What is the Precambrian?
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
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 The time-scale in your book (Figure 19.2) is far too detailed for our needs and already out-of-date!
“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
“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!)
•Growth of planet byaccretion:•gravitational energy•meteor bombardment•Earth homogenous mixtureof materials•Gravitational compression shrinks early Earth•Gravity concentrated denser materials (Ni & Fe) in center•Less densematerials (silicates) forced to outer layersThe Genesis EarthAccretion
Compositional Zones•Crust•Continental•Oceanic•Mantle•Core•Outer (Liquid)•Inner (Solid)The Genesis EarthDifferentiation
“Genesis” and HadeanBombardment•Meteorite impacts and crateringProterozoicArcheanHadeanGenesisFormation of MoonLateheavy bombardment (Earth and Moon)Oldest known rock 4.28 Ga
What was the Hadean like?
What was theHadean like?
Where did our Atmosphere come from?•Hadean atmosphere•atmosphere of H, He•Earth had no core, no magnetic field, no Van Allen Belts, so solar wind sweeps gases away, atmosphere lost
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- Geology, Geological history of Earth, Proterozoic, Archean, nt plate rge