【单选题】
Directions: In this section, you will read several passages.Each passage is followed by several questions based on its content. You are to choose ONE best answer,A、,B、,C、orD、, to each question.Answer all the questions following, each passage on the basis of what is stated or implied in that passage and write the letter of the answer you have chosen in the corresponding space in your ANSWERBOOKLET.
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 certainly 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 SouthAtlantic, where neither theAfrican nor theAmerican plate has a sinking part.
Another possibility is that the sinking 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 possible 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, theBlack Sea, and perhaps the North SeA、
B．Movement of mid-ocean ridges.
C．Sinking of ocean basins.
Differences in temperature under oceans and continents.
网考网参考答案：D
网考网解析：
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