.Ouractionsarenotwithoutconsequences.Everyday,ifweareattunedtothedynamicprocessesofourplanetaryhome,wecanexperienceeveryfacetofitsintertwinedcreativeforces.Andwecanthenunderstandhowdevastatinglychangeabletheworldcanbe,andhowutterlyindifferentitistoourfleetingaspirations.
Chapter11
TheFuture
ScenariosofaChangingPlanetEarth’sAge:Thenext5billionyears
Isthepastprologuetothefuture?ForEarth,theanswerisyes,andno.Asinthepast,Earthwillcontinuetobeaplanetofincessantflip-floppatterns
ofchange.Theclimatewillbecomewarmer,thencooler,overandoveragain.Iceageswillreturn,aswilltimesoftropicalextremes.Platetectonicswillpersist,shufflingcontinents,whileopeningandclosingoceans.Giantasteroidimpactsandmegavolcanoeswillonceagaindisruptlife.Butotherchangeswillbenew,andmanyofthemwillbeasirreversibleasthe
firstgranitecrust.Myriadlivingspecieswilldieout,nevertobeseenagain.Tigers,polarbears,humpbackwhales,pandas,gorillas—allaredoomed.It’sverypossiblethathumanswilldieout,too.ManydetailsofEarth’shistoryarelargelyunknown,perhapsunknowable.
Butourplanet’srichhistory,coupledwithnaturallaws,givesusasenseofwhatistocome.Let’sstartwiththelongviewandthenfocuscloserandcloseronmoderntimes.
Endgame:FiveBillionYearsfromNow
Earthisalmosthalfwaytoitsinescapabledemise.For4.5billionyears,theSunhasshonesteadily,gettingslightlybrighterthroughtimeasit“burns”throughitsvaststoreofhydrogenfuel.Foranotherfivebillionyears(moreorless)theSunwillcontinuetogeneratenuclearenergybyfusinghydrogenintohelium.That’swhatmoststarsdomostofthetime.Eventuallythehydrogenwillrunout.Somesmallerstarsreachingthisstage
justpeterout,shrinkinginsizewhilesendingoutmuchlessenergythanbefore.HadtheSunbeensucha“reddwarf”star,Earth’sultimatefatewouldbetofreezesolid.Life,ifitsurvivedatall,wouldbelimitedtoafewhardymicrobesdeepunderground,whereliquidwatercouldpersist.TheSunwon’tdieinthatpitifulway,though,foritismassiveenoughtohave
anuclearbackupplan.Rememberthateverystarmustbalancetwoopposingforces.Ontheonehand,gravitypullsthestar’smassinwardtomakeassmallasphereaspossible.Ontheotherhand,nuclearreactions,likeacontinuoussequenceofinnerhydrogenbombexplosions,pushoutwardandtrytomakethestarbigger.TheSun,initspresentstatelyhydrogen-burningphase,hasachievedastablediameterofabout870,000miles—thesizethathaspersistedfor4.5billionyearsandwillcontinuetopersistforabout5billionmore.TheSunislargeenoughthatwhenthehydrogen-burningphaseisfinallyover,
anew,franticallyenergetichelium-burningphasewillbegin.Helium,theby-productofhydrogenfusionreactions,canfusetootherheliumatomstomaketheelementcarbon,butthisnewsolarstrategywillhavecatastrophicconsequencesfortheinnerplanets.Becauseofthemoreenergeticheliumreactions,theSunwillswelllargerandlarger,likeacrazysuperheatedballoon,intoapulsatingredgiantstar.ItwillswellpasttheorbitoflittleMercury,engulfingthetinyplanet.ItwillswellpasttheorbitofourneighborVenus,swallowingthatsisterworldaswell.Itwillswelltomorethanonehundredtimesitspresentdiameter—evenpasttheorbitofEarth.TheexactdetailsofEarth’sendgamearemurky.Accordingtosomebleak
scenarios,theredgiantSunwillsimplyoverwhelmEarth,whichwillvaporizeinthesolaratmosphereandbenomore.OthermodelshavetheSunsheddingmorethanathirdofitspresentmassinunimaginablesolarwinds(whichwouldceaselesslyblastEarth’sdeadsurface).AstheSunbecomeslessmassive,Earth’sorbitwillmoveoutward—perhapsjustenoughtoavoidbeingengulfed.ButifwearenotengorgedbytheexpandingSun,allthatwouldremainofouroncebeautifulblueworldwouldbeanutterlybarrenorbitingcinder.Sparsesubsurfacemicrobialecosystemsmaypersevereforanotherbillionyears,butneveragainwillthelandbelushandverdant.
DesertWorld:TwoBillionYearsfromNow
Eversoslowly,eveninitspresentcalmhydrogen-burningstate,theSunisgettinghotter.Inthebeginning,4.5billionyearsago,theSunshonewith70percentofitspresentlight.TheGreatOxidationEvent2.4billionyearsagofoundaSunshiningwithperhaps85percentoftoday’sintensity.Andabillionyearsfromnow,theSunwillbebrighterstill.Foratime,perhapsformanyhundredsofmillionsofyears,Earth’sfeedbacks
maymoderatethechange.Moreheatmeansmoreevaporation,whichproducesmoreclouds,whichreflectsmoresunlightbackintospace.Moreheatmeans
fasterweatheringofrocks,whichconsumesmorecarbondioxide,whichlowerstheamountofgreenhousegases.AndsonegativefeedbackloopsmaykeepEarthhabitableforalongtime.Buttherewillcomeatippingpoint.SmallerMarsreachedthatcriticaltime
billionsofyearsago,asalmostallitssurfacewaterwaslost.Abillionyearsfromnow,Earth’soceanswillhavebeguntoevaporateatanalarmingrate,andtheatmospherewillhavebecomeaperpetualsauna.Noicecapsorglacierswillremain,aseventhepoleswillbecometropicalzones.Foraspanofmanymillionsofyears,lifemaythriveinsuchahothouseenvironment.ButastheSuncontinuestowarmandmorewatervaporenterstheatmosphere,hydrogenwillbelosttospaceatever-increasingrates,slowlydryingouttheplanet.Bythetimealltheoceansaredry,perhapstwobillionyearshence,Earth’ssurfacewillbebarrenandbaked;lifewillbeontheprecipice.
NovopangaeaorAmasia:250MillionYearsfromNow
Earth’sdemiseisinevitable,butit’savery,very,verylongwayaway.Projectionsintothelessremotefuturepaintamorebenignpictureofadynamicyetrelativelysafeplanet.Lookingforwardafewhundredmillionyears,thepastisindeedthekeytounderstandingthefuture.PlatetectonicswillcontinuetoplayacentralroleinchangingEarth.Today
thecontinentsarescattered.WideoceansnowseparatetheAmericas,EurasiaandAfrica,Australia,andAntarcticafromoneanother.Buttheselandmassesareconstantlyinmotion,atratesofroughlyaninchortwoperyear—athousandmilesevery60millionyears.Wecanestablishratherprecisevectorsforeverylandmassbystudyingtheagesofocean-floorbasalts.Basaltnearthemidoceanridgesisquiteyoung,justafewmillionyearsatmost.Bycontrast,basaltatcontinentalmarginsandsubductionzonesmaybemorethan200millionyearsold.It’safairlysimplemattertotakealltheseocean-floorages,playtheplatetectonicstapebackwardintime,andobtainaglimpseofEarth’sshiftingcontinentalgeographyduringthepast200millionyears.Fromthatinformation,itmayalsobepossibletoprojectplausibleplatemotionsmorethan100millionyearsintothefuture.Giventheirpresenttrajectoriesacrosstheglobe,itappearsthatallthe
continentsareheadedforanothercollision.Aquarter-billionyearsfromnow(moreorless),mostofEarth’slandwillagainformonegiantsupercontinent—alandalreadynamedNovopangaeabysomeprescientgeologists.However,theexactarrangementofthatfutureisstillamatterofdebate.
AssemblingNovopangaeaisatrickygame.It’seasytotaketoday’scontinentalmovementsandpredicttenortwentymillionyearsdowntheroad.TheAtlanticwillhavewidenedbyseveralhundredmiles,whilethePacificwillhaveshrunkbyanequalamount.AustraliawillhavemovednorthtowardSouthAsia,andAntarcticawillhaveshiftedslightlyawayfromtheSouthPole,alsointhedirectionofSouthAsia.Africaisalsoonthemove,inchingnorthwardtocloseofftheMediterraneanSea.Inafewtensofmillionsofyears,AfricawillhavecollidedwithsouthernEurope,intheprocessclosinguptheMediterraneanandpushingupaHimalayan-sizemountainrangethatwilldwarftheAlps.Sothemapoftheworldtwentymillionyearshencewillappearfamiliarbutskewed.Lookingasfarasonehundredmillionyearsintothefutureinthiswayisfairlysafe,andmostmodelersarriveatsimilargeographiesofaworldwheretheAtlanticOceanhasovertakenthePacificasthegrandestbodyofwateronEarth.Fromthatpointon,modelsdiverge.Oneschoolofthought,called
extroversion,assumesthattheAtlanticwillcontinueopeningandtheAmericaswilleventuallycrunchintoAsia,Australia,andAntarctica.Inthelatterstagesofthissupercontinentassembly,NorthAmericasweepsinfromtheeasttoclosethePacificandimpactJapan,whileSouthAmericawrapsaroundclockwisefromthesoutheasttosnuggleagainstequatorialAntarctica.It’samazinghowwellallthepiecesseemtofittogether.NovopangaeaisprojectedtobeoneimmenselandmassstretchingeasttowestalongtheEquator.Thecentralassumptionofthisextroversionvisionisthatthegreatmantle
convectioncellsthatunderlieplatemotionswillcontinuemoreorlessastheydotoday.Thealternativeview,calledintroversion,takestheoppositetackbyinvokingpastcyclesoftheopeningandclosingoftheAtlanticOcean.ReconstructionsofthepastbillionyearssuggestthattheAtlantic(oranequivalentoceanpositionedbetweentheAmericasonthewestandEuropeplusAfricaontheeast)hasopenedandclosedthreetimesonacycleofafewhundredmillionyears—aresultthatsuggestsvariableandepisodicmantleconvection.TherockrecordrevealsthatthemovementsofLaurentiaandothercontinentsabout600millionyearsagoformedtheAtlantic’spredecessor,calledtheIapetusOcean(namedfortheGreekTitanIapetus,fatherofAtlas).TheIapetusOceanclosedattheassemblyofPangaea.Whenthatsupercontinentbegantosplitapart175millionyearsago,theAtlanticOceanwasformed.Accordingtotheintroversionadvocates(probablybestnottocallthem
introverts),thestill-wideningAtlanticwillfollowthesamepattern.Itwillslow,stop,andreverseinabout100millionyears.Thensome200millionyearslater,
theAmericaswillonceagaincollidewithEuropeandAfrica.Atthesametime,AustraliaandAntarcticawillhavebeensuturedtoSoutheastAsiatocompletethefuturesupercontinentof“Amasia.”Thisgreatlandmass,shapedsomethinglikeasidewaysL,usesthesamepuzzlepiecesasNovopangaeabutthistimewiththeAmericasformingthewesternside.Forthetimebeing,bothsupercontinentmodels,extroversionandintroversion,
appeartohavemeritandarestillinplay.Andwhatevertheoutcomeofthisfriendlydebate,everyoneagreesthatEarth’sgeography250millionyearsfromnow,whilestrikinglydifferentfromtoday,willechopasttimes.ThetransientcollectionofcontinentsattheEquatorwillreducetheimpactsoficeagesandmoderatechangesinsealevels.Mountainswillrisewherecontinentscollide,whilepatternsofweatherandvegetationshiftandatmosphericlevelsofcarbondioxideandoxygenfluctuate.SuchchangeswillcontinuetobecentraltothestoryofEarth.
