Beyond acting as frames of reference, hot spots apparently influence the geophysical processes that propel the plates across the globe. When a continental plate comes to rest over a hot spot, material welling up from deeper layers forms a broad dome that, as it grows, develops deep fissures. In some instances, the continental plate may rupture entirely along some of the fissures so that the hot spot initiates the formation of a new ocean. Thus, just as earlier theories have explained the mobility of the continental plates, so hot-spot activity may suggest a theory to explain their mutability.

　　20. The primary purpose of the passage is to

　　describe the way in which hot spots influence the extinction of volcanoes

　　describe and explain the formation of the oceans and continents

　　explain how to estimate the age of lava flows from extinct volcanoes

　　describe hot spots and explain how they appear to influence and record the motion of plates

　　describe the formation and orientation of island chains in the Pacific Ocean

　　21. According to the passage, hot spots differ from most volcanoes in that hot spots

　　can only be found near islands

　　are active whereas all other volcanoes are extinct

　　are situated closer to the earth s surface

　　can be found along the edges of the plates

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