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M_W_Chapter_19-2_Archean_and_Proterozoic_short_version

M_W_Chapter_19-2_Archean_and_Proterozoic_short_version -...

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What is the Precambrian? All rocks below the Cambrian Adam Sedgwick called the “basement” or unfossiliferous rocks beneath his Cambrian rocks in Wales “pre-Cambrian” Turns out to represent 88% of geologic time (4.6 Ga - 545 Ma) and contains an important record of early life!
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Subdivisions of Precambrian Early concepts ( too simple!!! ): Hadean Eon: 4.6 to 4.0 Ga no surviving rocks, probably many meteorite impacts Archean Eon: 4.0 to 2.5 Ga mostly metamorphic rocks (“basement” of continents) Proterozoic Eon: 2.5 to 0.5 Ga early sediments, not obviously fossiliferous
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Precambrian Time Scale and Terminology Keeps being modified! The time-scale in your book (Figure 19.2) is far too detailed for our needs and already out- of-date!
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“Genesis” and the “Hadean” Time before preserved sedimentary record (boundary could change with new discoveries) – 4.6 Ga to about 3.85 Ga Accretion and early differentiation (oldest rocks dated are 4.28 Ga) Moon-forming giant impact (4.51 Ga) as boundary between Genesis and Hadean
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“Genesis” Term now being used for first 50 million years of Earth history (4.56-4.51 Ga) Accretion and differentiation 4.56 Ga is age of oldest meteorites, considered also to be birth of the Earth from cloud of planetesimals 4.51 age of moon – formed by impact between Earth and Mars-sized body (NOT MARS!)
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Growth of planet by accretion: gravitational energy meteor bombardment Earth homogenous mixture of materials Gravitational compression shrinks early Earth Gravity concentrated denser materials (Ni & Fe) in center Less densematerials (silicates) forced to outer layers The Genesis Earth Accretion
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Compositional Zones Crust Continental Oceanic Mantle Core Outer (Liquid) Inner (Solid) The Genesis Earth Differentiation
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PRECAMBRIAN Evolution! Atmosphere Crust-Mantle-Core Differentiation Tectonics Life
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“Genesis” and Hadean Bombardment Meteorite impacts and cratering Protero zoic Archean Hadean Genesis Formation of Moon Lateheavy bombardment (Earth and Moon) Oldest known rock 4.28 Ga
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What was the Hadean like? No rocks - no record Accretion: heavily bombarded by meteorites Hot: gravitation, decay of short-lived isotopes Molten Differentiation into crust, mantle, and core
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What was theHadean like? Poisonous atmosphere, no oxygen No ozone layer: lethal UV radiation No surface water Moon was much closer than today Intense volcanic activity Somecrust existed (detrital zircon crystals dated at 4.3 to 4.4 Ga found in Australia)
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Where did our Atmosphere come from?
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