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Unformatted text preview: GEOS 1004, SPOTILA Lecture 14, 10/20//09 PLATE TECTONICS - PLATE BOUNDARIES B. Subduction zones ( When you have plates come together which one goes down into the mantle) *where cold oceanic lithosphere sinks below the surface and into the asthenosphere. -driven by gravity, or density Rules the moho- boundary between the crust on top and mantle underneath it 1) plates sink because they are dense: any oceanic lithosphere sinks beneath continental lithosphere 2) continents never subduct; they collide 3) cold oceanic lithosphere is more dense, so old (has had time to cool) oceanic lithosphere subducts beneath young oceanic lithosphere -no oceanic lithosphere is >200 Myr old. Consequences and evidence of subduction: 1) trenches - slab is sinking, so it is low, or deep below sea-level; 10 km ocean depth 2) accretionary prism; oceanic sediment accumulates in soft, wet, deformed mass on edge of overriding plate 3) volcanoes; stratovolcanoes, partial melting of mantle and crust - melting caused by volatiles (water) coming off the subducting plate; lowers the melting temperature...
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This note was uploaded on 09/11/2011 for the course GEOG 1004 at Virginia Tech.