lecture13 - The History of Earth and Life Lecture 13 1...

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The History of Earth and Life Lecture 13 1 Lecture 13 - Ancient Rocks Precambrian Eon (= Cryptozoic Eon) Precambrian - "The time before Cambrian period" Cryptozoic - "The time of hidden (or obscure) life" The time before abundant fossils appeared in the rock record The first eon in the history of the Earth From ~4.6 to 0.54 billion years Represents over 85% of the Earth's History Subdivisions (Eras) of the Precambrian Eon Hadean (~4.6-4.0) - before oldest records Archean (4.0-2.5) - time of microplates and protocontinents Proterozoic (2.5-0.54) - full-sized continents develop Archean The oldest minerals and rocks are Archean in age 4.1–4.3 byr - The oldest zircon crystals: Derived from some crustal rocks that have been eroded away 3.96 byr - Acasta Gneiss, northern Canada: The oldest whole rock (preserved crust) 3.85 byr - Akillia Island, Greenland: The oldest metamorphosed sediments Banded Iron Formation Graywacke - immature sandstone with graded beds Carbonates (limestone) All of these rocks must have been deposited in water 3.5 byr - Morton Gneiss: The first direct evidence that continental crust was exposed to the atmosphere and underwent direct weathering 1. Two angular unconformities are present in a thick sequence of Archean sedimentary/volcanic rocks 2. The rocks containing some of the oldest recorded evidence of life come from underneath the younger of the two unconfomities Correlation of Archean Rocks
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This note was uploaded on 03/24/2008 for the course GEOS 1014 taught by Professor Sxiao during the Spring '08 term at Virginia Tech.

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lecture13 - The History of Earth and Life Lecture 13 1...

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