L I T E ... . ~, ~. , ~ ¯ , ° ¯ ’. ’ "i ’. ~" ’" :I:’ L , ~:.~i,~.~,~. "" , :.. A quarterly publication £or educatom and the pul)lic- contemporary geological topics, issues and evenls "~’.,#~%}?: !i’-t:’q~- .-..’.~:g;~:s ",.’:".’,:.~<~’,,.-..,.%" :.-. :.,. ~ ’,,’-, :’.; /,Y:.. ,~." : .~. ~.. :~, :%" ~.~. .’i.~; .-~: %% %;%%~ ’cRY.. ¯ ,’.., :; .".::" Y’ ~/,; .L" :.".: !:.’.’.i’. ":/";’, ;: >" i <:. . ~.,vi.. :... :...:......:.....,, p.-,.~.i.L ~;.~, )u.,~.. :..".’.."’...,.i~ :. ". ’.":’..... .t ~ ,’ : ’...::;: :.::."T,:. :i’ .,.-’:..’"I.F::: ;,;%" ":~.,.,:;,.’~,.: .:"’. :: :. !: ’ i:. ’q :{-. :.? "-.-:, .~.k : ;’:: L{;. :.. :~ :.": :.";:.~:..%~.;!,!:! ¯": : :;i:i;-![:;~.i/i.:.ii/:;: i.~:..i:i,L f/; ;i:iL! T f]:i :::/,: : }i::i::,;iii ~. ~ i i :/i:i di/,!:!:~i ~ :i!:if" y.~ ~ii~,! :) .}!~,: ,..,::i) ,: :.:.::-,..’.~, ~ . ~\~ILI,~ ~.... ii ~i."~::i;~i~’.: h; :1~ ::. ’ ",i.i :" ." .i . :.1.:~ .. :.:..:’,":
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L I T E... . ~,
~. , ~¯ , ° ¯
’. ’ "i ’. ~" ’" :I:’L ,~:.~i,~.~,~.
"" ,
:..
A quarterly publication £or educatom and the pul)lic-
contemporary geological topics, issues and evenls
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reduced activity, and perhaps isheading for dormancy. Changes ingeyser eruption patterns arecommon following an earthquake,and serve to illustrate thatYellowstone Park’s geyser regionsare a dynamic and complex weaveof thermal and geologic processes.
A few scientists speculate that somegeysers change their pattern oferuptions before earthquakes occur. Ifso, it might be possible to predictearthquakes using geyser behavior.This hypothesis is being tested bydetailed monitoring of eruptionbehavior of a few geysers inYellowstone. The outcome of the datacollection awaits more earthquakes.The evidence from previousobservations, however, is that the mostnoticeable changes in geyser behaviorbegin with earthquake shaking andchanges in natural plumbing.
The known geothermal areas in NewMexico may be affected by earthquakesas well. Although there are presentlyno geysers in the Jemez Mountains orthe Lightning Dock geothermal areas,changes in amounts and temperaturesof water from hot springs and hot wellsare documented in both areas and thewater is hot enough to flash intoeruption-causing steam under the rightconditions. Earthquakes could changethe plumbing systems to thegeothermal fields. Hot water moving tothe surface along a number of faults inJemez Canyon has shifted throughgeologic time to.its present location atSoda Dam. Remnants of previoustravertine spring deposits along thecanyon walls show where plumbingsystems used to be active.
SourcesElston, W. E., Deal E. G., and Logsdon, M.J.,
1983, Geology and geothermal waters ofLightning Dock region, Animas Valley, andPyramid Mountains, Hidalgo county, NewMexico: NewMexico Bureau o[ Mines andMineral Resources, Circular 177, 44 pp.
Golf, E, and Shevenell, L., 1987, Travertinedeposits of Soda Dam, New Mexico, and theirimplications for the age and evolution of the
Seismic Geyser evolved from a fumarole in 1963, spittingstones and killing trees with boiling water.
!
Castle Geyser cone in foreground, Giantess eruptingbeyond Firehole river, Upper Geyser ~asin.
Valles caidera hydrothermal system:
Geological Society of Am,erica, Bulletin, v. 99,pp. 292-302.
Haines, J., "Geyser Shakeup": BozemanChronicle, issue h)r May 24, 1994,Bozeman, Montana.
Hutchinson, R., August 3, 1994, personalcommunication,Yellowstone National Park,Yellowstone, Wyoming.
Marler, D. M., and White, D. E., 1975,~ismicGeyser and its bearing on the origin andevolution of gey~rs and hot springs ofYellowstone National Park: GeologicalSociety of America, Bulletin, v. 86,pp. 740-759.
