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 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 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 veers scientists have learned about radium, uranium, 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 beat 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 15 meters. At this rate the temperature 64km 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 plantic. 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.
When the melted rock moves with difficulty close to the earth surface, a volcano may be formed. The melted rock often contains steam and other gases under great pressure. If the rock breaks, 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.
A volcano may be formed if ______.