Days%207-8-Archean

Days%207-8-Archean - Hadean and Archean Time The Hadean is...

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Hadean and Archean Time
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The Hadean is the earliest eon of Earth history, from earth’s birth at 4600 Ma to 3800 Ma. The Archean is the next eon, from 3800 Ma to 2500 Ma Archean 3.8 Ga Hadean
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Outline of our discussion • Uniformitarianism • The hot early Earth/Radioactive heat • Formation of continental crust • When did plate tectonics begin? • Origin of life
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Outline of our discussion • Uniformitarianism • The hot early Earth/Radioactive heat • Formation of continental crust • When did plate tectonics begin? • Origin of life
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Uniformitarianism is the idea that the processes we observe today have acted in the past. Furthermore, the results of those processes will not change over time. For example, the blowing of sand across modern desert dunes yields a distinctive layered pattern called cross-bedding
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Therefore, ancient features we observe in rocks were caused by the same processes that form similar features today So these 180 million-year-old sandstones are interpreted to represent ancient sand dunes
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. . . this polygonal rock pattern on a sandstone is evidence of mudcracking 520 million years ago
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. . . and these marks on a 260 million- year-old sandstone were made by the footfalls of a rodent-like creature
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Clicker Question #1 Using uniformitarianist principles, what is the likely explanation for the pattern you see on the top of this sandstone? A. These are ripple marks from a river current B. These are burrows made by worms that dug through the sediment C. These are dikes of igneous rock created by an intrusion D. These are the veins on a fossil leaf E. These are mineral compositional layers typical of a sedimentary rock
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Today we will be telling the story of the Hadean and Archean eons, which represent earth’s earliest days As we already know, there were significant differences between the Earth then and the Earth now. Nevertheless, we will use uniformitarianism to interpret the events of this time, just as we will throughout the rest of this course.
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Outline of our discussion • Uniformitarianism • The hot early Earth/Radioactive heat • Formation of continental crust • When did plate tectonics begin? • Origin of life
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Soon after the moon-forming impact the earth’s surface was a magma ocean
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The heat for that magma ocean came from two sources The energy released by the frequent asteroid impacts during solar system formation . . . and that released by the decay of Earth’s radioactive elements US NRC
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The Earth was much hotter then because: 1. the rate of impacts was much higher 2. there were more radioactive elements available NASA
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will decay in a period of time known as the half- life . The half-life of each element is different. Earth began with many short-lived and long-
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This note was uploaded on 05/01/2010 for the course GEOL 1020 taught by Professor Pranter during the Spring '07 term at Colorado.

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Days%207-8-Archean - Hadean and Archean Time The Hadean is...

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