Milstein, M., "Large Geyser goes toSleep": Billings Gazette, issue forMay 3, 1994, Billings, Montana.
--story by D. Love and S. Welch
Summer 1994, Life Geolo~,,y New Mexico Bureau of Mines and Mineral Resources
Castle Geyser in steam phase
Lone Star Geyser
phok~s by David Love, Senior Envinmnu’ntalCa’ol~gi~t, NMBM&MR
.New Mexico Bureau of Mines and Mineral Resources Life Geolo~*y, Summer 1994
Have you ever wondered......How Earthquakes arcMeasured?Richard AsterAssistant Professor of GeophysicsNew Mexico Tech
Virtually everyone has seen pressreleases describing earthquakes aroundthe world where the size of theearthquake has been quoted as amagnitude. What exactly does this,refer to, and how do scientists assessand quantify the size of earthquakes?
As we saw in a previous article inLite Geology, almost all earthquakes arecaused by sudden fault slip in thebrittle upper few hundredkilometers of the Earth. As mostearthquake faults do not breakthe surface, most of what isknown about earthquakes isinferred from observations ofseismic waves that propagateaway from the earthquake sourceregion at depth and thus can berecorded ~t or near the Earth’ssurface.
The earliest semiquantitativemethod of assessing the size ofearthquakes is the intensity scale,first developed by the Britishengineer Robert Mallet to assessthe damage from a disastrous1857 earthquake io southern Italy.The simple idea is to measure theintensity of shaking, as indicatedby different types of structuraldamage. One can then plot on amap the intensity of shaking atdifferent locations and contourthe data to obtain a set of curves(isoseismal contours) that indicateapproximately equal degrees ofshaking (Fig. 1). An isoseismal contourplot thus gives an estimate of thespatial distribution of shaking. A-twelve-point intensity scale wassubsequently devised by the Italiangeologist Giuseppi Mercalli in 1902,where the degree of inferred shakingranges from I (detectable only withsensitive seismic instruments calledseismometers) to XII (essentially total
destruction). The intensity scale usedmost often in North America, theModified Mercalli intensity scale, wasdeveloped by H.O. Wood and ENeumann in the early 20t" century to fitconstruction conditions in California(California buildings have appreciablydifferent features than Eur(~pean ones!).The intensity scalej despite itssimplicity, is still a useful seismologicaltool today, particularly as a standardfor assessing the size of historicearthquakes.
Isoseismal contours also may beused to estimatethe location, depth,and size of an earthquake when noother data are available, as deeper
Figure 1--Diagram showing isoseismal con-tours, surface ruptt/re, fault surface, focus, andepicenter.
events will generally affect a wider areawith equal intensity. However, localground shaking is highly influenced bynear-surface and regional geology dueto attenuation (the rate at which seismicwave energy is converted into heat byfriction), resonance (local buildup seismic wave amplitudes due toconstructive interference), andamplification (soft materials whichpropagat~e seismic waves more slowlyinduce higher amplitudes to conserveenergy).
As an example of attenuation, notethat the 1886 earthquake in Charleston,South Carolina (estimated magnitude7.2) was felt over an area ofapproximately 8 million squarekilometers, while the ~imilarly-sized1989 Loma Prieta earthquake was feltover only about ! million squarekilometers of California as a result ofthe more attenuating rocks of the westcoast. The clearest examples ofresonance and amp!ification effects areseen in cities: This is because manymeasurements of intensity can be madein a small region and because certaintypes of .soft soil that amplify seismicdisturbances, such as "reclaimed"
wetlands, are common.Recent examples of extreme
local amplification include highlylocalized and very strong groundmotions observed in Mexico Cityduring the 1985 Mexicoearthquake (in areas built on anancient lake bed), during’the 1989Loma Prieta earthquake (in theMarina District of San Francisco),and the 1994 Northridgeearthquake (in parts of SantaMonica, California).
The most commonly quotedmeasure of earthquake size is themagnitude. The originalearthquake magnitude scale wasdeveloped by Charles Richter in1935 for California. If you want toimpress your friends, here is astrict definition of the RichterMagnitude:
The base 10 logarithm of themaximum seismic-wave amplitude (inmicrons, or one one-millionth of ameter) recorded on a standard Wood-Anderson seismograph at a distance of100 kin from the epicenter, where thestandard seismograph consists of a 5gram mass suspended on a torsion wirewith optical recording andelectromagnetic damping. It has aperiod of O.8 seconds and magnifiesground motion by a factor of 3000.Mathematically, the Richter, or local,
Summer 1994, Lite Geology New Mexico Bureau of Mines and Mineral Resources
k.
magnitude is
ML = log 10 ( 3000 u ,,,,
where um,, is the r~aximum grounddisplacement in microns.
