PASSAGE 3
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
5) liquid when it is subjected to small forces for long periods and that differences in temperature under oceans and continents are sufficient to produce convection in the mantle of the earth with rising convection currents under the mid-
10) ocean ridges and sinking currents under the con-tinents. Theoretically, this convection would carry the continental plates along as though they were on a conveyor belt and would provide the forces needed to produce the split that occurs
15) 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
20) 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 certainly does not occur along
25) lines broken by frequent offsets or changes in direction, as the ridge is. Also it is difficult to seehow 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
30)
since there is no intermediate trench, the two ridges must be moving apart. It would be odd ifthe rising convection currents kept exact pace with them. An alternative theory is that the sink-ing part of the plate, which is denser than the
35) 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 asinking part.
40)
* Another possibility is that the sinking plate cools the neighboring mantle and produces con-vection 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
45) Japan. These seas have a typical oceanic floor,except that the floor is overlaid by several kilo-meters of sediment. Their floors have probably been sinking for long periods. It seems possible that a sinking current of cooled mantle material
50) 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 in because, addi-tion to the enclosed seas that are developing at present behind island arcs, there are a number of
55) older ones of possibly similar origin, such as theGulf of Mexico, the Black Sea, and perhaps the North Sea.
7. According to the passage, the floor of the Black Seacan best be compared to a
(A) rapidly moving conveyor belt
(B) slowly settling foundation
(C) rapidly expanding balloon
(D) violently erupting volcano
(E) slowly eroding mountain
请教NN，文章中那段红色的字是什么意思？上面这个第7题题干中的那个短语是什么意思？ 谢谢。
是不是我提的问题太弱智了？都没人理。
幸好在msn上有几个GG帮忙，题干中的 be compared to 已经理解了。
但是，对于文中的那两个因果关系怎么能推出来还是不理解：为什么 no thench 就must be moving apart? 后面又为什么odd?
帮帮我阿，NN们
问题还没解决，帖子已经沉下去了，自己让它浮出来
请问为什么本题正确答案是B而是E呢?
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