Impact:TheNextFiftyMillionYears
Arecentsurveyofhowpeoplearemostlikelytodieratedasteroidimpactsprettylow—somethinglike1in100,000.That’sstatisticallyaboutthesameprobabilityasdeathbylightningoratsunami.Butthere’sanobviousflawinthispredictivecomparison.Lightningkillsonepersonatatimeaboutsixtytimesperyear.Asteroidimpacts,bycontrast,probablyhaven’tkilledanyoneinthousandsofyears.Butonereallybadday,onelittlethwackcouldkillalmosteveryoneallatonce.Chancesareexcellentthatyoudon’thavetoworry,normostlikelywillanyof
thenexthundredgenerations.Butwecanbeabsolutelysurethatanotherbigimpactofthedinosaur-killingvarietyiscomingsomeday,somewhere.Inthenextfiftymillionyears,Earthwillsufferatleastonebighit,maybemore.It’sallamatteroftimeandprobability.Themostlikelyculpritsareso-calledEarth-crossingasteroids—objectswithhighlyellipticalorbitsthatcrosstheplaneofEarth’smorecircularpatharoundtheSun.Atleastthreehundredofthesepotentialkillersareknown,andinthenextfewdecadessomeofthemwillpassuncomfortablyclose.OnFebruary22,1995,ajust-discoveredasteroidwiththebenignname1995CRwhizzedbywithinafewEarth-Moondistances.OnSeptember29,2004,asteroidToutatis,anelongated1.5-by-3-mileobject,passedevencloser.Andin2029asteroidApophis,a900-footdiameterrock,ispredictedtocrossmuchcloserstill,wellinsidetheMoon’sorbit.ThatunsettlingencounterwillirrevocablyaltertheApophisorbitandpossiblybringiteven
closerinthefuture.ForeveryknownEarth-crossingasteroid,thereareprobablyadozenormore
yettobespotted.Andwhenoneoftheseprojectilesisfinallyobserved,itwilllikelybemuchtoocloseforustodomuchaboutit.Ifwe’rethebull’s-eye,wemayonlyhaveafewdays’warningtosettleouraffairs.Drystatisticstellthetaleofprobabilities.Earthisstruckbyatwenty-five-footrockalmosteveryyear.Thankstothebrakingeffectsofouratmosphere,mostsuchmissilesexplodeandfragmentintolittlepiecesbeforehittingthesurface.Butobjectsahundredfeetormoreacross,whicharriveaboutonceeverythousandyears,causesignificantlocaldamage:inJune1908,suchanimpactorleveledaswathofforestneartheTunguskaRiverinRussia.Exceedinglydangeroushalf-milestonesimpactonaverageaboutonceeveryhalf-millionyears,whileasteroidsaslargeasthreemilesindiametermayarriveaboutonceeverytenmillionyears.Theconsequencesofanimpactwillvaryaccordingtothesizeandlocationof
theimpact.Aten-mileboulderwoulddevastatetheglobejustaboutanywhereithits.(Bycontrast,thedinosaur-killingasteroidof65millionyearsagoisestimatedtohavebeenaboutsixmilesacross.)Ifaten-mileobjecthitstheoceans—a70percentchance,giventhedistributionoflandandsea—thenallbutEarth’shighestmountainpeakswillbesweptcleanbyimmenseglobe-destroyingwaves.Nothingwillsurviveuptoafewthousandfeetabovesealevel.Everycoastalcitywillutterlydisappear.Ifsuchaten-mileasteroidhitsland,theimmediatedevastationmaybemore
localized.Everythingwithinathousandmileswouldbeobliterated,andmassivefireswouldsweepacrosswhatevercontinentistheunluckytarget.Forashorttime,moredistantlandsmightbesparedtheviolence,butsuchanimpactwouldvaporizeimmensequantitiesofrockandsoil,sendingSun-obscuringcloudsintothehighatmosphereforayearormore.Photosynthesiswouldallbutshutdown.Plantlifewouldbedevastatedandthefoodchainwouldcollapse.Afewhumansmightsurvivethehorror,butcivilizationasweknowitwouldbedestroyed.Smallerimpactorswouldcauselessdeathanddestruction,butanyasteroid
overafewhundredfeet,whetheritsmacksthelandorthesea,wouldcauseanaturaldisastergreaterthananythingwehaveknown.Whattodo?Shouldweignorethethreatastooremote,tooinsignificantinaworldthathassomanymoreimmediatelypressingproblems?Whatcouldwedotodivertabigrock?ThelateCarlSagan,perhapsthemostcharismaticandinfluential
spokespersonforscienceinthelasthalfcentury,thoughtalotaboutasteroids.Inpublicandprivate,mostfamouslyinhisepicTVseriesCosmos,headvocatedconcertedinternationalaction.Hesetthestagebytellingthevividtaleofthe
monksofCanterburyCathedral,whointhesummerof1178sawaviolentexplosionontheMoon—anasteroidimpactsoveryclosetouslessthanathousandyearsago.IfsuchanimpactoccurredonEarth,countlessmillionswoulddie.“TheEarthisaverysmallstageinavastcosmicarena,”hesaid.“Thereisnohintthathelpwillcomefromelsewhere.”Thesimplest,firststepinavoidingsuchaneventistolookashardaswecan
forthoseelusiveEarth-crossingdestroyers—toknowtheenemy.Weneeddedicatedtelescopes,automatedwithdigitalprocessors,tolocatetheEarth-crossingprojectiles,toplottheirorbitsandpredicttheirfuturepathways.Suchanendeavorisrelativelycheapandalreadyunderway.Morecouldbedone,butatleasttheeffortisbeingmade.Andwhatifwefoundalargerockthatisprojectedtosmashintousafew
yearsfromnow?ForSagan,alongwithothersinboththescientificandthemilitarycommunities,asteroiddeflectionisanobviousstrategy.Ifinitiatedearlyenough,evenasmallnudgebyarocketengineorafewwell-placednuclearexplosionscouldshiftanasteroid’sorbitsufficientlytochangeacollisioncoursetoanear-miss.Suchaneventualnecessityisreasonenoughforarobustprogramofspaceexploration,heargued.Inaprescient1993essay,Saganwrote,“SincehazardsfromasteroidsandcometsmustapplytoinhabitedplanetsallovertheGalaxy,iftherearesuch,intelligentbeingseverywherewillhavetounifytheirhomeworldspolitically,leavetheirplanets,andmovetosmallnearbyworldsaround.Theireventualchoice,asours,isspaceflightorextinction.”Spaceflightorextinction.Tosurviveinthelongrun,wemustjourneyoutward
tocolonizeneighboringworlds.FirstwillcomebasesontheMoon,thoughourluminoussatellitewilllongremainahostileplacetoliveandwork.NextisMars,wheremoreabundantresources—especiallylotsoffrozensubsurfacewater,butalsosunlight,minerals,andatenuousatmosphere—areathand.Itwon’tbeeasyorcheap;norisMarsdestinedtobecomeathrivingcolonyanytimesoon.Butsettling,andperhapsterra-forming,ourpromisingneighbormaywellbethenextessentialstepinourspecies’sevolution.Twoobviousobstacleswillprobablydelay,ifnotprevent,theestablishmentof
aMarsbase.Thefirstismoney.ThemanytensofbillionsofdollarsitwilltaketodesignandimplementaMarslandingisoutsidethemostoptimisticNASAbudget,eveninthebestoffinancialtimes.Acooperativeglobaleffortmaybetheonlyoption,butsuchamassiveinternationalprogramhasneverbeenattempted.Astronautsurvivalisanequallydauntingchallenge,forit’snexttoimpossible
toensureasaferound-triptoMars.Spaceisharsh,withmyriadsand-size
meteoritebulletstopiercethethinshellofeventhemostarmoredcapsuleandunpredictablesolarburstsoflethalpenetratingradiation.TheApolloastronauts,withtheirweeklongvoyagestotheMoon,weredamnedluckythatnothingbadhappened.ButavoyagetoMarswouldtakemanymonths;inthegambleinherentinanyspacemission,moretimemeansmorerisk.What’smore,norockettechnologyonthebookswouldallowaspaceshipto
carryenoughfueltogettoMarsandmakeitback.SomeinventorstalkofprocessingMartianwatertosynthesizeenoughfueltorefillthetanks,butthattechnologyisonlyadreamandprobablyalongwayoff.Perhapsthemorelogicaloption—onethatfliesinthefaceofNASAmoresbutisincreasinglypromotedinpassionateeditorials—isaone-waytrip.Werewetosendanexpeditionwithyearsofsuppliesinsteadoffuel,withsturdyshelterandagreenhouse,withseeds,withalotofoxygenandwater,andwithtoolstoextractmorelife-givingresourcesfromtheredplanet,thenanexpeditionmightjustmakeit.Itwouldbeunbelievablydangerous,butsoweremanyofthepioneeringhumanvoyagesofdiscovery—thecircumnavigationofMagellanin1519–21,thewestwardexplorationsofLewisandClarkin1804–6,andthepolarexpeditionsofPearyandAmundsenearlyinthetwentiethcentury.Humanshavenotlosttheirlusttoengageinsuchriskyventures.IfNASApostedasign-upsheetforthechanceataone-waytriptoMars,thousandsofscientistswouldvolunteerinaheartbeat.Fiftymillionyearsfromnow,Earthwillstillbeavibrantlivingworld,itsblue
oceansandgreencontinentsshiftedbutrecognizable.Thefateofourhumanspeciesismuchlesscertain.Perhapswewillbeextinct.Ifso,fiftymillionyearsismorethansufficienttoerasealmosteverytraceofourbriefdominion—everycity,everyhighway,everymonumentwouldhaveweatheredawaymillionsofyearsearlier.Alienpaleontologistswouldhavetosearchlongandhardtofindtheslightestnear-surfacetraceofourvanishedspecies.Butitisalsopossiblethathumanswillsurviveandevolve,movingoutwardto
colonizefirstourneighboringplanets,thenourneighboringstars.Ifso,ifourdescendantsmakeitintospace,thenEarthwillsurelybetreasuredasneverbefore—asapreserve,asamuseum,asashrineandplaceofpilgrimage.Perhapsonlybyleavingourworldwillhumanseverfullyappreciatetheplaceofourspecies’sbirth.
Earth’sChangingMap:TheNextMillionYears
Inmanyrespects,Earthamillionyearsfromnowwillnothavechangedvery
much.Thecontinentswillhaveshifted,tobesure,butprobablynomorethanthirtyorfortymilesfromtheirpresentrelativelocations.TheSunwillstillshine,risingeverytwenty-fourhours,andtheMoonwillstillorbitonceeverymonthorso.Butsomethingswillhavechangedalot.Inmanyplacesaroundtheglobe,
inexorablegeologicalprocesseswillhavetransformedthelandscape.Themostobviouschangeswillaffectvulnerablecoastlines.Oneofmyfavoriteplaces,CalvertCounty,Maryland,withitsmilesoffast-erodingMioceneEpochcliffsandtheirseeminglylimitlessstoreoffossils,willhavedisappearedcompletely.Afterall,thecountyisonlyfivemileswide,andit’sgettingskinnierbyalmostafootayear.Atthatrate,CalvertCountywon’tlastfiftythousandyears,muchlessamillion.Inotherstates,geologicalprocesseswilladdvaluablenewrealestate.Anew
ocean-floorvolcanooffthesoutheasterncoastofHawaii’sbigislandisalreadyalmosttwomileshigh(thoughstillsubmerged)andgrowinglargereveryyear.Amillionyearsfromnowanewisland,alreadynamedLoihi,willhaverisenfrombeneaththewaves.Ofcourse,olderextinctvolcanicislandstothenorthwest,includingMaui,Oahu,andKauai,willbecomecorrespondinglysmalleraswindandwaveserodethemaway.Speakingofwaves,scientistswhoexaminetherockrecordforhintsofwhatis
tocomeconcludethatEarth’sgeographywillbealteredmostdramaticallybytheadvancingandrecedingoceans.Changesintherateofriftvolcanismhavealong-termeffect,asgreaterorlesservolumesoflavasolidifyontheoceanfloor.Sealevelcandropsignificantlyduringlullsinoceanvolcanism,whenseafloorrockscoolandsettle,asmanysuspectoccurredduringthedramaticfallofsealeveljustbeforethelateMesozoicextinction.ThepresenceorabsenceoflargeinlandseasliketheMediterraneanandtheassemblyandbreakupofcontinentscausemajorchangesintheextentofshallowcoastalwaters,whichplayyetanotherimportantroleintheshapethegeosphereandbiospherewilltakeinthenextmillionyears.Amillionyearscorrespondstotensofthousandsofhumangenerations—
hundredsoftimesallofrecordedhumanhistory.Ifwesurvive,thenEarthmaybephysicallytransformedbyourevolvingtechnologicalprowessinwayswecannoteasilyimagine.Butifwedieout,Earthwilllikelysoldieronmuchasitistoday.Lifeonlandandintheseawillflourish;thecoevolutionofthegeosphereandbiospherewillquicklyreturntoitspreindustrialequilibrium.