Because of the base 10 logarithm inthe definition, each unit increase inmagnitude reflects a ten-fold increase inground motion. An increase of twomagnitude upits corresponds to aground motion increase of 102 or 100.The instrument calibration is such thatthe maximum seismogram amplitudeexpected from a Richter magnitude 5earthquake 100 km away is about 100mm (Fig. 2). This ~corresponds to maximum ground motion of about 33
duration magnitude scale is routinely ¯used to assess the size of earthquakes inNew Mexico by the New Mexico TechSeismological Observatory.
The Richter magnitude wasdeveloped as a rough measure ofearthquake size in California, whei’edistances between earthquakes andseismographs are commonly not morethan a few 10s of km. Many earthquakesaround the world, however, occur inremote regions such as under the. -oceans and are thus recorded only atteleseismic distances of 100s to 1000s ofkm.
Distant large earthquakes producetwo basic types of seismic waves that
can be seen on seismograms recordedvirtually anywhere on the Earth withsuitably sensitive instruments. Each ofthese waves has its own associatedmagnitude scale. Seismic body wavestravel deep into the Earth’s mantle andeven through its core before reaching aparticular station, while surface waves,travel near the Earth’s surface. Atteleseismic distances these two typesof waves can be quite distinct onseismograms, in contrast withseismograms recorded close to thesource, which are characteristically amore complicated superposition ofbody waves and surface waves.Surface waves are commonly thelargest signals on a teleseismic record
$microns, which is about 1/1000 of an¯ ~ [ body wave because they spread out over the (2-inch. The expected ground motion for a
[dimensional) surface of the earth,magnitude 7, on the other hand, is 100
p [ ’ while body waves must spread outtimes greater (3300 microns or about 1/ j~ body we~e "10 of an inch) which can easily be felt’l~’~ [L Figure 2--Seismograms of local andeven 100 km away.
Note that earthquake magnitudes can :l.m [ Ji. distantearthquakesshowingdifferences
l.ll~ati., in duration, arrival times of differentbe zero or negative, as the maximum~ wave types, and amplitudes, a)
ground displacement, u,~ of an m]1~[~q~w’r’’"’~~ seismogram recording a localML = 0 earthquake is 1/10 that of an[ "~ Jumble of surfece& wevea earthquake in southern California 15 kmML = 1 earthquake, and the ground bodyJumble of sur~moe & body waves
from epicenter, MD = 1.6;displacement from an ML = -1 t [11]1. (b)Seismogram’fromAdirondack, Newea, rfhquake is just 1/100 that of an
, , [ .... York, recording a ~leseismic earthquakeML = 1 earthquake, and so on.4,200 kilometers awa)6 off the coast ofSeismologists study earth~iul/kes 0 sI ~me2eco.o.~o~sbe0,nnm0o~,ueUe)25northern California, Ml=6.7.spanning the huge range of 11 orders of
magnitude (magnitudes from 1
eu /~/~’~ ~/1AIli t~"approximately-2 to 8). ~e/eal~Despite its simplicity the Richter P ~ ~~filllllllll[lllllll~l"magnitude doesn’t do too bad a job of Ix~/wave. Ix~l~ w.ve ~ UV~UUIIVl[
quantifying the relative size ofearthquakes, particularly when thereare many stations to provide i , , , i , . , ~ , , . i , , , i , , , i , , , i , , , ,measurements to average over.
