单项选择题
| Geologists have been studying volcanoes
for a long time. Though they have learned a great deal, they still have not
discovered the cause 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 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 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, or puttylike. In other words, the rock yields slowly to pressure but is not liquid. But if some change in the earth’s crust 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 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. Theses 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. Some of the material falls around the hole made in the earth’s surface. The melted rock may keep on rising and pour out as lava. In this way, volcanic ash, cinders and lava build up the cone-shaped mountains that we call volcanoes. |