Megavolcano:TheNextHundredThousandYears
Thesuddencatastropheofanasteroidimpactmaypalebesidethetime-releaseddeathofamegavolcanoorfloodbasalt.AccompanyingallofEarth’sfivegreatestmassextinctionintervals—includingtheonecontemporaneouswithabigrockfallingfromthesky—wasglobe-alteringvolcanism.Thisisnottobeconfusedwithrun-of-the-millvolcanicdeathanddestruction
initsvariousforms.TheseincludedramaticlavaflowslikethosesofamiliartoHawaiianislanderslivingontheslopesofKilauea—utterlydestructivetoanydwellingsintheirpaths,butalsolocalized,predictable,andeasilyavoided.Somewhatmoredeadlyinthisleagueofordinaryvolcanismaretheexplosionsandashfallsofpyroclasticvolcanoes,whichcanreleaseimmensequantitiesofincandescent,steam-drivenashthatracedownamountainsideatmorethanahundredmilesperhour,incineratingandburyingeverythingintheirpath.Cuethe1980explosionofMountSt.HelensinWashingtonStateandtheJune1991eruptionofMountPinatubointhePhilippines,bothofwhichwouldhavekilledthousandsofpeopleifpriorwarningshadn’tpromptedmassevacuations.Stillmoreominousisathirdtypeofvolcanismasusual:theejectionoflargequantitiesoffine-grainedashandtoxicgasesintothehighatmosphere.TheIcelandicasheruptionsofEyjafjallajökullinApril2010andGrímsvötnin
May2011wererelativelypuny,releasingmuchlessthanacubicmileofdebris.Nevertheless,theydisruptedEuropeanairtravelforseveraldaysandcausedhealthconcernsformanypeopleinnearbyregions.TheeruptionofLakiinJune1783—amongthelargesthistoriceruptions—releasedanestimatedfivecubicmilesofbasaltandassociatedashandgas,sufficienttoinducealong-lastingpoisonoushazeoverEurope.AquarterofIceland’spopulationdied,somerapidlyfromexposuretoacidicvolcanicgasesandmanymorefromstarvationduringthesubsequentwinter.Thedisasteralsoaffectedlandsmorethanathousandmilestothesoutheast,astensofthousandsEuropeans,mostintheBritishIsles,alsodiedfromLaki’sprolongedeffects.EvenmorepeoplediedfollowingtheAugust1883explosionofKrakatoaandtheresultanttsunamithatsweptnearbycoastlinesofJavaandSumatra.AndthecolossalApril1815eruptionofTambora,whichproducedanamazingtwelvecubicmilesoflava,wasthedeadliestofall.Morethanseventythousandliveswerelost,mostasaconsequenceofagriculturalfailureandsubsequentmassstarvation.Tambora’sinjectionofimmensequantitiesofsun-blockingsulfurcompoundsintotheupperatmosphereturned1816intotheNorthernHemisphere’s“yearwithoutasummer.”Suchhistoriceruptionstroublethemodernimagination,andforgoodreason.
Sure,theirdeathtollspalebycomparisonwiththehundredsofthousandsofindividualskilledbyrecentearthquakesintheIndianOceanandHaiti.Butthere’sanimportant,terrifyingdifferencebetweenearthquakesandvolcanoes.Thesizeofthelargestpossibleearthquakeislimitedbythestrengthofrock.Hardrockcantakeonlysomuchstressbeforeitsnaps;thatextremelimitcanproduceanextremelydestructive,butlocalized,earthquake—magnitudenineontheRichterscale.Volcanoes,bycontrast,havenoapparentupperlimitinsize.Infact,the
geologicalrecordholdsunambiguousevidenceoferuptionsahundredtimesgreaterthanthelargestvolcaniceventsinrecordedhumanhistory.Suchmegavolcanoeswouldhavedarkenedtheworld’sskiesforyearsandalteredthelandscapeovermillionsofsquaremiles,notthousands.Themostrecentmegavolcanoexplosion,TaupoontheNorthIslandofNewZealand26,500yearsago,mayhaveproducedmorethan200cubicmilesoflavaandash.TobainSumatra,whicherupted74,000yearsago,releasedanestimated672cubicmilesofejecta.Theconsequencesofanothersuchcatastropheonmodernsocietyarehardtofathom.Andyet,eventhesemegavolcanoes,thoughmuchgreaterthananycataclysm
inrecordedhistory,weredwarfedbythegreatfloodbasaltsthatcontributedtomassextinctions.Unlikeone-offexplosionsofmegavolcanoes,floodbasaltsrepresentasustainedintervalwiththousandsofyearsofintensevolcanicactivity.Thegreatestoftheseepisodes,allofwhichcoincidewithglobalmassextinctions,producedhundredsofthousandstomillionsofcubicmilesoflava.Thebiggestknownevent,nowrevealedbymorethanahalf-millionsquaremilesofbasaltflows,occurredinSiberiaduringEarth’sgreatestmassextinction,thegreatdying251millionyearsago.Thedemiseofthedinosaurs65millionyearsago,sooftenascribedtoanasteroidimpact,isalsocoincidentwithimmensefloodbasaltsinIndia—theDeccanTraps,almost200,000squaremilesinextent,representingmorethan120,000cubicmilesofnewrock.Thesevastsurfacefeaturescouldnothavecomefromsimplereprocessingof
thecrustanduppermantle.CurrentmodelsoffloodbasaltformationenvisionathrowbacktoEarth’searliestageofverticaltectonics,withgiantbubblesofmagmaslowlyrisingallthewayfromthesuperheatedcore-mantleboundary,crackingthecrust,spewingoutoverthecoldsurface.Sucheventsarenowrare.Onescenariopositsaroughlythirty-million-yearintervalbetweenfloodbasalts,inwhichcasewe’resomewhatoverdueforthenextbigone.Ourtechnologicalsocietywillcertainlyreceivefairwarningofsuchanevent.
Seismologistswillbeabletotrackthehot,moltenplumeasitrises.Wemay
havehundredsofyearstoprepareforthecalamity.Butshouldhumanityeverenteranothereraofmegavolcanism,therewouldbenothingwecoulddotostopEarth’smostviolentparoxysms.
IceFactor:TheNextFiftyThousandYears
Fortheforeseeablefuture,thebiggestdeterminingfactorinEarth’scontinentalcontoursisice.Onshorttimescalesofafewhundredorthousandyears,thedepthoftheoceansismostcloselytiedtothetotalvolumeofEarth’sfrozenwater,includingtheicecaps,glaciers,andcontinentalicesheets.It’sasimpleequation:thegreaterthevolumeofwatertiedupiniceonland,thelowerthelevelofthesea.Thepastiskeytopredictingthefuture,buthowcanwepossiblyknowthe
depthofhistoricoceans?Satelliteobservationsofoceanlevels,thoughincrediblyaccurate,arerestrictedtoaboutthepasttwodecades.Tidegaugemeasurements,thoughlessaccurateandsubjecttolocalidiosyncrasies,gobackperhapsacenturyandahalf.Coastalgeologistscanresorttomappingancientshorelinemarkers—raisedbeachterraces,forexample,whichcanbefoundinnear-shoreaccumulationsofsedimentsdatingbacktensofthousandsofyears,thoughsuchelevatedfeaturescanreliablyrevealonlyperiodsofhigherwaterlevels.Thepositionsoffossilcorals,whichmusthavegrownintheocean’sshallowsunshinezone,canpushtherecordevenfurtherback,butsuchrockformationscommonlyexperienceepisodesofuplift,subsidence,ortiltingthatconfusetherecord.Manyscientistsnowfocusonalessobviousindicatorofsealevel—the
variableratiooftheisotopesofoxygenintinyseashells.Suchratiostellusmuch,muchmorethanacosmicbody’sdistancefromtheSun,asdiscussedinchapter2.Becauseoftheirtemperature-sensitivenature,oxygenisotopesarealsothekeytodecipheringthehistoricvolumeofEarth’siceandthusancientsealevelchanges.Evenso,theconnectionbetweenicevolumeandoxygenisotopesistricky.By
farthemostabundantoxygenisotope,accountingforabout99.8percentofwhatwebreathe,isthelighteroxygen-16(witheightprotonsandeightneutrons).Aboutoneinfivehundredoxygenatomsistheheavieroxygen-18(witheightprotonsandtenneutrons).Thatmeansaboutoneineveryfivehundredwatermoleculesintheoceanisheavierthantheaverage.AstheSunheatsequatorialoceanwater,waterwiththelighteroxygen-16isotopeevaporatesabitfasterthanthatwithoxygen-18,ensuringthatwaterinlow-latitudecloudsisabitlighteron
averagethantheoceansfromwhichitcame.Asthecloudsrisetocoolerzones,waterwiththeheavieroxygen-18isotopecondensesintoraindropsabitfasterthanthatwithoxygen-16,ensuringthatthecloud’soxygenbecomesevenlighterthanitwasbefore.Bythetimethecloudsmovetowardthepoles,ascloudsinevitablydo,theoxygenintheirwatermoleculeshasbecomemuchlighterthaninthoseofoceanwater.Whenthesepolarcloudsreleasetheirprecipitationontoicecapsandglaciers,moreofthelighterisotopeislockedintheice,leavingtheoceansheavier.Duringtimesofmaximumglobalcooling,whenmorethan5percentof
Earth’swatercanbefrozensolid,theoceansbecomesignificantlyenrichedinoxygen-18.Duringtimesofglobalwarmingandglacialretreat,oxygen-18levelsintheoceansdecline.Soitisthatcarefullayer-by-layermeasurementsofoxygenisotoperatiosincoastalsedimentscanrevealhowEarth’ssurfaceice(orlackthereof)hasvariedthroughtime.SuchexactingworkisthedomainofgeologistKenMillerandhiscolleagues
atRutgersUniversity,whohavespentdecadesscrutinizingthethickaccumulationsofmarinesedimentsthatblanketcoastalNewJersey.Thesesediments,witharecordextendingbackonehundredmillionyears,containawealthofmicroscopicfossilshellscalledforaminifera,orforamsforshort.Eachtinyforampreservestheoxygenisotopecontentoftheoceanatthetimewhenitgrew.Layer-by-layermeasurementsofoxygenisotopesinNewJerseysedimentsthusprovideasimpleandaccurateestimateoftheicevolumethroughtime.Intherecentgeologicalpast,theicecoverappearstohaveebbedandflowed
constantly,withcorrespondinglylargechangesinsealevelonatimescaleofafewthousandyears.Attheheightofrecenticeages,morethan5percentofEarth’swaterhasbeentrappedinice,loweringsealevelsbyperhapsthreehundredfeetfromtheirpresentmark.Abouttwentythousandyearsago,suchaperiodoflowoceanlevelisthoughttohavecreatedalandbridgebetweenAsiaandNorthAmericaacrosswhatisnowtheBeringStrait—theoriginalpassagewayforhumansandothermammalstotheNewWorld.Duringthesameicyinterval,therewasnoEnglishChannel,asadryvalleyconnectedtheBritishIslestoFrance.Bycontrast,attimesofmaximumwarming,whenglacierslargelydisappearandicecapsrecede,sealevelshaverepeatedlyrisenbyasmuchasthreehundredfeethigherthantoday,submerginghundredsofthousandsofsquaremilesofcoastalareasaroundtheglobe.Millerandhiscolleaguesidentifiedmorethanahundredcyclesofglacial
advanceandretreatinthepastninemillionyears,withatleastadozensucheventsinjustthepastmillionyearsalone—changesthatpointtowildswingsof
asmuchassixhundredfeetinoceanlevel.Whiledetailsmayvaryfromcycletocycle,theseeventsareclearlyperiodicandarerelatedtoso-calledMilankovitchcycles,namedfortheSerbianastrophysicistMilutinMilankovitch,whodiscoveredthemaboutacenturyago.Herealizedthatwell-knownvariationsinEarth’sorbitaroundtheSun,includingourplanet’stilt,itsellipticalorbit,andaslightwobbleinitsrotationaxis,imposeperiodsofclimatechangewithintervalsofroughlytwenty,forty-one,andonehundredthousandyears.ThesevariationsallaffecttheamountofsunlighthittingEarthandthusexertaprofoundeffectonglobalclimate.Sowhatofthenextfiftythousandyears?Wecanbeconfidentthatsealevels
willcontinuetovarydramatically,withmanymorerisesandfalls.Attimes,verypossiblyoverthenexttwentythousandyears,icecapswillgrow,glacierswilladvance,andsealevelwilldecreasebytwohundredfeetormore—alevelthathasoccurredatleasteighttimesoverthepastmillionyears.Suchachangewillhavepowerfuleffectsontheworld’scoastlines.TheEastCoastoftheUnitedStateswillshiftmanymilestotheeast,astheshallowcontinentalslopeisexposed.AllofthegreatEastCoastharbors,fromBostontoMiami,willbecomehighanddryinlandcities.AnewiceandlandbridgewillconnectAlaskatoRussia,andtheBritishIslesmayonceagainbecomeapartofmainlandEurope.Meanwhiletheworld’smostproductivefishinggroundsalongthecontinentalshelveswillbecomedryland.Inthecaseofsealevel,whatgoesdownmustalsogoup.It’sverypossible,
somewouldsayverylikely,thatoverthenextthousandyearssealevelwillincreasebyahundredfeetormore.Suchariseintheoceans,arathermodestonebygeologicalstandards,wouldrenderthemapoftheUnitedStatesallbutunrecognizable.Ahundred-footriseinsealevelwouldinundatemuchofthecoastalplainoftheEastCoast,shiftingshorelinesuptoahundredmileswestward.Allthemajorcoastalcities—Boston,NewYork,Philadelphia,Wilmington,Baltimore,Washington,Charleston,Savannah,Jacksonville,Miami,andmore—willbesubmerged.LosAngeles,SanFrancisco,SanDiego,andSeattlewillalsodisappearbeneaththewaves.AlmostallofFloridawillbegone,itsdistinctivepeninsuladrownedinashallowsea.MostofDelawareandLouisianawillbeunderwater,too.Inotherpartsoftheworld,consequencesofahundred-footriseinsealevelwillbeevenmoredevastating.Entirecountries—theNetherlands,Bangladesh,theMaldives—willbenomore.Thegeologicalrecordisunambiguous:thesechangeswillhappenagain.And
ifEarthiswarmingrapidly,asmostexpertssuspect,thenwaterswillrisesoon,perhapsasrapidlyasafootperdecade.Merethermalexpansionofoceanwater
duringextendedperiodsofglobalwarmingcanincreaseaveragesealevelsbyuptotenfeet.Suchchangeswillchallengehumansociety,forsure,butwillhavelittleeffectonEarth.Afterall,itwouldn’tbetheendoftheworld.Justourworld.