Another scale that is used to measureearthquakes at local distances (typicallyless than 100 km) is the durationmagnitude, which assesses how longthe seismic waves rattle around in thecrust of the Earth before they becomeindistinguishable from the backgroundnoise. Its formula is
Md = logt0(d) + q
where d is the duration in seconds and qis a factor used to make Mdapproximately equal to ML. The
200 400 600 800 1000 1200 1400 1600Tlme (eeconds from beginning of quake)
Table 1--Moments and magnitudes of some famous earthquakes
surface wave average seismic momentmagnitude area of fault amount of slip scalar moment magnitude
Ms S (lenllth x width) U (m) Me iN-m) Mu,
Northridge (1994) 6.6San Francisco (1906) 8.2Alaska (1964) 8.5Chile (1960) 8.3
, 20 km x 14 km 1.4 1.2 x 10I* 6.7320 km x 15 km 4.0 6.0 x 102o 7.9500 km x 300 km 7.0 5.2 x 1022 9.1800 km x 200 km 21.0 0.4 x 1023 9.6
New Mexico Bureau of Mines and Mineral Resources Life Geology,. Summer 1994
over the (3-dimensional) interior.When thinking about thes(two types
of waves, imagine dropping astoneinto a pond. Sound waves from theimpact (which you don’t normally senseunless you are a fish) travel deep intothe pond, while surfa~:e waves are seentraveling away along the air/waterinterface. The sound-like waves inseismology are body waves, and theconcentric ripples moving away fromthe source (the rock) at the surface aresurface waves. The body wavemagnitude, is
mb = logl0(A/T) +
where the standard practice in the U.S.is to measure the maximum amplitude,A, in microns, of the initial teleseismicP-wave arrival, T is its period (typicallyless than 3 seconds), and Q is a termthat depends on distance and sourcedepth and minimizes the differencebetween m~ and other magnitude scales(there are always additive constants inthese formulas for this reason, and whyit’s called m~ instead of Mv no oneseems to know). The surface wave
’magnitude is, similarly,
Ms = log10(A/T)+l.66 logA+3.3
where A is the maximum surface waveamplitude and A is the source-to-receiver distance in degrees (e.g., quarter of the way around the Earthwould be D = 90°. Surface and bodywave magnitudes may differ by up toseveral units, as shallower earthquakesrelease more energy near the Earth’S;surface and hence generateproportionately larger surface wavesthan deeper earthquakes. Surface wavemagnitude values for severalearthquakes are shown in Table 1.
Note that all of these magnitudescales, although useful, are purelyempirical and relative. They alsocontain an additional problem in that,for technical reasons beyond the scopeof this article, they don’t do a very goodjob of discriminating between merely
big earthquakes and really bigearthquakes, as we’ll see. To obtain amore meaningful measure ofearthquake sources we must look moreclosely at the physics of fault slip.
It turns out that when a fault slips,the mechanical process is equivalent toa force operating on a lever arm of some
¯ length, or a mechanical moment (the"units are torque, or Newtons timesmeters). The seismic scalar moment ofan earthquake is just the size of thistorque, and is defined by
M0= ~Sp
where ~ is the average amount of slipon the fault, S is the area of the fault,and/~ is a scale factor with units of forceper unit area, or pressure, called therigidity. The rigidity describes howhard it is to bend the rocks in theearthquake source region with ashearing motion. Both ~ and S can beestimated from modern seismogramsand/or from ground rupture oraftershock patterns, while Ix isestimated from laboratoryrock-squeezing experiments.’ The scalarmoment defines a magnitude scalecalled the moment magnitude
M~ s 2/3 loglo Me - 6.0.
Table 1 shows some surface-wavemagnitudes and seismic moments for afew famous earthquakes.Note that thesurface-wave magnitude (Ms) valuesaren’t all that different for the three very.large earthquakes in the table (Ms isactually smaller for the Chile event thanfor the Alaska event) but the seismicmoment and its corresponding momentmagnitude show just how much largermechanically the Alaska/rod Chileearthquakes were than the SanFrancisco event (the Chile event is justabout as large an earthquake as theEarth is thought to be capable ofgenerating!). As the seismic momentbecomes more routinely estimated byearthquake monitoring agencies, youcan expect to see the moment
magnitude reported’ more frequently inyour favorite newspaper, especially forlarge earthquakes.
Because the seismic moment and themoment magnitude are determined bystraightforward calculations from thedimensions and amount of slip on thefault and from the stiffness of the rocksaround the fault, it is possible forpaleoseismologists to estimate the sizeof earthquakes that may have occurredon faults in the past. Furthermore,seismologists can use the sametechnique to assess the size of anearthquake that may occur on arecognized fault in the future. Toperform these calculations, it isnecessary to estimate the length andwidth of the fault in question as well asthe probable amount of slip and therigidity of t~e rocks near the fault zone.
A final measure of the size of anearthquake is the total amount ofenergy released in the slipping of thefault. This is expressible in terms of theseismic moment as~
E--M0(O/Ix) = 6S~
where ~ is the average shear stress (theforce per unit area pushing the faultalong in units of pressure) along thefault during the earthquake. Typically,~is on the order of 100 times thepressure of the atmosphere at theEarth’s surface, or in metric units, about10s Newtons per square meter. Theenergy released in a large earthquake isgigantic. For the Chile earthquake it is
E -, 10s x [800 x 10~] x [200 x 10s] x 21= 3.4 x 1017 Joules
For comparison, the energy releasedin a 1 megaton nuclear explosion is"only" about 8 x 1013 Joules, so a verylarge earthquake releases the energyequivalent to about 10,000 hydrogenbombs! Fortunately for the human race,most of this energy goes into fracturingand heating the rocks around the faultand only a few percent actually getsradiated away as seismic waves.
