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 (inconstancy).

　　17. The author believes that _____ .

　　[A] the motion of the plates corresponds to that of the earth’s interior

　　[B] the geological theory about drifting plates has been proved to be true

　　[C] the hot spots and the plates move slowly in opposite directions

　　[D] the movement of hot spots proves the continents are moving apart

　　18. That Africa and South America were once joined can be deduced from the fact that _____ .

　　[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

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