Chapter 11: The Archean Eon of Precambrian Time 4.6 to 2.5 BYA.

Chapter 11:The Archean Eon of Precambrian Time

Chapter 11:The Archean Eon of Precambrian Time

4.6 to 2.5 BYA

Origins: UniverseOrigins: Universe

Observationsred shift: expansion

looking back in time– stars & galaxies– quasars– cosmic

background

laws of physics

explanation: big bangformation of all matter from energy

elemental composition: 75% H and 25% He

agemethods– date the cosmic

background (using red shift)

– run the expansion backwards

– estimate the mass

10 to 20 billion years

Origins: solar systemOrigins: solar systemobservations

galaxies: hot, new stars in nebulae

other star systems & nebulae– composition

old stars: mostly H and He newer stars: mostly H and He with other, heavier

elements

– activity collapsing nebulae protostars planets

Origins: solar systemOrigins: solar system

more observationsour solar system– composition: H and He with other, heavier

elements– distribution

sun at center with most of mass planetary composition

all are differentmost dense element nearest sunleast dense elements farthest from sun

uniform rotation and revolution comets and asteroids

Origins: Solar systemOrigins: Solar system explanation: nebular hypothesis (fig p 293)

nebula formed of dust and gas {of previous star(s)}collapse due to disturbanceslow rotation increases as nebula collapsesmass collects at center of system– hot, dense gas begins fusion (sun ignites)

additional material collects around smaller centers of mass (planetesimals)– higher density elements condense near primary center of

mass– lower density material cleared from center by solar wind

planetesimals coalesce into planets

Origins: Solar SystemOrigins: Solar System

agemethods– solar fuel use– radiometric dating–Xe and Pu isotope studies

4.5 to 5 BYO

time to form 50 to 100 MY

Origins: EarthOrigins: Earth observations

layered interiorasteroid & comet compositionsother planetsother star systems

explanation: planetary accretion

homogeneous (fig p 294)– (1) accretion of

planetesimals– (2) melting– (3) differentiation into

layers

heterogeneous– (1) accretion of most

dense material while the nebula was hot and less dense stuff as the nebula cooled

Ni & Fe first peridotite later

– (2) limited differentiation later

– (3) atmosphere still accreting

Origins: MoonOrigins: Moon

observationsalmost no water

small metallic core

feldspar-rich outer layer

fast earth rotation

compositionally differs from Earth

explanation: glancing blowplanetesimal sideswiped earth

shortly after Earth’s accretion

Early Archean conditionsEarly Archean conditions

no rocks heavy impacting

very large impacts: alter rotationlarge impacts– disrupt surface– extinguish life– vaporize oceans

internal heat production - 2 to 3 X modern rate

late Archean rockslate Archean rocks Sedimentary (most are similar to modern types)

deep water marine (graywacke, BIFs, volcanic seds)terrestrial/shallow marinesome quartz sandstonesome carbonatesexamples: Witwatersand sequence/Pongola Supergroup

greenstone beltslocated in bands between felsic gneisseslow-grade metamorphic– mafic and ultra-mafic meta-volcanics (inc. pillow basalts)– some felsic volcanics– turbidites and mudstones

BIFs - interlayered chert and ironinterpretation: old ocean crust caught between colliding continents

late Archean: crust formslate Archean: crust forms oceanic crust (mafic)

forms from mantle materialdifferentiates as it coolsmay have melted and reformed several times

continental crust (intermediate-felsic)hot spots– segregation of molten rock– partial remelting of roots

subduction zones– water from subducting crust enters mantle– partial melting produces intermediate-felsic magmas

original differentiation– intermediate and felsic material floated to the top of the molten

earth

Archean tectonicsArchean tectonics early Archean

thin crust?small continentsmostly mafic crust?vigorous movementdisruption by impact

later Archeanmovement and impacts slowcratons form– 2.7 to 2.3 BYA– continents accrete as island arcs coalesce (greenstone belts)

plate core: shield & platform (oldest rocks)

– mountains form and weather (sedimentary rocks)