0
Summer 1994, Lite Geology New Mexico Bureau of Mines and Mineral Resources
magnitude 7 earthquake
magnitude 8 earthquake
magnitude 9 earthquake
Who really knows wherestones come from?
If you have ever applied yourself toyour garden in early spring, to dig out,once and for all, every rock-only toreturn the next season to find a newcrop of rocks to pick out-you canunderstand why people ponder thisuniversal problem. The June 1991 issueof the Badger Comm~’Tater, the maga-zine of the Wisconsin Potato andVegetable Grower’s Association~featured an insightful article on thisdilemma. Excerpts from the article,"Where stones come from," by JustinIsherwood, are reprinted with permis-sion and appear in italics. JustinIsherwood is a potato grower fromPlover, WI, and is a regul~ir columnistfor the Badger Common "Tater.
Farmers often ponder where stonescome from. It troubles some more thanothers, usually for the reason thosewho are not troubled do not own stonyland.
Farmers think about stonesbecause they have picked stones, pickedthem last year and the year before.Picked stones as far back as they canremember and their pa too. Everyspring more stones, stones where noneexisted before...In the farmships whystones reappear every spring isroutinely discussed. Village folk argueother things, troublesome subjects likenational debt, greenhouse effect, andacid rain. Transitory things likeapartheid, black holes and stock prices.
But in the glacial valleys, in thedim rural taverns late at night theytalk stones and why stones come backto the fields year after year like migrat-ing geese.
One farmer confesses he has takento painting the rocks he picks becausehe swears they look like the ones hepicked several years previous... Hebelieves stones come back, how he throws"em on a rock pile and they zoon’t’stay put,instead crawl offlike wounded dogs to licktheir wounds and return to the field...Stoneshave a homing instinct. Dump "era inRomania, he says, and they’ll find their way
back, which is why he is painting stones.Green one year, blue the next, white another.S, ome day when a blue stone shows up he,will have his proof
This is for the same reasons sea shells arefound halfway up Mount Everest, the wholeload of plate tectonics is driven by stonescrawling their way home. If people had le~stones plumb alone in the first place none ofthis would have happened. What with allthe coal dug out of the ground and shippedthousands of miles away, the copper andiron ore and gold...the surface of the earth iscrawling to beat heck.
There is always a Scientist or twoamong any farm coven, and mytho-logical stone lbre sets them off."...Stones don’t think, crawl, or getsquashed out of the ground .... Everfeel the bottom of a rock? Colder thandirt, ain’t it? Cold in May when thetop of the rock is hot enough to frysausages. Cold and wet. Sometimesafter weeks of warm weather you findice there. Tain~t the ice that does itthough, it’s the dark side of that rocksucking water that does it. Frost heavewon’t get you any altitude even if youwait a thousand years. But the coldside of a rock sucking water day afterday, season after season, brings rocksup from thousands of feet down. Weain’t never gonna be free of stones. Ifit was frost alone, we’d have won thewar a couple of generations ago. Itain’t ice, it’s them stones suckingwater just like a hydraulic cylinderlifts the stones out of the ground."
We encourage our readers towrite to us with scientific, orhumorous explanations aboutwhere stones-especially thereappearing kind-come from, or toshare other stories of folk geology.
Sotu’ce
Isherwood, J., (1991) Where stones comefrom, in Houlihan, T. (ed), BadgerCommon’ Tater: Wisconsin Potato andVegetable Grower’s Association, Antigo,Wisconsin, v. 43, no. 6, pp. 38-39.
New Mexico Bureau of Mines and Minerai Resources Lite Geology, Summer 1994
\
Ancient Lakes: A Tool For Understandirlg Climatic ChangeBruce AllenDepartment of Earth and Planetery Sciences,University of New Mexico
One of the attractions of living inthe southwestern United States is itsdry air and clear, blue skies that seemto linger for weeks on end. Althoughthunderstorms being sporadic rainsduring summer months, and cyclonesbearing moisture fromthe Pacific Ocean sweepthrough the regionduring winter months,the overall climate of theSouthwest is dry, ra factthat is borne ’out by thescarcity of large, naturallakes in the region.Geological evidenceindicates that such wasnot always the case. Ifwe could travel back intime about 20,000 years,during the height of thelast Ice Age, we wouldfind that many of theintermontane basins inthe Southwest wereoccupied by large lakes.These ancient lakes arecalled pluvial lakesbecause they formedwhen the climate waswetter than it is today.