Warming:TheNextHundredYears
Mostofusdon’tcaresomuchaboutafewbillionyearsinthefuture,orafewmillionyears,orevenathousandyears.Mostofusfocusonshort-termconcerns:HowwillIpayformykid’scollegeintenyears?WillIgetthatpromotionnextyear?Willthestockmarketgoupnextweek?What’sfordinner?Inthatcontext,wehavelittletoworryabout.Barringanunforeseen
cataclysm,Earthnextyear,nextdecade,willseemprettymuchasitdoestoday.Anydifferencesfromoneyeartothenextwillprobablybetoosmalltonotice,evenifweexperienceanuncharacteristicallyhotsummer,endureacrop-witheringdrought,orsufferanunusuallyviolentstorm.WhatisabsolutelycertainisthatEarthwillcontinuetochange.Present
indicatorspointtoacomingepisodeofglobalwarmingandmeltingglaciers,mostprobablyinfluencedandacceleratedbyhumanactivities.Overthenexthundredyears,theconsequencesofthiswarmingwillaffectmanypeopleinmanydifferentways.Inthesummerof2007,IparticipatedinaKavliFuturesSymposiuminremote
Ilulissat,afishingvillageonthewestcoastofGreenland,ashortdistancesouthoftheArcticCircle.Itwasafortunatechoicefordiscussingthefuture,aschangeswereoccurringimmediatelyoutsideourconferencecenteratthecozyArcticHotel.Forathousandyears,theharbor,locatednearthecalvingfrontofthemightyIlulissatGlacier,hadservedasarichfishingground.Forathousandyears,thefishermenresortedtoicefishinginthewinterastheharborfrozesolideveryyear.Untilthenewmillennium,thatis.In2000,forthefirsttime(atleastinathousandyearsoforalhistory),theharborwasopenandunfrozen.Themightyglacier,aUnitedNationsheritagesite,hasbeenrecedingatanastonishingrate—almostsixmilesinthreeyearsaftermanydecadesofstability.Anotherchange:forathousandyearsIlulissatandnearbynativevillageshadbeenfreeofinsectpests,butin2007andallsubsequentyears,aninfestationofmosquitoesandblackfliesarrivedinAugust.Theseareanecdotes,tobesure,butsotooaretheyunambiguousharbingersofsignificant,inexorablechange.Acrosstheglobe,similarchangesareoccurring.WatermenontheChesapeake
Bayreportconsistentlyhigherhightidesthanafewdecadesago.YearbyyearthenorthernSaharaDesertispushingeverfarthernorth,turningoncefertileMoroccanfarmlandintodust.Antarcticiceshelvesaremeltingandbreakingoffatincreasedrates.Averageglobalairandwatertemperaturesareontherise.It’sallpartofaconsistentpatternofwarming—apatternthatEarthhasexperiencedcountlesstimesinthepastandwillexperiencecountlesstimesinthefuture.Warmingcanhaveother,sometimesparadoxicaleffects.TheGulfStream,the
greatoceancurrentthatcarrieswarmwaterfromtheEquatortotheNorthAtlantic,ispoweredbythestrongtemperaturedifferencesbetweentheEquatorandhigherlatitudes.Ifglobalwarmingreducesthattemperaturecontrast,assomeclimatemodelssuggest,thentheGulfStreammayweakenorevenstop.Ironically,animmediateconsequencewouldbetomaketheBritishIslesandnorthernEurope,whereclimateismoderatedbytheGulfStream,muchcolderthanitistoday.Otheroceancurrents—forexample,thosefromtheIndianOceantotheSouthAtlanticpasttheHornofAfrica—wouldbesimilarlyaffectedandmightcauseasimilarshiftinthemildSouthAfricanclimateorachangeinthemonsoonrainsthatkeeppartsofAsiawetandfertile.Asicemelts,theseasrise.Somesoberprojectionssuggestincreasesofas
muchastwoorthreefeetinthenextcentury,thoughmuchfasterincreasesofseveralinchesperdecademayhaveoccurredfromtimetotimeaccordingtotherecentrockrecord.SuchaseachangewillaffectmanycoastalresidentsaroundtheworldandmaycauseheadachesforcivilengineersandbeachfrontpropertyownersfromMainetoFlorida,butafewfeetisprobablymanageableinmostpopulatedcoastalareas.Forawhile,foragenerationortwo,mostresidentsreallywon’thavemuchtoworryaboutwhenitcomestoencroachingseawater.Someanimalandplantspeciesmaynotfaresowell.Thelossofnorthern
polaricewillreducethefamiliarhabitatofpolarbears,addingchallengestoapopulationthatappearstobeshrinking.Arapidshiftofclimatezonestowardthepolesmayalsostressmanyotherthreatenedspecies,notablybirds,whichareparticularlysusceptibletoalterationsintheirmigratorynestingandfeedingareas.Onerecentreportestimatedthatanaverageglobaltemperatureriseofjustacoupleofdegrees,wellwithinpredictionsofsomeclimatemodelsforthenextcentury,couldtriggerextinctionratesamongbirdsapproaching40percentinEuropeandexceeding70percentinthelushrainforestsofnortheasternAustralia.Anothersoberinginternationalreportfoundthatnearlyoneinthreeoftheapproximatelysixthousandknownspeciesoffrogs,toads,andsalamandersissimilarlyendangered,mainlybytherapidwarmth-drivenexpansionofadeadlyamphibianfungaldisease.Whateverelsetranspiresoverthecoming
century,itdoesappearthatweareenteringatimeofacceleratedextinction.Certaintransformativeeventsofthenextcentury—someguaranteed,others
highlypossible—willbeinstantaneous:thedisruptionofagreatearthquake,theeruptionofamegavolcano,ortheimpactofamile-wideasteroid.Humansocietiestendtobeillpreparedfortheonce-in-a-centurystormorearthquake,muchlessforthetrulycatastrophiconce-in-a-thousand-yearsdisaster.AswereadEarth’sstory,weseethattheseshockingeventsarethenorm,inevitable,apartofthecontinuumofourplanet’shistory.Nevertheless,webuildourcitiesontheflanksofactivevolcanoesandonsomeofEarth’smostactivefaultzones,hopingthatinourtimewewilldodgethetectonicbullet(ifnotthecosmicmissile).Inbetweentheveryslowandveryfastarefluctuatinggeologicalprocesses
thatnormallytakehundredsorthousandsofyears—shiftsinclimates,sealevel,andecosystemsthatareusuallynoticeableonlyoverthespanofseveralgenerations.Theratesofsuchchanges,notthechangesthemselves,shouldbeourbiggestconcern.Forclimate,sealevel,andecosystemscanreachtippingpoints.Pushedtoofar,positivefeedbackloopscankickin.Whatnormallytakesathousandyearscouldtranspireinadecadeortwo.Complacencyiseasy,especiallywhenbolsteredbyflawedreadingsofthe
rocks.Forawhile,until2010,concernsaboutmoderntimesweresomewhatassuagedbyongoingstudiesofaparallelscenario56millionyearsago—oneofthemassextinctionsthatdramaticallyaffectedtheearlyevolutionandspreadofmammals.Thisharshevent,calledthePaleocene-EoceneThermalMaximum,orPETMforshort,sawtherelativelysuddendisappearanceofthousandsofspecies.ThePETMisimportantforourtimebecauseitisthemostrapidwell-documentedtemperatureshiftinEarthhistory.Arelativelyfastvolcano-inducedincreaseinconcentrationsofatmosphericcarbondioxideandmethane,thetwinheat-trappinggasesofthegreenhouseeffect,causedmorethanathousandyearsofpositivefeedbacksandacorrespondingepisodeofmodestglobalwarming.SomeresearcherssawthePETMasacloseparalleltotoday’sevents,bad,tobesure—withanalmost10-degreeriseinglobaltemperatures,arapidriseinsealevel,acidificationoftheoceans,andsignificantpolewardshiftsinecosystems—butnotsocatastrophicastothreatenthesurvivalofmostanimalsandplants.ShockingrecentdiscoveriesbyPennStategeologistLeeKumpandhis
colleaguesmayhavedestroyedanylingeringcauseforoptimism.In2008Kump’steamwasgivenaccesstoadrillcorefromNorwaythatpreservedtheentireintervalofthePETM—sedimentaryrocksthat,layerbylayer,documentedinexquisitedetailtheratesofchangeofatmosphericcarbondioxideandclimate.
ThebadnewsisthatthePETM—whathasformorethanadecadebeenthoughttobethemostrapidclimatedisruptioninEarth’shistory—wastriggeredbyatmosphericchangeslessthanatenththeintensityofwhatishappeningtoday.GlobalchangesinatmosphericcompositionandaveragetemperaturethattookmorethanathousandyearsduringthePETMextinctionscenariohavebeensurpassedinjustthelasthundredyears,ashumanshaveburnedimmensequantitiesofcarbon-richfuels.Thereisnoknownprecedentforsuchrapidchange,andnooneknowshow
Earthwillrespond.AtanAugust2011meetingofthreethousandgeochemistsinPrague,themoodamongclimatespecialistsfamiliarwiththenewPETMdatawassober.Thoughpublicpredictionsbythesecautiousexpertshaveremainedmeasured,thecommentsIheardoverabeerwerepessimistic,frightening.Ifgreenhousegasconcentrationsrisetoorapidly,noknownmechanismscanabsorbtheexcess.Willwarmingtriggeramassivemethanerelease,withallthepositivefeedbacksthatthatscenariomightentail?Willsealevelquicklyrisehundredsoffeet,asithassomanytimesinthepast?Weareventuringintoterraincognita—conductinganill-conceivedglobal-scaleexperimentonEarthperhapsunlikeanythathascomebefore.Whatthetestimonyoftherocksdoesrevealisthat,asresilientaslifeitselfis
andalwayswillbe,thebiosphereexperiencesgreatstressattippingpoints,duringtimesofsuddenclimateshifts.Biologicalproductivity,includingagriculturalproductivity,willmostcertainlyfallprecipitouslyforatime.Undersuchdynamicconditions,largeanimalslikeourselveswillpaythedearestprice.Thecoevolutionofrocksandlifewillcontinueunabated,tobesure,buthumanity’sroleinthatmultibillion-yearsagaremainsunknowable.Havewealreadyreachedsuchatippingpoint?Probablynotinthisdecade,
maybenotinourlifetimes.Butthat’sthethingabouttippingpoints—youcanneverbesureyou’reatoneuntilit’shappened.Thehousingbubblebursts.ThepopulaceofEgyptrevolts.Thestockmarketcrashes.Werealizewhat’shappeningonlyinretrospect,whenit’stoolatetorestorethestatusquo.NotthattherehaseverbeenanysuchthinginthestoryofEarth.
