HistGeolChapt8 - Chapter 8 Precambrian Earth and Life History Hadean and Archean The Precambrian lasted for 4 b.y 88 of estimated geologic time No

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Chapter 8 Precambrian Earth and Life History— Hadean and Archean The Precambrian lasted for 4 b.y., 88% of estimated geologic time. No rocks are known from the first 640 million years of geologic time, though evidence suggests their existence.
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Hadean Early Early Proterozoic Archean Middle Middle Late Late 2.5 4.0 3.4 3.0 1.6 .9 .64 Precambrian Terminology in North America Phanerozoic
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about 4.6 billion years ago Earth was a rapidly rotating, hot, barren, waterless planet, bombarded by comets and meteorites, with no continents, widespread volcanism yielded a thin atmosphere of volcanic gases, unable to shield from cosmic and UV radiation. As Earth cooled, water vapor in atmosphere condensed, infilling topographic lows as new oceans. 3
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Hadean crust was likely thin, unstable, and ultramafic. Rotation of the Earth is an hypo- thesized origin of vertical convection currents within the upper portions of the molten outer mantle. As Earth cooled, more crustal material solidified. Each time ultramafic and mafic crust was subducted, a little more silica and aluminum was “liberated” by partial melting. Re-erupted andesitic lavas helped “build” the earliest island arc systems. Collisions of island arc systems “built” the nuclei of earliest continents. Oldest zircons in metamorphic rocks age-dated @ 4.2 b.y. only remnants of earliest igneous rocks. Oldest rocks include 3.96 b.y. Acasta Gneiss in Canada, other rocks in Montana. 4
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Continuation of subduction and partial melting further contributed intermediate and later felsic rocks to the growing continents. Exposed Precambrian rocks are referred to as Precambrian shields ”. Surrounding the shields are platforms of covered Precambrian rocks. Shield + platform = craton. Most cratons have experienced little deformation since the Precambrian ended. Exposed North American craton = Canadian
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This note was uploaded on 03/27/2012 for the course GEOL 2413 taught by Professor Moody during the Spring '10 term at Texas A&M University, Corpus Christi.

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HistGeolChapt8 - Chapter 8 Precambrian Earth and Life History Hadean and Archean The Precambrian lasted for 4 b.y 88 of estimated geologic time No

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