New Mexico hadseveral large, pluviallakes. One such lakefilled the bottom of theEstancia Valley, east ofthe Manzano Mountainsin central New Mexico.The northern shorereached the outskirts ofwhat is now M0riarty,NM (Fig. 1). At itsmaximum extent, LakeEstancia was about 40miles long and 20 mileswide and covered the town sites ofEstancia and Willard with almost 100,feet of water (Fig. 1). Today, a fewmiles east of Willard, NM, huge cup-shaped depressions, or blowouts, somea mile in diameter, have been carved by
wind into the old lake sediments.Highway 60 passes through theexposed remains of ancient LakeEstancia (Fig. 2). The blowoufs nowcontain small, ephemeral salt ponds, orplayas" Salt from these playas washarvested by the people of the SalinasPueblos at Quarai and Gran Quivers.
Evidence for the expansion andcontraction of ancient lakes provides
IEstancla
1-40 )
Highest Shoreline ofAncient Lake Estanola
2’//~
- Modern~.. eo Salt Ponds
.T , (Playas)%
I
20 miles
Figure 1. Map sho~ving location of the Estancia basin and extentof former lake as indicated by stranded shorelines. ’
scientists with a powerful tool forunderstanding how the Earth’s~climate
. has changed over the past several tensof thousands of years. Past fluctuationsin lake level can be documented bymapping the elevation of shoreline
features such as beach ridges, spits, andsand bars that formed along themargins of pluvial lakes. Other morecomplete information about the riseand fall of lake level is obtained bystudying the sediments that weredeposited on the bottoms of the lakes.At Estancia, for example, when the lakewas freshened and expanded in size,aquatic worms and other bottom-
dwelling organismscolonized the lakebottom and mixed theuppermost layer of thebottom muds in theirsearch for food.Sediments that weredeposited duringhighstands of the lakeare relativelyhomogenous due tothis mixing (calledbioturbation). Duringlowstands of the lake,as salinity increased,bioturbating .organismswere eliminated fromthe lake bottom,resulting in delicatelylaminated sediments.
Still anotherindicator of highstandsand lowstands is theabundant remains oftiny crustaceans calledostracodes ("seedshrimp"). Ostracodesare used to estimatechanges i n the salinityof the lake in whichthey lived. Biologistsstudying modernostracodes havedetermined that certainspecies of ostracodescan tolerate and eventhrive in water thaf hashigh salinity. Other
species are able to tolerate only freshwater. Changes in the abundance andtypes of ostracodes in the deposits of ,Lake Estancia can be used to determinewhen the ancient/ake was freshenedand expanded in size, and when the
,2
Summer 1994, Lite Geology New Mexico Bureau of Mines at~d Mineral Resources
Figure 2--Wind deflation of the central basin began ~8,000 years ago,resulting in the formation of numerous blowouts that presently containephemeral salt pp’nds, or playas.
water became salty during Iowstandsof the lake.
The climatic history of LakeEstancia, as recorded by its sediments,can be read in the walls of the manyblowouts that excavated the floor of theold ~ake. The upper 15 feet ofsediments exposed in the blowoutscontains two thick beds of nearly pureclay that are bioturbated, indicatinghighstands of the lake. Radiocarbondating of the older of the two clay bedstells us that the first highstand begansuddenly about 20,000 years ago,forming the highest set of shorelinesthat now lie stranded along the marginsof the Estancia Valley (Fig. 1). Then, justas suddenly, about 15,000 years ago, theclimate changed dramatically and thelake dropped to a low elevation (Fig.4),salinity increased, bioturbationstopped, and only the most saline-tolerant slbecies of ostracodes survived
¯ in the lake water. This contraction of thelake lasted for about 1000 years and wasfollowed by another rapid expansionbeginning about 14,000 years ago (Fig.4). This last major expansion of thelake, however, was not as extensive orlong-lived as the earlier highstand, and
by 12,000 years ago the lake began itsfinal disappearance from the landscape.
The effects of dramatic changes inclimate continued to be imposed on thefloor of the Estancia Valley, and onother desiccated pluvial lake basins inthe Southwest, long after the lakesdried up. The large blowouts cut into’the lake sediments at Estancia (Fig. 2),for example, began to be excavatedroughly 8,000 years agoas southwesterly windsblew across theabandc)ned lake bottom.Dried pellets of clayand gypsum, scoopedout by the wind, weredeposited nearby asgiant dunes calledlunettes, in reference tothe crescent shape of thedunes. This process of"deflation" continueduntil the blowoutsreached a depth ofabout 30 feet below thefloor of the basin. Anoverall rise in the watertable during the pastfew thousand years duE’to relatively wetterclimatic conditions hasreversed the trendtowards deflation of the
basin floor, and the bottoms of theblowouts are beginning to fill in withsediment.