Epilogue
Climateschange,sealevelschange,therainsandwindschange,thedistributionsoflifeacrossthesurfaceandwithintheseaschange.Rocksandlifecontinuetocoevolve,astheyhaveforbillionsofyears.HumanscannomorestopglobalchangethanwecanalterEarth’strajectorythroughthecosmos.NorcanwedestroylifeonEarth,norevenstopitsinexorableevolution.Life
hasensconceditselfineverynicheoftheglobe.LifeaboundsinfrozenArcticice,inboilingacidpools,inrock-encasedporesmilesunderground,andonwind-bornedustspecksmilesabovetheground.Whateverstupiditywemightinflictuponourselves—whetherwecauseglobaltemperaturestoriseadozendegrees,orpoisonourairandwater,ordecimatefishstocksinthesea,orevenunleashourcollectivenucleararsenalsinaglobalholocaust—lifewillgoon.Humansmaydisappearforever,butmicroscopiclifewillscarcelymissabeat.Forbillionsofyearstocome,EarthwillcontinuetowhirldailyonitsaxisinitsannualodysseyabouttheSun.Forbillionsofyears,ourswillstillbealivingplanetofblueoceans,greenlands,andswirlingwhiteclouds.Fromspace,Earthwillbenolessbeautifulthanitistoday,humansorno.Makenomistake.Therecannotbetheslightestdoubtthathumanactivitiesof
thepastcenturyhaveinitiateddramaticchangesinatmosphericcomposition,andchangesinclimatemustfollowassurelyasthelawsofphysics.Theconcentrationsofcarbondioxideandmethane,bothefficientgreenhousegases,haveclimbedatratesunmatchedinhundredsofmillionsofyears.Suchchangesareamplifiedbyourrapiddenudingoftropicalrainforests,ourefficientconsumptionofsealife,andourincessantdestructionofhabitatsacrosstheglobe.Thankstoouractions,Earthwillgethotter,icewillmelt,oceanswillrise.Butthat’snothingnewforEarth.Why,then,shouldwecareifhumanactionsspeeduptheprocessofchange?Forone,imaginethesufferingunleashedonaworldwheresealifeundergoes
massdeathoragriculturaloutputissuddenlyhalved.Whatofthemillionsquaremilesofthemostproductivefarmlandthatwillbeflooded,theseaportsdrowned,thelivelihoodslost?Imaginethesufferingofabilliondisplacedandhomelesshumans.Ifwecaretoact,itissurelynotinorderto“savetheplanet.”Earth,after
survivingmorethan4.5billionyearsofconstant,extravagantchange,doesn’tneedsaving.Perhapssomeethicistswillfocustheireffortsinsteadonsavingthewhalesorpolarbears,fortheirlosswouldbepermanentandundeniablysad.Buteventheextinctionofthesegreatbeasts,orofelephantsorpandasorrhinosoramillionotherspeciesbothcharismaticandmundane,isbutatemporarylosstoEarth.Newgreatandwondrousbeastswillinevitablyevolve,inageologicalmoment,tofillthosevacantniches—perhapsinnomorethanamillionyears.Largemammalslikeourselvesmaysuffermassextinction,butothervertebrates,maybethebirds,willtakeourplace.Maybepenguins,whichhaverecentlybeenshowntoevolveparticularlyrapidly,willmorphandradiatetofilltheniches:whalelikepenguins,tigerlikepenguins,andhorselikepenguins.Maybethepenguinswilldevelopbigbrainsandgraspingfingers.Whateverwedo,Earthwillcontinuetobeavariegatedlivingworld.No,ifwechoosetoworry,itshouldbefirstandforemostforourhuman
family,foritiswewhoaremostatrisk.Earthisagreatwinnowerofwasteanderror.Lifeinitsgrandeurwillgoon,buthumansociety,atleastinitspresentprofligatemode,maynotmakethecut.Wehumanshavethesoberingpotential,througheitherourthoughtlessactionsorourequallythoughtlessinaction,toheapuntoldsufferinganddestructionuponourownspecies.Aswecontinuetoalterourhomeworld—our“palebluedot,”toquoteCarlSagan—atafasterandfasterrate,thetimeremainingforeffectiveactionslipsaway.Earthisnotsilentonthispoint;itsstoryistheretoreadintherichrecordof
therocks.Forthousandsofyears,wehavebeenwiseenoughtoseekoutthestoryofEarthinanefforttoknowourhome.Letushopewelearnitslessonsintime.
Acknowledgments
Dozensoffriendsandcolleagueshavecontributedtotheconceptanddevelopmentofthisvolume.Iamespeciallyindebtedtofourscientistswhoembracedtheconceptofmineralevolutionin2008,initsearlystages.MineralogistRobertDowns,alongtimefriendandcollaborator,providedhisconsiderableexpertiseonthenatureanddistributionofminerals.PetrologistJohnFerryofJohnsHopkinsUniversity,whomI’veknownsinceourdaysingraduateschool,contributedasophisticatedtheoreticalframeworkforthenewapproachtomineralogy.GeobiologistDominicPapineau,aformerGeophysicalLaboratorypostdoctoralfellowandnowonthefacultyofBostonCollege,wasoneoftheearliestcontributorstoandmostperceptiveandconstructivecriticsofthemineralevolutionidea,inspiteoftheobjectionsofhisotherCarnegiementors.GeochemistDimitriSverjenskyofJohnsHopkinsUniversity,myclosestprofessionalcolleagueforthepastseveralyears,broughtawealthofideasandinsightstotheconceptualdevelopmentofmineralevolution.Thesefourfriendsweretheearliestchampionsofthemineralevolutionidea,andallhavebeenarticulateandeffectivecollaborators.Thisbookwouldnothavebeenpossiblewithouttheirhelp.WegainedinvaluableinsightsfromPrecambriangeologistWouterBleakerof
theGeologicalSurveyofCanada,meteoriteexpertTimothyMcCoyoftheSmithsonianInstitution,andbiomineralogyauthorityHexiongYangoftheUniversityofArizona.Theyjoinedintheinitialpublicationoftheseideas.SubsequentcollaborationswithDavidAzzolini,AndreyBekker,DavidBish,RodneyEwing,JamesFarquhar,JoshuaGolden,AndrewKnoll,MelissaMcMillan,JolyonRalph,andJohnValleyhaveamplifiedtheconceptandledusintoexcitingnewdirections.IamespeciallyindebtedtoEdwardGrew,whosestudiesoftheevolutionofthemineralsoftherareelementsberylliumandboronhavetakenthefieldtoanewquantitativelevel.Thisbookcouldnothavebeenundertakenwereitnotformymanycolleagues
intheorigins-of-lifefield.SpecialthanksgotoHendersonJamesCleaves,GeorgeCody,DavidDeamer,CharleneEstrada,CarolineJonsson,ChristopherJonsson,NamheyLee,KatarynaKlochko,ShoheiOno,andAdrianVillegas-Jimenez.IhavealsobenefitedimmeasurablyfromcollaborationswithHarvard
paleontologistAndrewKnollandseveralofhisassociates,notablyCharlesKevinBoyceandNoraNoffke,aswellasNeilGupta.IhavereceivedunflaggingsupportfrommycolleaguesConnieBertka,
AndreaMagnum,andLaurenCryanattheDeepCarbonObservatory,aswellasJesseAusubeloftheAlfredP.SloanFoundation,whichhasprovidedgeneroussupporttolaunchthisglobaleffort.Theyhavebornemuchofthebruntofmydistractionswhileworkingonthisbook.MycolleaguesatGeorgeMasonUniversity,especiallyRichardDiecchio,HaroldMorowitz,andJamesTrefil,haveengagedinnumerousstimulatingdiscussionsthroughoutthedevelopmentofthemineralevolutionconcept.IamalsogratefultoRussellHemley,directoroftheGeophysicalLaboratory,whohasofferedunqualifiedsupportandencouragementforthisproject.Manyscientistsofferedinvaluableadviceandinformationduringtheresearch
forthisbook.IthankRobertBlankenship,AlanBoss,JochenBrocks,DonaldCanfield,LindaElkins-Tanton,ErikHauri,LindaKah,LynnMargulis,KenMiller,LarryNittler,PeterOlson,JohnRogers,HendrickSchatz,ScottShepard,SteveShirey,RogerSummons,andMartinvanKranendonk.IamgratefultotheVikingeditorialandproductionteamfortheirenthusiasm
andprofessionalisminthedevelopmentofthisbook.AlessandraLusardifirstchampionedthebookandprovidedcriticaladviceinitsdevelopmentalphase.LizVanHooseprovidedinvaluableeditorialguidanceandusheredthemanuscripttoitsfinalstatewithcreativity,efficiency,andgoodhumor.I’dalsoliketothankBruceGiffordsandJanetBiehl.TheoriginalideaforthisbookwasdevelopedincollaborationwithEric
LupferofWilliamMorrisEndeavor,whohasprovidedthoughtfulanalysis,timelyadvice,andconstantsupportateverystageofthisproject.Iammuchinhisdebt.MargaretHazenhashelpedmethroughoutthedevelopmentofthemineral
evolutionidea,fromlongbeforeitsfirstarticulationonDecember6,2006,toitspresentationinthisvolume.Herkeeneyeandinfectiousenthusiasminthefield,herinformedadviceandincisivecritiquesonallmymanuscripts,andherspontaneousjoyandgentlesympathyinresponsetothesuccessesandfailuresofanintenseresearchcareerhavesustainedthiseffort.
Indexabiogenicpetroleum,vs.biological,217–18
abyssalplains,113accretion:ofgraniteislands,124ofplanetesimals,19,124ofplanets,54
aceticacid,143achondrites,19,20,24acidrain,251acids,83Adam(biblical),29adaptation,233
Africa,186,191,195,235asoriginofhumanbeings,255AfricanPlate,121–22agriculture,90,109alanine,141albedo,211–12Aldrin,Buzz,34algae:green,152,155,198,206,213,232,236,242seealsomicrobialmatsAlps,114,174
aluminum,11,12,57–60,61,66,69,72,103,127inplagioclasefeldspar,711
Amasia,seeNovopangaeaAmazonia,210amber,248AmericanMineralogist,4–5,201American-Russianrivalry,217–18aminoacids,132,135,136,141,152aslefthanded,140
ammonia,12,84,141,199ammonites,240,254fossilized,252–53
amphibians,247–48,278
amphibole,59,104Anbar,Ariel,178–79Andes,114angularmomentum,47,50–51animals,227–31andedibleplants,247endangered,257,278first,227–30,235–37hardshelled,236–37,238land,236large,237,249,252–54softbodied,236,238,240
anorthite,66–67,71–72Antarctica,191,195,235,261glacialdrillcoresfrom,2,28icein,16,91,277matchingcoastlineof,109meteoritesin,15–16,19,20
AntarcticPlate,121antennasystems,161Anti-AtlasMountains,238Apexchert,Australia,166–67,168,172Apollomissions,32,33–40,66,87,266Apollo12LunarSamplePreliminaryInvestigationTeam,35Apophis,263AppalachianMountains,2–3,28,234aquifers,90Arabiandesert,17ArcheanEon:biospherein,149,151,152
ArcheanEon(cont.)fossilsfrom,165iceagein,224
ArcticHotel,276argon,29Armstrong,Neil,34arsenic,11asbestos,57ash,72–73Asia,235
AsteroidBelt,19,23asteroids,9,14,23,36,54,61,84,135,149,271incollisionwithEarth,253,254,257,263–65,278incontinentformationtheory,106–7deflectionstrategiesfor,265asmoons,32seealsodinosaurs,extinctionofastronauts,survivalof,266–67
AtlanticOcean,113–18correlatingcoastlinesof,108–10,111expansionof,115–16,118,119,120,184,232,261–62formationof,235,253riftvalleyof,114–17
atmosphere,5,80,155,206,210,224,230,236,253,262early,94–95,103,190,197future,260ofMars,25ofTitan,24ofTriton,24ofVenus,25seealsoglobalwarming;GreatOxidationEvent;greenhouseeffectatomic
number,38atoms,6,8,11,56inheatproduction,62
Australia,17,189,195,235,261cratonsin,169–70,186fossilsearchesin,169,172,173,174,176matchingcoastlineof,109
AustralianNationalUniversity,169autocatalyticnetwork,144,146Avzyanformation,198azurite,177
Baltica,190–91,195,210bandedironformations(BIFs),156,197,199,206,223,225Barghoorn,Elso,172basalt,65,99,104,271inEarth’scrust,1–3,71–76,179magneticsignalof,116meltingof,103
inmeteorites,20inoceanfloor,115–16,118,260inoriginofcontinents,124inrockyplanets,14,71,127subductionof,124–25involcaniceruptions,72–73weatheringof,99
bases,83–84beaches,79,103,106,126microbialmatson,172–76
Bean,Alan,34bedrock,212Belt-Purcellsupergroup,194,196BeringStrait,274beryl,202–3beryllium,78,103,183,202–3graphof,203
BigBang,5,6,7–8,10,12,54timingof,29
BigCottonwoodCanyon,Utah,48“bigsix”elements,56–60,65,79,102BigThwack,31,42–52,53,56,66,95,98binary(double-star)systems,22biogenesis,131–32biologicalpetroleum,vs.abiogenic,217–18biomass,burialof,210–11,227biomineralization,237–38biomolecules,experimentswith,132–36,140biosphere,seecoevolution,ofgeosphereandbiospherebirds,54,278assurvivingdinosaurs,254
