2015年6月gmat閱讀機經：火山

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　　火山

　　Earth’s surface consists of rigid plates that areconstantly shifting and jostling one another. Platemovements are the surface expressions of motionsin the mantle—the thick shell of rock that liesbetween Earth’s crust and its metallic core. Althoughthe hot rock of the mantle is a solid, under thetremendous pressure of the crust and overlyingrock of the mantle, it flows like a viscous liquid. Themantle’s motions, analogous to those in a pot ofboiling water, cool the mantle by carrying hotmaterial to the surface and returning cooler materialto the depths. When the edge of one plate bends under another and its cooler material isconsumed in the mantle, volcanic activity occurs as molten lava rises from the downgoing plateand erupts through the overlying one.

　　Most volcanoes occur at plate boundaries. However, certain “misplaced” volcanoes far fromplate edges result from a second, independent mechanism that cools the deep interior ofEarth. Because of its proximity to Earth’s core, the rock at the base of the mantle is muchhotter than rock in the upper mantle. The hotter the mantle rock is, the less it resists flowing.Reservoirs of this hot rock collect in the base of the mantle. When a reservoir is sufficientlylarge, a sphere of this hot rock forces its way up through the upper mantle to Earth’s surface,creating a broad bulge in the topography. The “mantle plume” thus formed, once established,continues to channel hot material from the mantle base until the reservoir is emptied. Thesurface mark of an established plume is a hot spot—an isolated region of volcanoes anduplifted terrain located far from the edge of a surface plate. Because the source of a hot spotremains fixed while a surface plate moves over it, over a long period of time an active plumecreates a chain of volcanoes or volcanic islands, a track marking the position of the plumerelative to the moving plate. The natural history of the Hawaiian island chain clearly shows themovement of the Pacific plate over a fixed plume.

　　Q22:

　　The passage is primarily concerned with discussing

　　A. the composition of Earth’s mantle

　　B. how the Hawaiian Islands were created

　　C. what causes Earth’s surface plates to move

　　D. two different mechanisms by which volcanoes are formed

　　E. why most volcanoes occur at plate boundaries

　　Q23:

　　It can be inferred from the passage that a chain of volcanoes created by a mantleplume would most likely be characterized by

　　A. a curved outline

　　B. constituent volcanoes that differ from each other in age

　　C. occurrence near a plate boundary where one plate bends under another

　　D. appearance near many other volcanic chains

　　E. rocks with a wide range of chemical composition

　　Q24:

　　The author’s reference to the Hawaiian Islands serves primarily to

　　A. provide an example of a type of volcanic activity that does not occur elsewhere

　　B. identify the evidence initially used to establish that the Pacific plate moves

　　C. call into question a theory about the source of the volcanoes that created the HawaiianIslands

　　D. illustrate the distance from plate edges at which volcanoes typically appear

　　E. provide an example of how mantle plumes manifest themselves on Earth’s surface

　　Q25:

　　According to the passage, a hot spot on Earth’s surface is an indication of which of thefollowing?

　　A. An untapped reservoir of hot rock in the base of the mantle

　　B. Volcanic activity at the edge of a plate

　　C. Solid mantle rock under tremendous pressure

　　D. The occurrence of a phenomenon unique to the Pacific plate

　　E. A plume of hot mantle rock originating near Earth’s core

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