Some of the earliest attempts toestimate past climates were done in themid-1900’s by using the Estancia Valleyand its shorelines to construct ahydrologic-balance model. In itssimplest form, the hydrologic-balancemodel keeps track of the volume of
’water that enters and leaves the lake.Lake Estancia, as for many of thepluvial lakes in the Southwest, did notoverflow and the removal of water fromthe lake was largely throughevaporation from the lake surface.Inputs of water into the lake includeddirect precipitation on the lake surface,runoff from the surrounding drainagebasin, and ground-water discharge.These early studies suggested thathighstands were the result of bothlower temperatures (lower evaporationrates) and increased precipitation.
A topic that is of great interest andimportance today is how quickly largechanges in climate, such as thoserecorded by Lake Estancia, may occur.The first good evidence that the Earth’sclimate has undergone large andabrupt changes in climate has come
"̄~.- .,~,-._..,,..~..... ". "r ",, ¯.--’o A.:: ,, ,. .~,"~ "=,q’ ..... ,. " "’ ."I" .¯ ¯ ,,,
Figure 3--Photograph of a blowout (large depressionexcavated by the wind) in the Estancia basin. Layeredsediments in the wall of the blowout were depositedin ancient Lake Estancia during the last Ice Age.
New Mexico Bureau of Mines and Mineral Resources Lite Geology, Summer 1994
6200
, ~ 6100
!6000
20
Estancla Valley, NM,Lake-Level andWater-Table Fluctuations
Elevation of Basin Floor
Deflation
Infllllng
,/
16 12 8 4 0Thousands Of Years Ago
Figure 4--Flu(~tuations in lake level and water table in the Estancia basinoccurred during the last 20,000 years. Notice a rapid rise in lake level-20,000 years ago and a rapid fall ~15,000 years ago, followed by several .other high lake stands.
from cores collected from theGreenland Ice Cap. Ice cores extendfrom the present back through the lastIce Age and indicate that dramatic shiftsin temperature and other climaticvariables, at that high latitude, occurredwithin a few years to a few decades.The abrupt shifts in climate suggestthat the earth’s climatic system iscapable of "flipping" b~ack and forthbetween relatively stable states in veryshort periods of time.
Our detailed study of the Estancialake sequence suggests that rapid shiftsin climate, similar to thoserecorded inhigh-latitude ice sheets, also occurredover the southwestern United Statesduring the last Ice Age. The changefrom a low to a high lake stand about20,000 years ago, for example, wasaccompanied by a large increase in therunoff of water and sediment being.carried to the lake in stream channels.Suddenly, these streams began carryinglarge quantities of sediment, includingsand-sized quartz grains, which werethen spread, out over the surface of thelake. Thin concentrations of quartzgrains, found in mudsfar from shore,can be traced directly back to thestream channels that carried them.When this major runoff episode
Summer .1994, Life Geology
freshened the lake, just as suddenly,ostracodes that live in fresh waterreproduced in great numbers. The -’lake/during that single climaticepisode -20,000 years ago, rose to itshigh shoreline within a few decadesand during the next 5,000 yeats it wassustained near its highest elevation byother brief episodes of increasedprecipitation (Fig. 4).
Although most of the moisture thatreached Lake Estancia during bl"iefepisodes of in,eased precipitationprobably’ came from the Pacific Ocean,it is unclear what may have triggeredthe sudden changes in precipitation.
The accumulating evidence for largeand abrupt changes in climate and for ametastable climatic system is causingclimatologists to reconsider scenariosfor global climate change. Prospects for"global warming" from theintroduction of carbon dioxide into theatmosphere have been highlypublicized, but the prediction of themagnitude and time-scale ior such ahuman-caused event is hampered by anincomplete understanding of, naturalclimatic variability. Records of "
¯ paleoclimate from different parts of theworld that resolve climatic changes ona time scale of decades, such as the onebeing reconstructed for Lake Estancia,will be needed in order to understandhow the earth’s climatic systemoperates and to predict future changesdue to both natural variability andhuman activity.
Suggested ReadingAllen, B.D., and Anderson, R.Y.i 1993,
~ Evider)ce from western NorthAmerica for rapid shifts in climateduring the last glacial maximum:Science, v. 260, pp. 1920-1923.
, I
Bradley, R.S., 1985, QuaternaryPaleoclimatology: Boston, UnwinHyman, 472 pp.
Smith, G.I., and Stneet-Perrott, EA.,1983, Pluvial lakes of the westernUnit~.<i States, in Porter, S.C., ed.,Late Quaternary Environments ofthe United States, v. 1, The latePleistocene: University ofMinnesota Press, Minneapol/s, "-pp. 190-212.