birthdefects,139blackcherts,172seealsoApexchert;GunflintchertblackEarth,53–76
blackflies,276blackholes,252blackshale,170,172,173,178–79,187,241blacksmokers,inoriginsoflife,133–34,136Blankenship,Robert,161–62blood,ironin,58,59
blueEarth,77–101“bomb”(experimentalpetrologydevice),65,92“bombs,”volcanic,72“boring”billion,181–205,206,222DarkAgesanalogyfor,182feedbackloopsin,208mineralogyof,200–204asperiodofslowchange,184,195,205
boron,78,183,202Boyce,Kevin,243–45brachiopods,238,241,249Brasier,Martin,vs.Schopf,166–67Brazil,191,195BritishMuseum,166Brocks,Jochen,170–71bryozoa,238,249Buick,Roger,181,199BurgessShale,2240butane,150butterflies,229
Cabeuscrater,88calcite,216calcium,11,12,56,57–60,66,69,72,83,99,103,126,127,236California,Universityof:atBerkeley,160atLosAngeles,165,220atRiverside,226atSanDiego,157
CaliforniaCurrent,90Callisto,84,127Cambrianboundary,26Cambrian“explosion,”27,237,254CambrianPeriod,238Cameron,Alastair,44Canada,fossilsearchesin,169CanadianRockies,115,237canali,85Canfield,Donald,198,199
CanfieldOcean,198CanterburyCathedral,265capturetheory,33,39–40carbon,11,12,28,38,79–80,127,149,151,255inbiomass,210–11,227inclimateandenvironment,219deep,219–21lightvs.heavy,220asmostimportantelement,219organicvs.inorganic,176inplantfossils,245–46recyclingof,80instars,9–10asvitaltolife,9,130–32,142
carbonate,83,126,151carbondioxide,12,25,78,82,99,100,126,190,224,262,279,282inphotosynthesis,153,159
carbon-14,160carbonicacid,99,190CarboniferousPeriod,247,248,251carbonmonoxide,12CarnegieInstitution,37,64–65,88,110,135,138cells,228–29self-assemblyof,137–39
cephalopods,fossilized,252–53cerium,155Challenger,HMS,112Chamberlin,RollinT.,111Charon,iceon,84chemicalbonding,56,59chemicalreactions,first,11–13ChesapeakeBay,277fossilsof,26–27
China,191,195,198,230chirality,139–41chlorine,78,83chlorofluorocarbons(CFCs),158chlorophyll,161chloroplasts,228
chondrites,15–21,24,25,28asrecordofvolatiles,79–80
chondrules,15–16,183Choubertella,239chromium,37–38,58cinnabar,204citricacidcycle,143–44,146clams,238,249clay,21,38,91,93,106,190,227,246onMars,86–87
Clementinemission,87climate,210–12,222currentchangesin,231future,257moderatefluctuationsin,232unstable,212–15
Clinton,Bill,166clouds,90,100–101co-accretiontheory,33,39coal,241,248coastlines,alterationsin,268,274–75,277seealsocontinentalcorrelations;continentalshelvesCody,George,245
coevolution,ofgeosphereandbiosphere,3–6,131,148,151,180,226–27,233,246,251,256,269,280,281–83
author’sviewson,5,138–39,177–78,201–2Collins,Mike,34Columbia(Nena;Nuna;Hudsonland),191–92,194,196,203,204comets,14,84,149commonalityofspin,14compositecratons,187conduction,61–62,63,122Congocraton,209Conrad,Charles,Jr.,33–34continentalcorrelations:coastline,108–10,113,114,196hypothesestoexplain,111seealsoplatetectonics“continentaldrift,”109
continentalshelves,251,253,260continents,75,98–99,232
asteroidimpacttheoryof,106–7coastlinecorrelationsof,108–10,113,114,196cratonsasfoundationsof,186–87driftingof,seeplatetectonicsgraniteasfoundationfor,102,103riseof,105,124–26
convection,62–63,107,118,122,123,255convectioncells,123,125convergentplateboundaries,191copper,11,156,177coralreefs,asevidenceofshortdays,48corals,238,249fossilized,3,48,241,272
core:ofEarth,35,38,44,45,58,68,70,94,102ofMoonvs.Earth,38,45ofplanetesimals,19,20ofplanets,60
core-mantleboundary,68–70,271corundum,12cosmicclumping,8–9cosmicnovelty,10–11cosmicrays,70,86,162cosmochemistry,55–56Cosmos,265cratons,186–88worldwidedistributionof,186,190–91,209–10
Creationism,29crinoids,241,249crust:ofEarth,53–76,102–26,150,194;seealsobasalt;graniteoceanic,113,119–
21,123crystallization,5ofEarth’scrust,64–67
crystals,57formationof,12,64asrecordofoceanformation,95–98
Cubanmissilecrisis,120cyanobacteria,166,170cyclotrons,160
D″(Ddoubleprime)layer,69–70Daguinaspis,239Darwin,Charles,33,133Darwin,GeorgeHoward,33,40,108Deamer,David,137,142DeepCarbonObservatory(DCO),219–21deep-Earthexperiments,216“deephotbiosphere,”218–19deeptime,25–30deepwatercycle,91–99Deimos,32deserts,3meteoritesin,17–19
deuterium,229Devonianperiod,48diabase,73diamondanvilcell,92–93,216diamonds,5,12diatoms,161dinosaurs,243,252–54extinctionof,26,27,52,54,254–55,264,271
dip,185disk,inSolarSystemformation,13–14,15divalentiron,58–59DNA,138,144,152,170,225,228Dopplershift(redshift),28–29Doushantuoformation,230,237drillcores,2,169,279dunite,66,67dust,cosmic,13DuToit,James,110
Earth:ageof,6,25–30,31,53,77,102,127,154,181,206,232,257asalteredbylife,151,154,276–80,281–83collisionswith,31–32,42–43,54,61;seealsoTheia-Earthcollisioncooling
of,seeglobalcoolingcoreof,35,38,44,45,58,68,70,102crustof,53–76,102–26;seealsobasalt;granitedeterminingtimescaleof,27–
28
diversificationof,75,78–79,103dominantelementsof,56–60dynamicnatureof,2–6,128,183–84,193,205,206,233,257–80formationof,7–30,31–32,55futureof,257–80,281–83in“Goldilocks”zone,25habitabilityof,25,259housemetaphorfor,207–8asindifferenttohumanlife,256,268–69,275,280interconnectedsystemsof,207interiorof,62–63,64,67–70,125internalpressureof,68,70internaltemperaturesof,70,75layeredstructureof,58,68living,150–52magneticfieldof,seemagneticfieldmantleof,66–67,68,72,102Moon’sdistancefrom,46,48Moonvs.,38,41orbitof,47–48,274projectedendoflifeon,22,260rotationof,47–48,49,50–51,75sevencoretruthsof,6sizeof,42solidificationof,51;seealsocrystallizationsurfaceof,74–76,98–99,178–79Theia’scollisionwith,seeTheia-Earthcollisionasthree-dimensional,6tiltof,45transformativepowersof,256uniquenessof,127–28universebefore,7–13warmingof,seeglobalwarmingwateron,25,38,87,89–90seealsoEarthhistoryEarth-crossingasteroids,263–64
Earthhistory:blackphaseof,53–76bluephaseof,77–101“boring”billionphaseof,181–205grayphaseof,102–26greenphaseof,232–56predictionoffuturebasedon,3,260,272redphaseof,254–80
asrevealedincratons,186–87asrevealedinmeteorites,15–18asrevealedinrocks,2–3,4,6,77,283shorterdaysin,47–48,75stasisin,6,181–82,184,192–95,200–201,204,205,208transformationaleventsin,6,31,52,53–54,153,183–84,212,249–52,253,
278;seealsospecificeventswhitephaseof,206–31seealsocoevolutionofgeosphereandbiosphereEarth-Moonsystem,47
earthquakedetection,120–21earthquakes,256,270,278California,122depthof,120–21future,270,278undersea,115,120–21
Earthrisephoto,1,6Earthsystems,asinterconnected,6EastAfrica,109EastPacific,ridgesunder,115eclipses,49EdiacaranPeriod,237,238asanimaldominated,229–30
eggs,fossilizedembryosof,230electricity,conductorsof,60electronmicroprobe,38,244electrons,8,11,54,56,150elements:“bigsix,”56–60,65,71,79,102dominantEarth,56–60oflife,10–11,130periodictableof,217asproducedinstars,9–11rare,11,78,103,202
elements(cont.)ofrocks,11spaceasseededby,10–12seealsocosmochemistryelephants,254,282
endosymbiosis,228–29vs.Darwinianevolution,228
energy,8,129
forlife,130,151,152,162,163,199Eofallotaspis,238Eratosthenes,1–2,3,6erosion,79,189inmountainforms,3,28,234
ethanol,64Euclid,1,122eukaryotes,170,206EurasianPlate,121Europa,23waterandiceon,84,127
Europe,191EuropeanSpaceAgency,24evaporation,90evaporiteminerals,87Everest,Mount,108,237evolution,142accelerationof,233Darwinian,130,132,146,228ofhumanbeings,255oflivingthings,125,151,233,247–48mineral,177
Ewing,Maurice,115exobiology,135ExobiologyProgram,NASA,129expanding-Earthmodel,111,120experimentalpetrology,64–65extinctions,249–52extraterrestriallife,searchfor,23,24,25,84,85,129,134–35,161,218eyes,237,241Eyjafjallajökull,269
facies,241faintSunparadox,99–101,224Farquhar,James,157feedbackloops,207–8asacceleratingchange,279inclimatemoderation,209–12,259inglobalwarming,208,215–17,231,280
methanein,221inmicrobesandclay,227insnowballearth,213–14
feldspar,57,66,96,126ingranite,103
ferns,fossilized,241firstlight,8–10fish,247fissiontheory,32–33,40,108Flagstaff,Ariz.,observatoryin,85floodbasalts,271Fogel,Marilyn,175–76,245foraminifera(forams),273forests,emergenceof,246,248fossils:ofanimals,230incarbonatecliffs,237chemical,168incontinentalcorrelations,110,125controversyoveroldest,166–67indeterminingageofEarth,26–27asevidenceforphotosynthesis,164–67fish,247frog,240ofhard-shelledanimals,236–39magnetismof,185Martian,165–66microbial,149,166–71multicellular,228atMuslimshrine,239Myocene,268oldest,165–67,168,170–73plant,242–45asrecordofEarth’shistory,241,249asrecordoftransitiontoterrestriallife,242,247–48smallest,168–71,198ofsoft-bodiedanimals,240sparseevidenceoflossin,250,254spore,242
trilobites,238–39,241,243,249–50,252,254frogs,278fossilized,240
fungus,245,246futureEarth,257–80asdesertworld,259–60endgameof,258
gabbro,73galaxies:formationof,13retreatingfromEarth,28–29
Ganymede,23Gaoyuzhuangformation,198garnet,57,58,59,91gasgiantplanets,22–24,102compositionof,14vs.rockyplanets,14–15
gasgiants,47,84Gaskiersglaciation,222,236genetics,asfoundationforlife,128–31,144geochemistry,vs.organicchemistry,138GeographicalSocietyofAmericaBulletin,118geologicalcalendars,27geologicalprocesses,gradual,2–3geology,3,26,109GeophysicalLaboratory,CarnegieInstitution,37,64–65,175,216,219,225,
243,245geosphere,seecoevolution,ofgeosphereandbiospheregeysers,75,95glaciation,26,223–25,254,255seealsoiceages;snowballEarth;specificglaciations
glaciers,2,89,181,195,276Earth’shistoryrecordedin,222sedimentsin,212
globalcooling,54,60–63,71,78,95,151,211,273globalwarming,273–75,277anomaliesin,41–42feedbackloopin,208,215–17warningsabout,231,275–80,281–83
glucose,160gold,11,78,81,109Gold,Thomas(Tommy),218–19“Goldilocks”zone,Earthin,25Gondwana,234–35Gosse,Philip,29Gould,StephenJay,240granite,127asbuoyant,104–6,124compositionof,103–4inEarth’scrust,102–26,179formationof,103,125iceberganalogyof,105
graphite,asfirstmineral,12gravitationalclumping,44–45gravity,24,45innebulaformation,5inplanetesimalformation,21,25inSolarSystemformation,13,30instarformation,8–10seealsotidalbulge;tidesgrayEarth,102–26
greatapes,255GreatBarrierReef,Australia,3GreatDepression,112GreatDying,249,250–52,271causeof,250–51
GreatOxidationEvent,154–80,197,203,259rocksasevidenceof,155–58second,225–27skepticismabout,156third,248–49timeof,158,181
greenEarth,232–56fivechangesin,232–33
greenhouseeffect,279–80infaintSunparadox,100onVenus,84
greenhousegases,61,101,215,222,282asendoficeages,224
feedbackloopsin,208,211Greenland,191fossilsearchesin,169,176glaciersof,2,3,28,109
GreenRiverShalepreserve,Wyo.,2,27Grew,Ed,202–3Grotzinger,John,192–93groundwater:onEarth,89onMars,86
GulfStream,42,277Gunflintchert,172,198
HadeanEon,54–55,77Earth’ssurfacein,74–76,98–99life’sbeginningsin,132,149,151
hafnium,19Hales,Stephen,159half-life,28,96“handedness,”139–41hard-shelledanimals,26,27Hauri,Erik,88,93HawaiianIslands,3,42formationof,2volcanicactivityon,28,70,72,268
Hazen,Margee,240–41heat:inEarth’sinterior,60–63,70,74–76,100,122,255inmeteorites,19
