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Unformatted text preview: Lecture 25 3/12/10 Ice Age What happens when ice accumulates, and how it enables identification of past ice ages?- Greenland 2 mile thickness- Antarctica 2+ mile; unusually thick- Oceanic ice is not as thick- Ice sheet likely to start from the form of snow- Glacier: river of ice moving slowly- Moraine: accumulation at the tip of the glacier- Ice melts, retreats, and leaves the sediments from the moraine- Till material: gravel, cobbles- Erratic: a boulder without a source carried by a glacier How common are the great ice ages? Have been at least 3 ice house time periods (snowball earth, end Ordovician, carboniferous-permian) Therefore, must be quite rare Why warm and cold alternate during an ice age? Milankovitch Cycles: 3 elements of planetary motion- Eccentricity of the orbit: our planets orbit changes- Tilt of planet: goes back and forth from between 21.5 to 24.5 deg- Wobble of the axis: when axis incline, must wobble When combine wave shape of eccentricity, tilt, and precession, the wave matches ice age timescale...
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- Spring '08