~J
New Mexico Bureau of Mines and Mineral Resources
.e
¯ ,h 1. g hL I T E SEARTH SCIENCE UPDATE
Teacher Resources:How to Teach with Topographic Maps,’for grades 5 through 10, is a 24-pagebooklet containing student activities,and includes a topographic map by theU.S. Geological Survey. Teachers andstudents can learn to read topographicmaps, and even construct a topo map oftheir own school yard. Contact:National Science TeachersAssociation,Publication Sales;1840 Wilson Blyd.,Arlington, VA 22201; phone (800) 722-6782 or (703) 243-7100. The price $7.95 plus $3.75 shipping.
Su’rface of the Earth coior~relief mapposter measures 31 x 43" and is acomputer-generated color image of thetopography of the world. The price is$20.00 which includes shipping (forU.S.A. orders). To order, request ReportMGG--5 from National GeophysicalData Center, NOAA E/GC4, 325Broadway, Boulder, CO 80303--3328;phone (303) 497--6338.
Creative Dimensions 1994 CatalogofScience kits, materials and books listsan abundance of fo~il and mineralactivity and investigative kits. For afree copy of this catalog, write to:Creative Dimensions, P.O. Box 1393,Bellingham, Washington, 98227.
Science Fare 1994 Catalogoffers a rangeof science supplies, including rockspecimens and mineral study kits. For afree catalog, write to: Science Fare, 8246Menaul Blvd NE, Albuquerque, NM87110.
Note: Please mention Lite Geology whenordering any of the materials listed above,
I
Upcoming EventsNovember3-5,1994New Mexico Science Olympiad FallWorkshop for teachers will be held onthe campus of New Mexico Tech inSocorro. For More Information callVannetta Perr~ (505) 835-5678.
November 18 and 19,1994The Fall Conference of the New MexicoScience Teachers Association and NewMexico Math Teachers Associationwill be held in Albuquerque at theHilton Hotel. For moreinformation, contact Cindy Lauster,11520 Pase6 del Oso NE,Albuqueruqe, NM 87111.
Lite Geology evolves:Lite Geology began as a small Earth-science publication designed and scaled forNew Mexico. Our subscription list has grown tremendously during the past twoyears, and now includes a large number of out-of-state readers. In order to keep upwith the demand for this publication from outside of New Mexico, we will charge$4.00 per year for out-of-state readers, which covers the cost of printing andmailing Lite Geology. The subscription year begins with the Fall issue, and endswith the Summer issue to correspond with the acaderdic year. This current issue,Summer 1994, is thelast of the free issues that you will receive if you resideoutside of New Mexico. We thank all of our readers for their enthusiastic support,and hope that all of you will continue to subscribe. If ybu have questions aboutyour subscription, please cal! Theresa Lopez at (505) 835-5420. Thanks! ---ed.
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New Mexico Bureau of Mines and Mineral Resources Lite Geology, Summer 1994
" ~k I t4
geologlst studies hot sprlngs...As a follow-up to our last issue, we have continued to explore
earthquakes. We hope that our readers will be more able to interpretnews about earthquakes-why they occur, how they are measured,;andhow they influence local geology, such as in the geyser fields ofYellowstone. Although New Mexico does not have geysers to explore,there are a few geothermal areas in the state that have hot springs. TheNMBM&MR has about a dozen publications that discuss New Mexico’sgeothermal resources. These Bulletins, Circulars, and Hydrologic Reportsare listed in the current price list available through the NMBM&MRPublications Office, (505) 835-5410. See you next issuel
Lite Geology Staff
hot sp, rings
L I T E
is published quarterly by New MexicoBureau of Mines and Mineral Resources(Dr. Charles E. Chapin, Director and StateGeologist), a division of New Mexico Tech(Dr. Daniel H. Lope~, President).
Purpose: to help build earth scienceawareness by presenting ed’ucato~ andthe public with contemporary geologictopics, issues, and events. Use Lite Geologyas a source for ideas in the classroom orfor public education. Reploduction isencouraged with proper recognition ofthe source. All rights reserved oncopyrighted material reprinted withpermission within this issue.
Lite Geology Staff InformationEditor: Susan J. WelchGeological Editors: Dr. Dave Love and
Dr. Charles ChapinEducational Coordinator: Barbara PoppGraphic Designer: Jan ThomasCartoonist: Jan Thomaswith inspiration
from Dr. Peter MozleyEditorial Assistants: Toby Click and Lois
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~lailing Add~ssNew Mexico Bureau of Mines and MineralResources, Socorro, NM 87801. Phone(505) 835-5420. For a subscriptioninformation, please call or write. LifeGeology is printed on recycled paper.
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