heattransfer,61–63Heezen,Bruce,114–15,120helium,5,8,10,11,13,14,24,54,102onEarth,25,56inSun,22,258
heliumnucleus,9hematite,59,151high-pressureminerology,91–92Himalayas,108,114,234,235Hoffman,Paul,212–14,222–23Holmes,Arthur,105,106
hominids,255Homohabilis,255Homosapiens,254Hood,Mount,194hopanes,152,169–71,218hotspots,70,106hotsprings,75HudsonRiver,73humanbeings:asadaptableandresilient,255–56appearanceinNorthAmericaof,26,255,274commonprimateancestorof,155first,254–56nomadic,256possibleextinctionof,257,267,275,280technological,256
humpbackwhales,257Huygenslander,24hydrocarbons,218–21hydrogen,5,11,13,14,24,54,102,142,163onEarth,25,38,56,260lightvs.heavy,220instarformation,8–10inSun,22,258inwater,80–81,83,93
hydrogenbomb,9hydrogenmolecules,11–12hydroniumions,83,90,99hydrons,83hydroxylgroups,83,90
IapetusOcean,262ice,4,13,72,82Antarctic,16Arctic,4inEarth’sfuture,271onJupiter’smoons,23nearEquator,213atpoles,214
propertiesof,81–82seealsoglaciation;glaciersiceages,212–14,251,253,255future,257,273–74
icebergdetection,112icebergs,ingraniteanalogy,105icebridges,255icecaps,89Iceland,72,269–70ice-skating,82igneousrocks,36,96,188IlulissatGlacier,Greenland,276“InceptionoftheClayMineralFactory,The”(Kennedy),227“incompatibleelements,”69IndexFossilsofNorthAmerica(Shrock),239India,189,235matchingcoastlineof,109
IndianOcean,ridgesunder,115,119infantsurvival,coldtemperaturetheoryof,255infraredradiation,63,122,161infraredspectroscopy,93insects,245,247,249,276interglacialperiods,255intermediateocean,181,196–99,204Intrepid,34introversion,262invertebrates,247Io,23ionicbonds,59–60ionmicroprobe,88,93ions,59,83,90,99iron,9–11,13,14,72,99,103–4,127,153,177,199,225bivalentandtrivalent,58–59incoreofEarth,35,38,44,58,70onEarth,56,58–60aselectrondonor,58–59,150greenorbluecolorof,58–59ofMoonvs.Earth,39,40,41,45inoceans,197redcolorof,59
threechemicalrolesof,58–59seealsobandedironformations(BIFs);rustisostasy,105–6
isostaticreadjustment,105isotopefractionation,157–58isotopologs,220–21
JackHills,Australia,95,97,104Japan,261Jefferson,Thomas,15Jeffreys,Harold,110–11jellyfish,228,230,236Joggins,NovaScotia,240Joyce,Gerald,129–30JuandeFucaRidge,magneticmapof,118–19,122Jupiter,20,102asgasgiant,14,22–23moonsof,23,32,84,127,135
Kaapvaalcraton,186Kah,Linda,192–93Kalaharicraton,209Kamen,Martin,160Kauffman,Stuart,144KavliFuturesSymposium,276Kennedy,Martin,226–27Kenorland(Superia),189–90,204Kepler,Johannes,50Knoll,Andy,175,192–93,243Kump,Lee,279
Laki,269–70LamontGeologicalObservatory,ColumbiaUniversity,114landbridges,110,273–75landplants,242landslides,4“Laser-RamanImageryofEarth’sOldestFossils”(Schopf),167lasers,93Laurentia,190,191,194,195,210,234–35,262lava,72–73
lawofmotion,Newton’s,110lead,29,182atomicstabilityof,28,96
leaves,245–48lemonjuice,83LewisandClarkexpeditions,267life,75,147–49anaerobic,162animal,228–30buildingblocksof,131–36,152chemicalfoundationof,128chiralityand,139–41definitionsof,128–30distinguishingtraitsof,128diversityof,248–49,254Earthasappropriateenvironmentfor,25extremophile,134human,254–56ingredientsfor,149magnetosphereinsurvivalof,70multicellular,228–29originsof,12,127–53rawmaterialsfor,130–31reproductionashallmarkof,141–47selectionand,137–39asself-sustainingandevolving,128–29watercyclein,91seealsocoevolution,ofgeosphereandbiosphere;extraterrestriallife,search
forlightcarbon,210–11,246lightning:energyand,164inoriginoflife,132–33,136
lime,58limestone,4,151,153,173,187,237fossilsin,241
LimitedTestBanTreaty,seismologyand,120limonene,139lipids,134,136,137,152,218as“boring,”182
lithium,5,8,78,103“LittleGreenMen”(LGM),218Loihi,268Lowell,Percival,85lowermantle,68LUNA20,37lunareclipses,49LunarHighlands,66lunarmodules,34LunarProspector,87–88LunarReceivingLaboratory,Houston,34–35LunarSampleBuilding,JohnsonSpaceCenter,Houston,37
McRae,Mount,Australia,170,178–79Madagascar,189Magellan,267magma,32,33,35,44,64,72onMoon,49–50,66tidesof,seetidalbulgewaterin,91
magmachambers,20,73magmaoceans,51,61,69,71,78coolingof,66
magnesium,9,11,12,14,56,57–60,72,99,126,127magnesiumsilicates,68magneticfield,116–19,188flipsin,117–19andfossilmagnetism,185keyfindingsabout,117–18
magnetite,116magnetometer,116–17magnetosphere,70malachite,177mammals,54,255extinctionof,279–80,282first,253,254
mammoths,255manganese,150,156,177,225mantle,ofEarth,66–67,68,72,91,102electronsin,150
replicationexperimentsfor,92–94temperatureandpressurein,92
mantleconvection,123–24mantles,ofplanetesimals,20maps:colortopographic,114ofJuandeFucaRidge,118–19,122ofNewWorld,108ofpastandfutureEarth,184–86,189
Margulis,Lynn,228–29MarianaIslandsoceantrench,108Marinermissions,85Marinoanglaciation,222,230Mars,19,53,56,102,104,201basalton,71“canals”on,85Earth’sprojectedcolonizationof,266–67meteoritesfrom,18missionsto,85moonsof,32,33asmostEarth-likeplanet,25photosof,87redcolorof,163,178asrockyplanet,14searchforlifeon,85,135sizeof,43tectonicson,127wateron,85–87,127,163,178,259
Mars(Lowell),85MarsandItsCanals(Lowell),85MarsastheAbodeofLife(Lowell),85MarsOdysseyspacecraft,86MarsReconnaissanceOrbiter,NASA,86Mason,Ronald,118–19,122massextinctions,255,269,271,279–80,282complacencyin,279asdifficulttodocument,250,254opportunityafter,252seealsospecificevents
mass-independentfractionation,158massspectrometers,156,221mastodons,255Mauritania,103mega-tsunami,253megavolcanoes,94,257,269–71,278melting,ofrocks,72Mendeleev,Dimitri,217MendocinoFractureZone,119Mercury,22,102,104basalton,71deathof,258elementsof,56ashotanddryplanet,84asrockyplanet,14asuninhabitable,24
mercury(element),11,64,177threestatesof,203
MesoproterozoicEra,181–205MesozoicEra,54,252metabolism:chemistryof,135–36asfoundationforlife,128–31,144
metallicbonding,60meteorbombardment,32,74meteorites,14commercialvalueanddealingof,17–18iron,15,17,20Martian,18,165–66microbesdiscoveredin,165–66mineralformationin,19,21inMoonbombardment,37recordofEarth’shistoryin,15–18,55typesof,20seealsoachondrites;chondritesmethane,12,82,101,150,164,208,220,
224,231,282deep-Earth,215–16labproductionof,216sourcesof,215–19,220
methaneclathrate,216–17mica,21,38,57,58,59,93,103microanalysis,ofplantfossils,243–45microbes:andclay,227“deephotbiosphere”of,218iniceage,214andmethane,215photosynthesisin,162,210
microbialmats,171–72,174–76,192author’sresearchon,175–76wrinkledsurfaceof,174–75
Mid-AtlanticRidge,20,113–18,121MiddleEast,fossilsearchesin,172Milankovitch,Milutin,274Milankovitchcycles,274MilkyWay,13Miller,Ken,273–74Miller,Stanley,132–33,135,138,142Millerites,133Miller-Ureysparkscenario,132–35,140author’sscenariovs.,136
“MineralEvolution,”4,201mineralevolution,4,71,104,177,201,226,285–87cartoonof,201testsofhypothesisof,201–3seealsocoevolution,ofgeosphereandbiospheremineralogy:paradigmshiftin,4asreframedbyauthor,201
mineralogy,traditional,200–201seealsomineralevolutionmineralrevolution,181–205
minerals:author’scatalogof,177–78colorful,180incontinentalcorrelations,110,125Earth’smostcommon,71evaporite,87asevidenceoflife,177asevidenceofoceans,97
formationoffirst,12asformedinmeteoritesandplanetesimals,19,21formedthroughrock-waterinteractions,86–87ashostsforwater,91,94increaseddiversityof,203–4andlife,seecoevolution,ofgeosphereandbiosphereoriginof,4,177–78ur-minerals,12,21
mining,109,110MioceneEpoch,268Mirovia,196–98,209MIT,35,71,192,2329author’stenureat,105,108
mitochondria,228MobileQuarantineFacility,NASA,34moissanite,12molecular“letters,”145–46molecules,11–12inoriginoflife,132–49self-replicating,141–47
molybdenum,178–79,199,225monkeys,255MontagneNoire,FrenchAlps,174Moon:ageof,45anomaliesof,42basalton,71concurrentformationofEarthand,39,97coreof,38cyclesof,49distancefromEarthto,46,48asdry,38,87elementsof,56explosionon,265formationof,31–52,53,55,61,78,96,106,183askeytowateronEarth,87–89landingson,1,32,33–40,55maresof,71mineralcompositionof,38,104orbitof,41,47,49,50,267
peculiaritiesof,32–33primordialvs.current,46–48
Moon(cont.)asrecedingfromEarth,50–51,75,78rotationof,47sizeof,32,45surfaceof,1,66,87theoriesabout,32–33,38–40tidesand,48–52wateron,87–89
Moondust,35–36author’sinspectionof,37–38
Moonrocks,32,33–40,88author’scontactwith,34–36,37,88asrecordofMoon’sorigin,35–46,66vs.Earthminerals,38
moons,84ofgasgiants,23,32orbitsof,14varyingsizesof,32seealsoMoonMorocco,17–18,277
mosquitoes,276motion,Newton’slawsof,157mountainbuilding,66,73,105,106,187,189,191,195,196,203,234,261mountains,undersea,112–15“multipleworkinghypotheses,”32Murchisonmeteorite,137,142mutation,128,130,147mycorrhiza,246
Namibia,172NASA,88inastrobiology,165–66budgetof,34,266lifedefinedby,129,130insearchforlifeonotherworlds,23,130,134–35,165
NationalAcademyofSciences,229,245Nature,170,198,201nautilus,chambered,252nebulas,15
formationof,5,13inSolarSystemformation,13–14,15
neon,9NeoproterozoicEra,181“NeoproterozoicSnowballEarth,A”(Hoffman),212,222Neptune,23,102asgasgiant,14moonsof,24,32,41
neutrons,8,38neutronspectroscopy,86,87–88neutronstars,218“NewModelforProterozoicOceanChemistry,A”(Canfield),198NewScientist,The,201Newton,Isaac,110,157nickel,150,156,1771955CR(asteroid),263nitrate,151nitrogen,11,12,38,51,80,126,127,142,199,225inphotosynthesis,152asvolatilecomponentofEarth,78,79–80
nitrogenase,199nitrogenoxides(NOX),164NobelPrizes,201Noffke,Nora,171–76,192“nominallyanhydrous”minerals,91NorthAfrica,desertsof,3,17NorthAmerica,190,191arrivalofhumansin,26,255,274cratonsin,186
NorthAmericanPlate,121,122NorthEquatorialCurrent,909NorthPacificCurrent,90novelty,molecular,145–46Novopangaea(Amasia),260–62nuclearfusionreactions,9–10,23,54,258nuclearweapons,120nuclei,ofatoms,11nucleotides,145
ocean-floortopography,112–15,118,119oceanridgesystems,crustgeneratedat,115,116,119oceans,127,197bluecolorof,98changesinlevelof,5,233,241–42,251,253,255,268,271–75,277crustof,113,119,121–22currentchangesin,5first,94–99formationof,77–101,103,104future,259,271–77onJupiter’smoons,23measuringdepthof,272–73mineralsdissolvedin,126pHof,99salinityof,83,98–99sedimentsin,273inshapingofEarth,79stratified,226superocean,192,196,234–35aswaterrepositories,89seealsospecificoceans
Ockham’srazor,42oil,asrenewableresource,218olivine,12,20,37,57,58,91,92,96basalt,73informationofEarth’scrustandmantle,66–67,68,72,94
Olympia,Mount,194Omphalos(Gosse),29opal,87Opportunityrover,86orbits:anomaliesof,41ofMoon,41,47inSolarSystem,14
ore,110,125,182–83,197ofstars,10
organelles,228–29origins-of-liferesearch,132–35author’s,135,137–39
selectionin,137–39OriginsoftheContinentsandOceans,The(Wegener),110oxaloaceticacid,143oxidation,150oxygen,11,12,14,127,142,255inanimalmetabolism,248–49atmospheric,80,155,206,210,224,230,236,253,262onEarth,56aselectronreceptor,56–60,150,180andfirstanimals,228,231inhumanbeings,180isotopesof,38–39,40,272–73inlife,153,228,231inmeasurementofoceandepth,272–73inoceans,198originof,158–62inphotosynthesis,152,154,179productionof,9,162–64,170–71risein,seeGreatOxidationEventscarcityof,74,126,150,154,156,162,
