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  3. No very satisfactory account of the mechanism that caused the formation of t

No very satisfactory account of the mechanism that caused the formation of t

游客2024-01-13  32
问题     No very satisfactory account of the mechanism that caused the formation of the ocean basins has yet been given. The traditional view supposes that the upper mantle of the earth behaves as a liquid when it is subjected to small forces for long periods and that differences in temperature under oceans and continents are sufficient to produce con- vection in the mantle of the earth with rising convection currents under the mid-ocean ridges and sinking currents under the continents. Theoretically, this convection would carry the conti- nental plates along as though they were on a conveyor belt and would provide the forces needed to produce the split that occurs along the ridge. This view may be correct: it has the advantage that the currents are driven by temperature differences that themselves depend on the position of the continents. Such a back-coupling, in which the position of the moving plate has an impact on the forces that move it, could produce complicated and varying motions.
    On the other hand, the theory is implausible because convection does not normally occur along lines, and it cer- tainly does not occur along lines broken by frequent offsets or changes in direction, as the ridge is. Also it is difficult to see how the theory applies to the plate between the Mid-Atlantic Ridge and the ridge in the Indian Ocean. This plate is growing on both sides, and since there is no intermediate trench, the two ridges must be moving apart. It would be odd if the rising convection currents kept exact pace with them. An alternative theory is that the sinking part of the plate, which is denser than the hotter surrounding mantle, pulls the rest of the plate after it. Again it is difficult to see how this applies to the ridge in the South Atlantic, where neither the African nor the American plate has a sinking part.
    Another possibility is that the sink- ing plate cools the neighboring mantle and produces convection currents that move the plates. This last theory is attractive because it gives some hope of explaining the enclosed seas, such as the Sea of Japan. These seas have a typical oceanic floor, except that the floor is overlaid by several kilometers of sediment. Their floors have probably been sinking for long periods. It seems possi- ble that a sinking current of cooled mantle material on the upper side of the plate might be the cause of such deep basins. The enclosed seas are an important feature of the earth’s surface, and seriously require explanation because, in addition to the enclosed seas that are developing at present behind island arcs, there are a number of older ones of possibly similar origin, such as the Gulf of Mexico, the Black Sea, and perhaps the North Sea. [br] The author refers to a "conveyor belt " in line 16 in order to
选项 A、illustrate the effects of convection in the mantle.
B、show how temperature differences depend on the positions of the continents.
C、demonstrate the linear nature of the Mid-Atlantic Ridge.
D、describe the complicated motions made possible by back-coupling.
E、account for the rising currents under certain mid-ocean ridges.
答案 A
解析 L16提到“传送带”是为了：A．正确。描绘地幔中对流的影响。“conveyor belt”产生是原文as though引出的一个比喻，前半句主句说明的“地幔中对流可使板块移动”，正是A选项所说。B．指出温差是怎样依赖于大陆位置。此现象原文提过，但与此处题干所问无关。C．指出中大西洋海岭的线性特征。原文未提此内容。D．描写挂钩关系可能产生的复杂运动。挂钩关系在后文L23才提到过，不是“conveyorbelt”所要说。E．解释海岭下的上升流，易混。此内容就在上句(L11—12)出现过，但和A选项比较，A选项是直接对应“conveyor belt”的，此选项则与“conveyor belt”这一解释无关。
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