Archean air and waterArchean air and water atmosphere

origin– (1) outgassing– (2) accretion of comets

composition– (1) water vapor– (2) H, HCl, CO, CO2, N– (3) no oxygen (very reactive, combines with iron in water)

oceansorigin– (1) outgassing & comets– (2) earth cooled & water condensed– (3) salts from volcanoes and weathered rocks

composition appx. same as today

Late Archean lifeLate Archean life

fossilssingle-celledsmall: prokaryoticstromatolites

conditionsfrequent to occasional bombardmentno oxygenno UV protectionenergy sources: sun, internal heat, bombardmentocean full of chemicals

life beginslife begins steps

synthesize amino acidsassemble RNAassemble cell

characteristicsneed energy and building materialslocation– underwater?– underground?– mid-ocean ridges?

life habits– chemosynthetic (1st)– consumers (2nd)– photosynthetic (3rd)

Chapter 12: Proterozoic Eon of preCambrian Time

Chapter 12: Proterozoic Eon of preCambrian Time

2.5 BYA to 544 MYA

Proterozoic Plate tectonicsProterozoic Plate tectonics continents assemble, develop primary

features central craton– original “microcontinents”– shield - eroding– platform - collecting sediment

orogenic belts– mountain ranges

interior (old, now part of craton) exterior (young, around edge of craton)

– orogenies weld large continental masses together

Proterozoic Plate tectonicsProterozoic Plate tectonics

history and appearance of typical orogen cross sections p. 319suite of rocks preserve record– rifting & spreading– passive margin – approching continental mass/island arc

100's of millions of years of erosion - planed off mountains leaving igneous, metamorphic and sedimentary suites exposed on flat land

Proterozoic Plate tectonicsProterozoic Plate tectonics Laurentia (North America), maps p. 331,

332, 335craton: Canadian Shield, Interior Lowlands– at least six microcontinents assembled between 1.95

and 1.85 BYA

orogenic belts– interior (Proterozioc): Wopmay, Trans Hudson,

Grenville, et.al.– exterior (Phanerozoic): Cordilleran, Ouachita,

Appalacian

failed rift– Mid-continent Rift (fig p 335) - 1.3 to 1.0 BYA– Keweenawan Supergroup: mafic intrusions and

extrusions>continental seds in grabens

Proterozoic Plate tectonicsProterozoic Plate tectonics supercontinent(s) assemble and break apart

Rodinia (figs p 332, 336)– Mesoproterozoic? - complete by 1.0 BYA– continents assemble around Laurentia– collision and orogeny (ie. Grenville Orogeny - 1.2 to 1.0 BYA)

rifting and separation of Rodinia– Pacific Ocean opens

extensive deposition, esp. in failed rifts (of triple junctions) Belt Supergroup et.al. (map p. 335, x-section p. 337)

South American & African cratons assemble2nd supercontinent assembles?– south and east of Laurentia– Neoproterozoic

Proterozoic LifeProterozoic Life

Fossilsmicro & macro

limited– poorly exposed–missing

Proterozoic LifeProterozoic Life

chemical evidence early lifedistinctive organic compounds: indicate types of lifeatmospheric & oceanic oxygen builds– source: photosynthesis– removal of sinks esp. Fe and C– rocks that contain minerals uraninite & pyrite

pre 2.3 BYA rocks would break down in presence of free oxygen

– banded iron formations (BIFs) 3.5 to 1.9 BYA

– continental red beds after 2 BYA

extensive bioturbation of ocean floor begins

Proterozoic LifeProterozoic Life

prokayoticbacteria & cyano bacteria (Kingdom Monera)

(very limited internal structures, very small)

(from Archean)

stromatolite colonies

seafloor covered with biotic “carpet”

Proterozoic LifeProterozoic Life early eukaryotic

Kingdom Protista(also from Archean?)key developments– cytoskeleton (flexible cell wall)– assembled from symbiotic Monerans (fig p 321)

“host cells”, “mitochondial bacteria”, cyanobacteria

– genetic drift and lateral gene transfer

types– acritarchs - single-celled algae

Proterozoic LifeProterozoic Life multi-cellular life (metazoans)

algae (seaweed)trace fossils– post 570 BYA– animals: moving, feeding, burrowing– oldest - simplest– later - increase in variety and complexity– indicate soft-bodied, multicellular life

soft-bodied animals– cnidaria– Ediacaran fauna (may contain unnamed Phyla)– annelida– arthropoda– mollusca

skeletal fossil - cloudinia

Proterozoic Ice agesProterozoic Ice ages tillite deposits record

Paleoproterozoic: appx 2.3 BYANeoproterozoic– 4 advances (?) between 850 and 600 MYA– deposits within 30 degrees of EQ– snowball earth?

buildup of ice change in C isotope ratios deposition of BIFs effect on life?