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

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

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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).
The passage is mainly about______.
选项 A、the features of volcanic activities
B、the importance of the theory about drifting plates
C、the significance of hot spots in geophysical studies
D、the process of the formation of volcanoes
答案C
解析 从文章第1段的内容可知，地球上散布着一百多个相互独立并且面积不大的火山活动区，地质学家称之为热点。随后说明了热点的特点。文章第2段说明了板块漂移理论，最后一句话指出：根据热点密度的分析可以看出——非洲板块静止不动了3，000万年；从文章第3段的内容可知，热点的重要性并不仅限于作为参照体系这一作用上，它们对推动板块地球上移动的地质物理过程也产生了重大影响；然后具体说明了热点如何促使形成了新海洋；从文章的最后一句话可知，正如早期的理论解释了大陆的移动那样，热点理论或许可以解释大陆的易变性。据此可知，本文主要讲
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