Question

Geologists have been studying volcanoes(火山)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, uranium, thorium, 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 might 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 degree F for each 50 feet. At this rate the temperature 40 miles below the earth's surface would be over 4,000 degrees F. This is much hotter than 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. In other words, the rock yields(屈服)slowly to pressure but is not liquid. But if some change in the earth's outer layer releases the pressure, the rock melts. Then the hot, liquid rock can move up toward the surface.

4. The subject of this passage is the________.

A. formation of volcanoes

B. results of volcanic action

C. heart of the earth

D. causes of the earth's inside heat

5. The cause for the heat in the interior of the earth is probably________.

A. radioactive elements

B. the great pressure of the earth

C. not determined

D. the heat remaining from the formation of the earth

6. If the temperature at the earth's surface is 20°F, the temperature in a coal mine 500 feet below the surface would, in degree, be________.

A. 40 B. 30 C. 50 D. 120

7. Why doesn't the rock deep in the earth melt as a liquid at such high temperature?

A. Because the temperature isn't high enough to melt it.

B. Because it may be plastic.

C. Because the pressure of the rock above is strong enough to stop it melting.

D. Because there isn't oxygen inside the earth.

Answer 1

4. D。主旨题,从全文的意义分析,文章的主题是"地球内热成因"。 5. A。细节题,第一段最后一句即本题答案出处。6. B。计算题,按"the temperature rises about 1 degree F for each 50 feet"的比率计算可得。

 7. C。细节题,第二段第五句即本题答案出处。