问答题 The Theory of Continental Drift has had a long and turbulent history since it was first proposed by Alfred Wegener in 1910. (46) Vigorously challenged yet widely ignored, the theory had languished for half a century, primarily due to its lack of a plausible mechanism to support the proposed drift. With the discovery of sea-floor spreading in the late 1950's and early 60's, the idea was reinvigorated. Plate tectonics is now almost universally accepted. Many details of the mechanism are to be worked out.
The surface of the Earth is divided into approximately six large plates, plus a number of smaller ones. The plates are' bounded by an interconnected network of ridges, transform faults, and trenches. Ridges, also called spreading centers, occur where two plates are moving away from each Other. As the plates separate, hot molten mantle material flows up to fill the void. (47) The increased heat resulting from this flow reduces the density of the plates, causing them to float higher, thus elevating the boundaries by many thousands of feet above the colder surrounding sea floor. (48) Ridges on the ocean floor form the longest continuous ranges of mountains on the planet, but. only in a very few places on the Earth do these mountains rise above the ocean surface.
New sea floor is constantly being created along spreading centers. Obviously somewhere else old sea floor must be going away. This occurs in trenches, also called subduction zones. Trenches occur along the boundary between two plates that are moving towards each other. (49) Where this occurs, one plate is bent downwards at about a 400 angle and plunges under the other plate's leading edge, eventually to melt back into the liquid mantle below. As the subducting plate is heated back up to mantle temperatures, certain minerals in the plate melt sooner than others. (50) Minerals that melt at lower temperatures and are lighter than the surrounding material tend to rise, melting their way up through the overriding plate to erupt as volcanoes on the ocean floor. As these volcanoes grow, they rise above the ocean surface to form lines of islands along the leading edge of the overriding plate. Numerous islands of Micronesia and Melanesia in the western Pacific were created in this way.

【正确答案】这个理论缺乏可信的漂移手段来证明其所提出的论点,受到有力的挑战,被人们忽略,半个世纪里衰弱无力。
【答案解析】
【正确答案】这种流动所产生的不断增加的热量降低了板块的密度,使板块再向上漂移,将边界提升,从而使其超出周围较冷海底数千英尺的高度。
【答案解析】
【正确答案】洋底上的山脊形成了地球上最长的绵绵山脉,但这些山脉只有在很少的几个地方才上升到海平面以上。
【答案解析】
【正确答案】在海沟出现的地方,一个板块向下弯曲至40°左右的倾角插入另一个板块前缘的下面,最后熔汇于下面的流体地幔中。
【答案解析】
【正确答案】在较低温度下熔化且比周围物质较轻的矿物容易上升,向上熔穿覆盖的板块,海底火山就这样爆发了。
【答案解析】