7. Precambrian

7. Precambrian - 339 Hadean bombardment Early Earth History...

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1 339 Hadean bombardment Early Earth History: The Pre- Cambrian 4.5-0.5 bya: Hadean Archean Proterozoic Key Events of Hadean (4.5-3.8 by) •Earth Accretes (4.55-4.60 by) •Differentiates •Moon forms •Early rocks, Oceans and Atmosphere (anoxic) form •‘Magma ocean’ in mantle, thin skin tectonics Time Line Valley: U. Wisconsin Constraints for Early Earth Models 1. Firm dates (know bold ones) Earth and meteorites are 4.55-4.60 by Age of moon (0.03-0.07 by younger than Earth) Oldest mineral grain is a zircon (4.4 by) -> solids present at surface, probably liquid water Oldest rocks are gneisses (4.0 by, maybe 4.3 by?) Metamorphosed sedimentary rocks (3.8 by) = ocean for sure Oldest fossils (3.5 by) 2. Physical and Chemical Composition of starting materials and final product Balances for heat and mass constrain models Accretion of Planets Earth accretes about 4.55 by ago Requires about 0.05-0.10 by Core Forms to Make Layers in Earth (seismic waves define density structure) crust (5-60 km thick) mantle (2900 km thick) core (3500 km radius) outer 2/3 is liquid inner 1/3 is solid Earth differentiates within <100 my Formation of the Moon A B C debris in orbit moon collision ejected debris debris accretes Fig. 17-2. Cartoon illustrating formation of the moon Moon forms from giant impact 0.03-0.09 by after Earth Low lunar Fe -> moon forms after Earth core formed
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2 Lunar Crater 0 100 200 300 400 500 600 0 1 2 3 4 5 Lunar Crater History Relative crater rate Time (billion years) before present after Lang and Whitney (1991) Frequent impacts during Hadean Temperature Distribution in Earth 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 6000 Temperature (°C) Depth (km) outer (liquid) inner (solid) core melts observed mantle partial melt mantle melts core mantle ….. 'magma ocean' in upper mantle, thin skin tectonics Heat Production in the Earth 0 1 2 3 4 5 0 1 2 3 4 Relative heat production Time before present (by) Assumes present Th/U=4, K/U = 10000
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This note was uploaded on 11/30/2010 for the course GEOL 125Lxg at USC.

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7. Precambrian - 339 Hadean bombardment Early Earth History...

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