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Chapter_19_-_Earth_History_2007

Chapter_19_-_Earth_History_2007 - Chapter 19 Review Earth...

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Chapter 19 Review   Earth History (1) The decay of radioactive elements and heat released by colliding particles aided the melting of Earth's  interior, allowing the denser elements, principally iron and nickel, to sink to its center. As a result of this  differentiation, Earth's interior consists of shells or spheres of materials, each having distinct properties. (2) Earth's primitive atmosphere probably consisted of water vapor, carbon dioxide, nitrogen, and several trace  gases that were released in volcanic emissions, a process called out- gassing. The first life forms on Earth,  probably   anaerobic   bacteria,   did   not   need   oxygen.   As   life   evolved,   plants,   through   the   process   of  photosynthesis, used carbon dioxide and water and released oxygen into the atmosphere. Once the available  iron on Earth was oxidized (combined with oxygen), substantial quantities of oxygen accumulated in the  atmosphere. About 4 billion years into Earth's existence, the fossil record reveals abundant ocean-dwelling  organisms that require oxygen to live. (3) The Precambrian spans about 88 percent of Earth's history, beginning with the formation of Earth about  4.5 billion years ago and ending approximately 540 million years ago with the diversification of life, which  marks the start of the Paleozoic era.  It is the least understood span of Earth's history. 
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