1 / 26

Chapter 11: The Archean Eon of Precambrian Time

Chapter 11: The Archean Eon of Precambrian Time. 4.6 to 2.5 BYA. Observations red shift: expansion looking back in time stars & galaxies quasars cosmic background laws of physics. explanation: big bang formation of all matter from energy elemental composition: 75% H and 25% He age

lysandra-rose
Download Presentation

Chapter 11: The Archean Eon of Precambrian Time

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. Observations red shift: expansion looking back in time stars & galaxies quasars cosmic background laws of physics explanation: big bang formation of all matter from energy elemental composition: 75% H and 25% He age methods date the cosmic background (using red shift) run the expansion backwards estimate the mass 10 to 20 billion years Origins: Universe

  3. Origins: solar system • observations • 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

  4. Origins: solar system • more observations • our solar system • composition: H and He with other, heavier elements • distribution • sun at center with most of mass • planetary composition • all are different • most dense element nearest sun • least dense elements farthest from sun • uniform rotation and revolution • comets and asteroids

  5. Origins: Solar system • explanation: nebular hypothesis (fig p 293) • nebula formed of dust and gas {of previous star(s)} • collapse due to disturbance • slow rotation increases as nebula collapses • mass 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

  6. Origins: Solar System • age • methods • solar fuel use • radiometric dating • Xe and Pu isotope studies • 4.5 to 5 BYO • time to form 50 to 100 MY

  7. observations layered interior asteroid & comet compositions other planets other 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: Earth

  8. Origins: Moon • observations • almost no water • small metallic core • feldspar-rich outer layer • fast earth rotation • compositionally differs from Earth • explanation: glancing blow • planetesimal sideswiped earth • shortly after Earth’s accretion

  9. Early Archean conditions • no rocks • heavy impacting • very large impacts: alter rotation • large impacts • disrupt surface • extinguish life • vaporize oceans • internal heat production - 2 to 3 X modern rate

  10. late Archean rocks • Sedimentary (most are similar to modern types) • deep water marine (graywacke, BIFs, volcanic seds) • terrestrial/shallow marine • some quartz sandstone • some carbonates • examples: Witwatersand sequence/Pongola Supergroup • greenstone belts • located in bands between felsic gneisses • low-grade metamorphic • mafic and ultra-mafic meta-volcanics (inc. pillow basalts) • some felsic volcanics • turbidites and mudstones • BIFs - interlayered chert and iron • interpretation: old ocean crust caught between colliding continents

  11. late Archean: crust forms • oceanic crust (mafic) • forms from mantle material • differentiates as it cools • may 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

  12. Archean tectonics • early Archean • thin crust? • small continents • mostly mafic crust? • vigorous movement • disruption by impact • later Archean • movement and impacts slow • cratons 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)

  13. Archean 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) • oceans • origin • (1) outgassing & comets • (2) earth cooled & water condensed • (3) salts from volcanoes and weathered rocks • composition appx. same as today

  14. Late Archean life • fossils • single-celled • small: prokaryotic • stromatolites • conditions • frequent to occasional bombardment • no oxygen • no UV protection • energy sources: sun, internal heat, bombardment • ocean full of chemicals

  15. life begins • steps • synthesize amino acids • assemble RNA • assemble cell • characteristics • need energy and building materials • location • underwater? • underground? • mid-ocean ridges? • life habits • chemosynthetic (1st) • consumers (2nd) • photosynthetic (3rd)

  16. Chapter 12: Proterozoic Eon of preCambrian Time 2.5 BYA to 544 MYA

  17. Proterozoic 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

  18. Proterozoic Plate tectonics • history and appearance of typical orogen • cross sections p. 319 • suite 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

  19. Proterozoic Plate tectonics • Laurentia (North America), maps p. 331, 332, 335 • craton: 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

  20. Proterozoic 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 assemble • 2nd supercontinent assembles? • south and east of Laurentia • Neoproterozoic

  21. Proterozoic Life • Fossils • micro & macro • limited • poorly exposed • missing

  22. Proterozoic Life • chemical evidence early life • distinctive organic compounds: indicate types of life • atmospheric & 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

  23. Proterozoic Life • prokayotic • bacteria & cyano bacteria (Kingdom Monera) • (very limited internal structures, very small) • (from Archean) • stromatolite colonies • seafloor covered with biotic “carpet”

  24. Proterozoic 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

  25. Proterozoic 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

  26. Proterozoic Ice ages • tillite deposits • record • Paleoproterozoic: appx 2.3 BYA • Neoproterozoic • 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?

More Related