1. Which of the following is not expected very often near a “textbook” subduction zone (that is, near a subduction zone that is so perfect and free of confusing complications that you would use it in a textbook to teach students)? A) Stratovolcanoes formed by explosions and lava from the slab being subducted. B) Piles of sediment scraped off the slab being subducted. C) Push-together earthquakes and faults as the subducting slab squeezes the rocks. D) Explosive volcanoes fed by melt from the slab being subducted. E) Slide-past (or transform, with horizontal but no vertical movement) earthquakes and faults such as the San Andreas. Feedback: Most of the action at subduction zones is “push-together”, including push-together earthquakes and faults, scraping off of sediment to make piles as one side moves under the other side, and volcanic explosions that contribute to layered volcanoes, or “stratovolcanoes”. Slide-past motion is not dominant, intermediates between pure subduction and pure slide-past motion do exist, but are not “textbook” cases of subduction. 2. Old, cold ocean floor sinks at subduction zones. Why does this cause melting to feed volcanoes? downgoing slabs. causing the rocks below to be warmer than elsewhere in the mantle. Feedback: 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.
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- Fall '08
- Correct Answer, Volcano, St. Helens, Basalt, GEOSC 010, Quiz, RockOn #3, 3