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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.
If the temperature at the earth’s surface is 20 F. , the temperature in a coal mine 500 feet below the surface would, in degrees, be______.
选项 A、30
B、40
C、120
D、500
答案A
解析 本题是一道根据文章具体细节推断类题。问如果地球表面的温度是20华氏度，那么在地球表面以下500英尺的矿井里的温度是多少。文章第二段指出，每向下挖50英尺，温度就升高1华氏度，假设地球表面的温度是20华氏度，那么500英尺深的煤矿下的温度应是20+500÷50=30华氏度。因此，本题的正确答案应是A。
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