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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.
From the information given in the passage, most minerals would melt fastest______.
选项 A、at 4 000 F. , at sea level
B、at 4 000 F. , 5 000 feet below sea level
C、at 4 000 F. , 5 000 feet above sea level
D、in the absence of oxygen
答案B
解析 本题是一道具体细节题。问从文章所给的信息来看，大多数矿物质在什么条件下融化得更快。根据文章第二段，不管地热的原因是什么，我们确切知道往地下挖得越深，地球内越热。在深矿井和油井中，每向下挖50英尺，温度就升高1华氏度。以这种比率在地球表面下40英里的地方，温度将是4 000华氏度以上。这已足够熔化岩石了。因此，本题的正确答案应是B“在海平面以下5 000英尺的地方，温度是4 000华氏度时”。
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