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  3. Geologists have been studying volcanoes for a long time. Though they have learned a great deal, they still have not discovered t

Geologists have been studying volcanoes for a long time. Though they have learned a great deal, they still have not discovered t

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问题     Geologists have been studying volcanoes for a long time. Though they have learned a great deal, they still have not discovered the causes of volcanic action. They know that the inside of the earth is very hot, but they are not sure exactly what causes the great heat. Some geologists have thought that the heat is caused by the great pressure of the earth’s outer layers. Or the heat may be left from the time when the earth was formed. During the last sixty years scientists have learned about radium (镭), uranium (铀), thorium (钍), and other radioactive elements. These give out heat all the time as they change into other elements. Many scientists now believe that much of the heat inside the earth is produced by radioactive elements.
    Whatever the cause of the heat may be, we do know that the earth gets hotter the farther down we dig. In deep mines and oil wells the temperature rises about 1 F. for each 50 feet. At this rate the temperature 40 miles below the earth’s surface would be over 4 000 F.. This is much hotter necessary to melt rock. However, the pressure of the rock above keeps most materials from melting at their usual melting points. Geologists believe that the rock deep in the earth may be plastic, or puttylike. In other words, the rock yields slowly to pressure but is not liquid. But if some change in the earth’s crust (地壳) releases the pressure, the rock melts. Then the hot, liquid rock can move up toward the surface.
    When the melted rock works its way close to the earth’s crust, a volcano may be formed. The melted rock often contains steam and other gases under great pressure. If the rock above gives way, the pressure is released.
    Then the sudden expansion of the gases causes explosions. These blow the melted rock into pieces of different sizes and shoot them high in the air. Here they cool and harden into volcanic ash and cinders (煤灰,余烬). Some of this material falls around the hole made in the earth’s surface. The melted rock may keep on rising and pour out as lava (熔岩,火山岩). In this way, volcanic ash, cinders, and lava build up the cone-shaped (圆锥形的) mountains that we call volcanoes.
The general idea of the last paragraph of this passage is______.
选项 A、heat at earth’s core
B、causes of volcanoes
C、roles of pressure
D、formation of volcanoes at earth’s surface
答案D
解析 本题是一道段落大意类问题。问本文最后一段的主题是什么。仔细阅读一下最后一段内容，地球岩石内部这种气体的突然膨胀会导致爆炸，从而使熔岩以不同大小的体积喷涌到空中。在空中熔岩遇冷变成火山灰和火山渣，这些物质有些落回到地面的岩洞周围，熔岩也许会以岩浆的形式继续涌出。这样，火山灰、火山渣和岩浆形成了我们称之为火山的锥形山峰。因此，本题的正确答案应是D“地球表面火山的形成”。
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