文章作者 100test 发表时间 2008:01:10 13:30:03
来源 100Test.Com百考试题网
提出问题No very satisfactory account of the mechanism that caused the formation of the ocean basins has yet been given. The traditional view*1*4*6(-) 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*1E under oceans and continents are sufficient to produce convection in the mantle of the earth with rising 理论一convection currents under the mid-ocean ridges and sinking currents under the continents. Theoretically, this convection would carry the continental plates along as though they were on a conveyor belt*3*7A(比喻) (conveyor belt: n. 传送带) and would provide the forces needed to produce the split that occurs along the ridge. This view may be correct(小 ): it has the advantage that the currents are driven by temperature differences that themselves depend on the position of the continents. Such a 优点back-coupling互相挂钩,互为因果, 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(大-) because(原因) convection does not normally occur along 缺点一lines*6A, and it certainly does not occur along lines broken by frequent offsets (an abrupt bend in an object by which one part is turned aside out of line) or changes in direction, as the ridge is. 缺点二:不能适用于某个板块Also(第二个原因) it is difficult to see how the theory applies to the plate between the Mid-Atlantic Ridge and the ridge in the Indian Ocean*5D. This plate is growing on both sides*5, and since there is no intermediate trench, the two ridges must be moving apart. It would be odd if(欲擒故纵的反证法) the rising convection currents kept exact pace with them. An alternative theory is that the 理论二sinking part of the plate, which is denser than the hotter surrounding mantle, pulls the rest of the plate after it. 缺点:不能适用于另一个板块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 理论三sinking plate cools the neighboring mantle and produces convection currents that move the plates. This last theory is attractive( ) because(原因) 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*2*7B. Their floors have probably been sinking for long periods. It seems possible that( ) a sinking current of cooled mantle material on the upper side of the plate might be the cause of(问题解决了) 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*2B*7, and perhaps the North Sea.(跑题的延伸性内容)
1. According to the traditional view*1