177,190,199,209,251,253inwater,80–81,83,93
ozonelayer,158,209,236collapseof,251
PacificOcean,33,89–90,108,119,261floorof,118–19riftvalleyof,117
PacificPlate,121,122Paleocene-EoceneThermalMaximum(PETM),279–80paleomagnetism,116–17paleontology,164molecular,168–71
PaleoproterozoicEra,181PaleozoicEra,massextinctionin,250Palisadescliffs,73pandas,257,282Pangaea,109–10,234–35,249,251breakupof,253
Panthalassa,235
Papineau,Dominic,225–26penguins,282peridot,58peridotite,67,68,71–74,103periodictable,56permafrost,86,216,231perovskite,68petrochemistry,55petroleum,debateoversourcesof,217–19,220PhanerozoicEon,232–33Phobos,32Phoenixlander,86phosphate,151,153,226,246phosphorus,11,142,151asessentialforlife,225–26
photosynthesis,91,152–53,154–80,225,243,244,253,264carbonin,210–11fossilevidencefor,164–67iniceage,214inmicrobialmats,172originsof,161–62,164oxygenic,163oxygenin,158–59,179
PhotosystemsIandII,162Pickering,WilliamHenry,108Pilbaracraton,169–70,186Pinatubo,Mount,269plagioclasefeldspar,71–72planetarysystems,evolutionof,31planetesimals,23,31,33collisionsof,19–20,25formationof,19–21,183inplanetformation,42
planets:distributioninSolarSystemof,14–15formationof,12orbitsof,14rotationof,14vs.stars,23
plantfossils,242–45author’sresearchon,243–45
plants:edible,247endangered,278flowering,253leafless,244leaved,245–48
platetectonics,3,28,75,107–15,183,184–85in“boring”billion,184–90,191,194continentalcorrelationsin,108–10asdisparagedbygeologists,110–11earlyevidenceof,108–11forcebehind,115–19,122–23fossilrecordof,185future,257,260–62,267–69iniceage,214–15onotherplanets,127platesin,121–22inpreservationofEarth,232threestagesof,233–35asunifyingprinciple,125
Pluto,iceon,84plutonium,11,19polarbears,257,278,282poles,explorationof,267potassium,29,61,103radioactiveisotopeof,28
“powercrystals,”57Prague,geochemists’meetingin,280prebioticchemistry,132–33prebioticsynthesis,142prechronism,29Press,Frank,240pressure:inmantle,92–93innuclearfusion,9
primates,255primordialsoup,133–34,144,147PrinciplesofPhysicalGeology(Holmes),105
Proceedings,229,245propane,150proteins,144ProterozoicEon,206proto-continents,124proto-Earth,31,43–44,183volatilesof,94
protons,8,9,38,54,56Prototaxites,245–46pulsars(LGM),218pyrite,60,155pyroxene,37,58,67,71–72,91,92,94,96,103pyruvicacid,143
quantummechanics,157quarks,8quartz,57,87,106,126ingranite,103,104
“QuestioningtheEvidenceforEarth’sOldestFossils”(Brasier),167
radar,86radioactiveelements,61radioactiveisotopes,decayratesof,28radiometricdating,28,96,117Raff,Arthur,118–19,122rain,5,82,90,99,190monsoon,277
Rainier,Mount,121,194recycling:ofatoms,6ofstarmatter,13
reddwarfs,21redEarth,154–80redox(reductionandoxidation)reactions,135,151,153,160,198redshift(Dopplershift),28–29regolith,35–36replication,141–47autocatalyticnetworkmodelfor,144,146citricacidmodelfor,143–44,146
RNAworldmodelfor,144–47reptiles,240seealsodinosaursrhenium,178–79
Rhyniechert,Aberdeenshire,Scotland,243–44Richterscale,270riftvalley,Mid-Atlantic,114–17“ringoffire,”120,121rings:ofSaturn,23–24ofUranusandNeptune,24
rivers,89RNA,138,152,225RNAworld,144–47vs.lipidworld,138
Rochelimit,45rocks,248incratons,187–88Earth’shistoryasrevealedby,2–3,4,6,77,233,283asevidenceforGreatOxidationEvent,155–58faciesof,241freezingandmeltingof,71–72lifeassourceof,4,177ofoceanfloor,115–16oldest,63–67,164,169oxygenin,56–57silicon-oxygenbondsin,57assourceoflife,4,177seealsocoevolutionofgeosphereandbiosphere;minerals;Moonrocks;
petrochemistryRockyMountains,3,90,105,108rocky(terrestrial)planets,24–25,71,102compositionof,14,19,56,127vs.gasgiants,14–15granitein,127asrelativelydry,84wateron,84–85
Rodinia:assemblyof,195–96,204breakupof,208–12,232,234
Rogers,John,191rootsystems,246
Ruben,Samuel,160rubidium,radioactiveisotopeof,28rubisco,160ruby,12,58Rumsfeld,Donald,77nRussian-Americanrivalry,217–18rust,59,87,135,150,160inEarth’ssurface,178–79
RutgersUniversity,273
Sagan,Carl,265,283SaharaDesert,17,18,277St.Helens,Mount,121,269salt,98–99SanAndreasFault,122sandstone,187fossilsin,173–74,241
SantaFeInstitute,144sapphire,12,58Saturn,23,102asgasgiant,14iceon,84moonsof,23–24,32,84,101,135ringsof,23–24,45
Scandinavia,glacialdepositsin,2Schauble,Edwin,220Schiaparelli,Giovanni,85Schopf,J.William“Bill,”165–66,168,171,172vs.Brasier,167
Science,88,132,166,170,175,178,201,212,227sciencefiction,Marsin,85Scott,Henry,216ScrippsInstitutionofOceanography,118ScrippsResearchInstitute,129secularvariation,116–17sedimentarybasins,210sedimentaryrocks,73,96,165,185,196fossilsin,238,242asrecordofEarth’shistory,2,27,149,188,242,279
seismology,68–69,119,120selection:inevolution,142molecular,137–39
selenium,78sharks,27,247shells,fossil,272–73shrinkingEarthmodel,111Shrock,Robert,239–41Siberia,191,195silicates,20,57,91,236silicon,9,11,12,14,56,72,103,127aselectrondonor,57–60
silicon-oxygenbonds,57–60,68sills,73silver,11,78,182Singer,Maxine,77nsingle-crystalX-raydiffraction,37single-starsystem,22singularity,7SkeletonCoast,Namibia,213Slavecraton,186“slushball”Earth,223SmithsonianInstitution:meteoritesstoredin,16moonsamplesin,34trilobitesin,238
snails,241,249snowballEarth,212–14,230,254,255debateover,222–23mysteryof,223–25
snowball-hothousecycles,206–31,232sodium,83,103soil,246solarabsorption,100–101solareclipses,49SolarSystem:ageof,7,25,28anomaliesin,41,45–46assemblyof,21–24
chancein,54colonizationof,266commonoriginof,14,41cosmiccluestooriginof,13–21distributionofplanetsin,14–15Earth’sprojectedcolonizationof,266formationof,13–14,15,22,28,30assingle-starsystem,22timebefore,28–29
solarwind,14,25,36,70,88,259sonar,112–113SoussRiver,238SouthAfrica,189,224fossilsearchesin,169,172,173,174,176
SouthAmerica,191,195,235SouthAmericanPlate,121SovietUnion,37spacetravel,future,266spiders,245,247,250Spiegel,Der,201spinel,37Spiritrover,86sponges,238,2327starfish,241stars:elementsasproducedin,9–10formationof,5,8–10,54vs.planets,23sizeandlongevityof,21seealsosupernovasstasis,6,225inanimalevolution,236during“boring”billion,181–82,184,192–95,200–201,204,205,208,222
steroids,152stratosphere,4StrelleyPool,Australia,167stromatolites,149,172,173,225,236Sturtianglaciation,222subduction,260oldcrustswallowedby,121–24,186
submarinetechnology,112,116sugars,136,139,141,152,161asrighthanded,140
sulfate,151,153sulfur,9,11,38,60,127,142,151,251inGreatOxidationEvent,156–58inintermediateocean,198–99inoceans,198recyclingof,80smellof,199,205,206asvolatilecomponentofEarth,78,79–80
Summons,Roger,169–71,173SummonsLab,MIT,169Sun:birthof,13–14brightnessof,99–101,258,259dominationofSolarSystemby,21,30,47inEarth’sendgame,258–59ignitionof,14,15–16,24projectedlifespanof,21–22,258rotationof,14
sunlight,152–53,256,267,274leavedplantsin,246inphotosynthesis,160
supercontinentcycle,109,118,183,184–90,204modelsfor,191newmineralsand,204
supercontinents,modelsforfuture,261–62supercratons,187Superiorcraton,186supernovas,5,10–13,21,54surfacewatercycle,89–91humaninfluenceon,90
sutures,252suturezones,189Sverjensky,Dimitri,177,179swamps,248Sweden,27symbiosis,ofcells,228–29
syntheticbiology,129Szostak,Jack,146–47
tagatose,139tantalum,103Taupo,270Taurus-LittrowValley,37teeth,237telescopes,84,87,265insearchforMartianlife,85starsviewedthrough,22
temperature:iniceage,214inmantle,92–93inmethaneformation,220innuclearfusion,9andPETM,279insnowball-hothousecycles,221–23,230ofVenus,25
template-directedsynthesis,137–38,139terrestrialbiosphere,232–56terrestriallife,242–45,247thalidomide,139–40Tharp,Marie,114Theia-Earthcollision,43–44,45,53–54,61,64,75,78,89,106,183thermocouple,92,93thermodynamics,secondlawof,61,122thorium,61tidalbulge,49–51,61,75tidalrhythmites,48tides,48–52,277tigers,257tillites,212,213,222,224time,immensespansof,25–30Tiout,Morocco,fossilsat,238–39Titan,23–24,101,135Titanicdisaster,112titanium,37–38,58Toutatis,263
transitionzone,68trilobites,243,249author’scollectionof,238–39,241,252extinctionof,250,254
Triton,24,41trivalentiron,59tsunamis,270tundra,5tungsten,61TunguskaRiver,264turquoise,177Tyler,Stanley,172
Ukrainiancraton,190–91“ultra-lowvelocityzones,”69–70ultraviolet(UV)radiation,101,136,157–58,162,236,251UnitedPlatesofAmerica,190UnitedStatesGeologicalSurvey,120universe,7–13ageof,28expansionof,8originof,6,7–8
uppermantle,68Ur,189–90,194,195,210UralMountains,198uraninite,155uranium,11,29,61,78,96,103,156,177,183radioactiveisotopeof,28
Uranus,23,102anomalousrotationof,41,46asgasgiant,14moonsof,24,32
Urey,Harold,132–34ur-minerals,12,21
Vaalbara,189VanHelmont,JanBaptista,159–60varvedeposits,27vascularplants,242
Venus,1–2,53,71anomalousrotationof,41,45–46,51atmosphereof,84deathof,258–59elementsof,56greenhouseeffecton,208lossofwateron,24–25,46,51,84orbitof,24projecteddemiseof,22asrockyplanet,14tectonicson,127
vertebrates,247rodentlike,254
verticaltectonics,105–7,125,271vs.platetectonics,108
Vikingmissions,85volatiles:ascomponentsofEarth,79,94inHadeanEon,74aslost,79–80,94ofMoonvs.Earth,40,41asproducedanddispersedbystars,79involcanicactivity,78
volcanicactivity,volcanism,2,28,98,107,127,150,194basaltin,72–73incarbondioxideproduction,211future,268inHadeanEon,74Hawaiian,28,70,72heatproducedby,61hotspotsof,70Icelandic,72lunar,37,47,49,89Mesozoic,253Paleozoic,251volatilesin,78waterin,91inYellowstone,70
volcanoes,4,75,256earthquakesunder,121
historiceruptionsof,269–70onIo,23undersea,116,119,123–24,268ventsof,4seealsomegavolcanoes;specificvolcanoes
wadsleyite,68,94WaroftheWorlds,The(Wells),85Warrier,SantoshMadhava,191water,79,82,101,255abundanceof,84–89chemistryof,80–84densityof,82distinctivepropertiesof,81–83onEarth,25,38,89–91inEarth’score,94asessentialtolife,81,84,101aslackingonMoon,38,87lackofpurityof,82–83onMars,25,85–87,89,163moleculesof,12onotherplanets,23–25,84pHof,83–84inphotosynthesis,152,159polarityof,81–82recyclingof,80subsurface,178surfacetensionof,81–82Venus’slossof,24–25,46,51
watercycles,seedeepwatercycle;surfacewatercyclewatervapor,84asvolatilecomponentofEarth,78,79,80,95
weathering,180,227,259ofbasalt,99,246continental,213,225,230
Wegener,Alfred,109–11,115,118Wells,H.G.,85WestAfricancraton,2029whales,27,254,282“WhiffofOxygenBeforetheGreatOxidationEvent?,A”(Anbar),178
whiteEarth,206–31WhyAren’tBlackHolesBlack?(HazenandSinger),77nWilliamson,Donald,229WonderfulLife(Gould),240Wones,David,35–36,71WorldHeritageSite,SharkBay,Australia,173WorldWarI,109,112WorldWarII,112,239WorldWideStandardizedSeismographicNetwork(WWSSN),1120–21worms,228,230,236,247
Yellowstone,70Yilgarncraton,186Yoder,HattenS.“Hat,”Jr.,65,92Young,Ed,220–21YucatánPeninsula,asteroidcollisionat,253
Zhao,Guochun,191Zimbabwecraton,186zinc,11,182zircon,21,96–98,103asevidenceofgraniteformation,104asoldestcrystals,97
zirconium,78,96,104
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