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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 cause for the heat in interior of the earth is______.
选项 A、radioactive elements
B、the great pressure of the earth
C、not determined
D、the heat remaining from the formation of the earth
答案C
解析 本题是一道具体细节题。问地球内部的热量的导因是什么。利用查阅式阅读法，我们可以在文章的第一段中找到相关信息。这段指出，地质学家知道地球内部极热，但不清楚导致地热的确切原因。有些地质学家认为是由于地球外表层的巨大压力而引起的，或是地球形成之时留下来的热量。到最近60年里科学家了解了镭、铀、钍等放射性元素。这些物质在转化成其他元素时一直都在释放热量。因此，现在许多科学家认为地内热是由放射性元素引起的。所以，本题的正确答案应选C“人们还不能确定到底是什么原因”。
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