Geosci Unit 4 notes.docx - Unit 4 Main Topic in Unit 4...

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Unit 4 Main Topic in Unit 4 Plate Tectonics III: Obduction o In subduction, denser side sinks under less-dense side o But continents and island arcs are too low-density to go down—” You can’t sink a continent” o When they run into each other, OBDUCTION results, with folding, push-together (thrust) faulting, and thickening o This makes the biggest mountain ranges—Appalachians (still high after 200 million years), Himalaya, etc. o Can even push older rocks on top of younger ones A Little History o Appalachians formed as proto-Atlantic closed o Had subduction-zone volcanoes with big eruptions, island arcs colliding with continent, etc. o This ended when Africa and Europe hit the Americas and pushed up the Appalachians (Great Smokies, State College, PA) o When the push-together ended, the great, hot pile of the Appalachians spread under its own weight, with Death-Valley-type faulting o Thinning with spreading reduced pressure on mantle, inducing rising and melting (convection), opening Atlantic The Three Basic Tectonic Styles o PUSH-TOGETHER: subduction (Olympic, Crater Lake, Mt. St. Helens) or obduction (Great Smokies) o PULL-APART: rifting/spreading/sea-floor-production (Death Valley) o SLIDE-PAST: faulting (San Andreas) o Can have intermediates (push-together while sliding past, or pull-apart while sliding past) o The three types of plate boundaries, plus hot-spot activity poking up through plates, give the great majority of mountain-building, earthquakes, volcanoes, etc. Meanwhile, Out West: o As Atlantic opens, Asia and Americas approach each other, narrowing the Pacific o Subduction under western US started with cold rock, but as continent moved toward the Pacific spreading ridge, more buoyant rock was forced down, scraped along under US rather than sinking deep, and rumpled up the lithosphere to make Rockies, etc., far inland o Where subduction zone reached and swallowed ridge, rock is no longer going down under the west; subduction zone was push-together plus slide-past, and the slide-past remains as the San Andreas Fault o Where and when the push-together of the subduction ended, the pile of the western US spread under its own weight, giving Death Valley faulting.
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