1Precambrian-Geology - PRECAMBRIAN before the Cambrian...

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PRECAMBRIAN before the Cambrian period” - refer to the whole period of earth's history before the formation of the oldest rocks with recognizable fossils in them. It is now also known as the Cryptozoic or "obscure life" eon. It is not very well understood because: 1) Precambrian rocks are deep-seated (they are called basement rocks) and tend to be poorly exposed at Earth’s surface. 2) The rocks are severely metamorphosed. 3) The rocks contain few fossils (correlation is difficult). Divided into 3 eons: 1) Hadean - 4.5-4.0 bya. 2) Archean - 4.0-2.5 bya. 3) Proterozoic - 2.50-0.6 bya.
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THE HADEAN (4.5 – 4 BYA) - began with the formation of the earth from dust and gas orbiting the Sun - volcanoes blast off all over the place, and the rain of rocks and asteroids from space never ends. - The air is hot, thick, steamy, and full of dust and crud. - Any rocks that do form from cooling lavas are quickly buried under new lava flows or blasted to bits by yet another impact - There is no rock record preserved for the Hadean. The major events of this time include: 1) Solar nebular processes. 2) The formation of Earth by accretion. 3) Differentiation of Earth into the crust, mantle, and core.
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THE ARCHEAN ("ANCIENT" OR "PRIMITIVE") - begins about a billion years after the formation of the earth. - most of the carbon dioxide is gone, having been chemically changed into limestone and deposited at the bottom of the ocean. - The continents have not formed yet. The islands are carried over the surface of the earth by the movement of rock deep in the earth's interior THE PROTEROZOIC ("EARLY LIFE") - It began about two billion years after the formation of the earth and lasted about another two billion years - The earth's interior has cooled some more, and there are fewer volcanoes than in the Archean. - Plate Tectonics are still very fast and continental collisions are frequent - In fact, geologists date the beginning of the Proterozoic Eon by the age of the oldest continental rocks that have not been reheated or chemically altered.
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Precambrian (Archean and Proterozoic) Rocks What kinds of Precambrian rocks exist?
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  • Spring '16
  • Historical Geology, Geological history of Earth, Proterozoic, precambrian rocks

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