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Geo Chap 12 - March 4 2008 Chapter 12 Page 1 Geological...

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March 4, 2008 Chapter 12; Page 1 Geological Catastrophes: Chapter Twelve I. Short Term Climate Change: Thousands of Years A. Variations in amount of solar radiation due to variations in Earth’s orbit 1. Eccentricity of orbit 2. Tilt of Earth’s axis 3. Precession of tile (wobble) B. How it’s measured/observed: 1. Ice cores and marine sediment cores a) Oxygen isotope measurements 18 O/ 16 O ratios b) Higher 18 O, cooler climate, more 16 O tied up in ice/snow 2. Glacial deposition 3. Fossils 4. Tree rings C. Glacial Advance and Retreat 1. Last 1 million years : about 10 glacial advances, retreats 2. Worldwide glacial advances lasting almost 100,000 years 3. Followed by retreats lasting decades to few thousand years—much faster than advance 4. Caused by cycles in Earth’s orbit around Sun affecting amount of solar energy received by Earth a) Changes postulated in 1920s by Serbian astronomer Milankovitch and supported recently by Greenland ice cores D. Milankovitch 1. Milankovitch defined changes in Earth’s orbit, tilt and wobble changes in amount of solar radiation received by Earth a) Amount of solar radiation at high latitudes during summers determines how much snow remains from winter to next winter, allowing glaciers to grow E. Changes in Earth’s Orbit: 1. Eccentricity of orbit around Sun: varies every 100,000 years from circular to elliptical (less solar radiation received when elliptical) as dominant control of glacial advance and retreat 2. Tilt of Earth’s axis: 21.5 – 24.5 o off vertical in 41,000 year cycle 3. Precession of tilt: direction of tilt changes (wobble in spin of toy top) in double cycle of 23,000 and 19,000 years F. Around 20,000 years ago : 1. Glaciers at maximum extent, covered 27% of today’s land a) Virtually all of Canada, part of northeastern U.S. 2. Seawater necessary to build glaciers lowered sea level 130 m 3. Current Ice Age continues (current glacial retreat) a) 10% continents still buried under ice b) If ice melts, sea level would rise 65 m G. At 7,000 years ago : 1.
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