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Basaltic hot-spot-type volcanoes. D.Basaltic mid-ocean-ridge-type volcanoes. E.Pull-apart earthquakes and faults often occur at pull-apart basaltic mid-ocean ridges, which are not subduction zones. Slide-past also occurs on the planet, but not primarily at subduction zones, which also are not hot spots. But subduction does lead to scraping of mud off the descending slabs, making piles such as the Olympic Peninsula. Points Earned:0/1 Your Response:C Old, cold ocean floor sinks at subduction zones. Why does this cause melting to feed volcanoes? 2.Slabs quickly become the hottest things in the mantle because of friction from the subduction. A.Subduction zones weaken the mantle so that hot spots can rise along the downgoing slabs. B.Water taken down subduction zones lowers the melting temperature in and near the slabs. C.Sediments scraped off downgoing slabs pile up, as at Olympic, trapping the Earth’s heat beneath and causing the rocks below to be warmer than elsewhere in the mantle. D.Subduction zones weaken the mantle so that convection cells from the deep mantle can rise along the downgoing slabs. E.Throw a little dry flour in a warm oven, and not much happens. Add some water, or better, some water and some carbon dioxide from yeast, and things happen in a hurry. The subduction zone takes water, and carbon dioxide in shells and other things, down to lower the melting point and feed volcanoes. Friction does warm the down-going slabs, but slabs start off way colder than the rocks into which they move, and remain colder for a while. Sliding your cold feet along the sheets when you get into bed on a winter night may warm your toes a little by friction, but if you happen to share the bed with a significant other, putting your tootsies on that persons bare belly will tell you that frictional heating takes a while! The scraped-off pile of sediment traps a tiny bit of heat, but not too much; the downgoing slab makes the nearby mantle colder than normal, not warmer. And nature tends to separate regions where something is flowing one way from regions where the flow is reversed; if the flows are too close together, one will drag the other along and change its direction. Hot spots occasionally ride along on spreading ridges, because both involve rising, but not on subduction zones. Points Earned:0/1 Your Response:E
Look at the picture above, from the coast of Olympic National Park. What happened here? 3.Glaciers coming down from the high peaks of Olympic National Park ground over the surface of the rock, carving the grooves we see. A.The pocket knife was confiscated by government agents when an absent-minded geologist tried to board an airline, and the government agents heaved it onto this rock. B.First Mt. Mazama and then Mt. St. Helens blasted rocks and ash and dust through the air, which fell as layers, with coarse at the bottom, fine on top from the first eruption, then coarse and fine again from the next eruption, and so on.