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Unformatted text preview: Ch. 8: Precambrian Earth and Life History The Archean Eon Key Words- Subdivisions of the Precambrian Archean Proterozoic- Shields, Platforms, and Cratons- Greenstone belts, granite-gneiss complexes- Atmosphere and hydrosphere- First organisms o Stanley Miller Experiment o Stromatolites _____________________________________________________________ 1 The Precambrian (Archean and Proterozoic Eons)- Over 4 billion years- About 88% of all geologic time- No rocks known for first 640 million years- Geologic record begins at _____________billion years o Rocks often altered by metamorphism, deformed, deeply buried, few fossils The Eoarchean- 4.6 billion to 3.6 billion years before present- Earth accreted from planetisimals- Differentiation into core, mantle, and some crust (Acasta Gneiss 3.96 b.y.; zircon minerals 4.4 b.y.)- >> Meteorites- >> Volcanic activity- Atmosphere formed- Surface water (by 4.4 b.y.?) 1) 2) 3) 4) Accretion -> Cratons Continental Foundations...
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