2/3/10 4:03 PMGEOSC 10: Geology of the National Parks (Web, merged), SP10Page 1 of 8Rock On #3Submitted by sjk5180 on 2/3/2010 3:54:42 PM Points Awarded10Points Missed5Percentage67%1.Which of the following is commonly expected near a “textbook” subduction zone (that is, near a subduction zonethat is so perfect and free of confusing complications that you would use it in a textbook to teach students)?2.Old, cold ocean floor sinks at subduction zones. Why does this cause melting to feed volcanoes?A.Pull-apart earthquakes and faults.B.Basaltic mid-ocean-ridge-type volcanoes.C.Slide-past (or transform, with horizontal but not vertical movement) earthquakes and faults.D.Basaltic hot-spot-type volcanoes.E.Andesitic stratovolcanoes.Pull-apart earthquakes and faults often occur at pull-apart basaltic mid-ocean ridges, which are not subduction zones. Slide-past also occurs on theplanet, but not primarily at subduction zones, which also are not hot spots. But subduction does lead to layered thick-lava-flow/blown-up-bitsstratovolcanoes of andesitic composition.Points Earned:0/1Correct Answer:EYour Response:DA.Sediments scraped off downgoing slabs pile up, as at Olympic, trapping the Earth’s heat beneath and causingthe rocks below to be warmer than elsewhere in the mantle.B.Water taken down subduction zones lowers the melting temperature in and near the slabs.C.Slabs quickly become the hottest things in the mantle because of friction from the subduction.D.Subduction zones weaken the mantle so that convection cells from the deep mantle can rise along thedowngoing slabs.E.Subduction zones weaken the mantle so that hot spots can rise along the downgoing slabs.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, andthings happen in a hurry. The subduction zone takes water, and carbon dioxide in shells and other things, down to lower the melting point and feedvolcanoes. 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 thebed 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 ofsediment traps a tiny bit of heat, but not too much; the downgoing slab makes the nearby mantle colder than normal, not warmer. And nature tendsto 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 theother along and change its direction. Hot spots occasionally ride along on spreading ridges, because both involve rising, but not on subduction zones.
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