2. 外语
  3. Scattered around the globe are more than 100 small regions of isolated volcanic activity known to geologists as hot spots. Unlik

Scattered around the globe are more than 100 small regions of isolated volcanic activity known to geologists as hot spots. Unlik

admin2017-07-31  43
问题     Scattered around the globe are more than 100 small regions of isolated volcanic activity known to geologists as hot spots. Unlike most of the world’s volcanoes, they are not always found at the boundaries of the great drifting plates that make up the earth’s surface; on the contrary, many of them lie deep in the interior of a plate. Most of the hot spots move only slowly, and in some cases the movement of the plates past them has left trails of dead volcanoes. The hot spots and their volcanic trails are milestones that mark the passage of the plates.
    That the plates are moving is not beyond dispute. Africa and South America, for example, are moving away from eath other as new material is injected into the sea floor between them. The complementary coastlines and certain geological features that seem to span the ocean are reminders of where the two continents were once joined. The relative motion of the plates carrying these continents has been constructed in detail, but the motion of one plate with respect to another cannot readily be translated into motion with respect to the earth’s interior. It is not possible to determine whether both continents are moving in opposite directions or whether one continent is stationary and the other is drifting away from it. Hot spots, anchored in the deeper layers of the earth, provide the measuring instruments needed to resolve the question. From an analysis of the hot spot population it appears that the African plate is stationary and that it has not moved during the past 30 million years.
    The significance of hot spots is not confined to their role as a frame of reference. It now appears that they also have an important influence on the geophysical processes that propel the plates across the globe. When a continental plate come to rest over a hot spot, the material rising from deeper layer creates a broad dome. As the dome grows, it develops seed fissures(cracks); in at least a few cases the continent may break entirely along some of these fissures, so that the hot spot initiates the formation of a new ocean. Thus just as earlier theories have explained the mobility of the continents, so hot spots may explain their mutability(inconstant).
That Africa and South America were once joined can be deduced from the fact that______.
选项 A、the two continents are still moving in opposite directions
B、they have been found to share certain geological features
C、the African plates has been stable for 30 million years
D、over 100 hot spots are scattered all around the globe
答案B
解析 本题可参照文章的第2段。从中可知，因为有新物质深入洋底，两个大陆距离愈来愈远；互补的海岸线以及似乎横跨海洋的地质地貌特征提示人们——这两块大陆曾经连接在一起；人们既不能确定是否这两块大陆在向相反的方向移动，也不能确定是否其中的一块大陆静止不动，而另一块大陆正在远它而去；根据热点密度的分析可以看出，非洲板块静止不动了的3，000万年。据此可知，人们之所以认为非洲和南美洲曾经连在一起，是因为它们的某些地质特征相似。B项与文章的意思相符，因此B项为正确答案。
转载请注明原文地址:https://kaotiyun.com/show/YiFO777K
0

考博英语

相关试题推荐
随机试题
最新回复(0)