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, uranium, thorium and other radioactive elements. These give ont 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 element.

Whatever the cause of the heat may be, we do know that the earth get hotter the farther down we dig. In deep mines and oil wells the temperature rises about 1 for each 50 feet. At this rate the temperature 40 miles below the earth’s surface would be over 4,0007 This is much hotter than necessary to melt rock. However, the pressure of the reek above keeps most materials from melting at their usual melting points. Geologists believe that the rock deep in the earth may be plastic, or putty-like. In other words, the rock yields slowly to pressure but is not liquid. But if some change in the earth's crust re leases 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 crust, a volcano may be formed. The melted rock often contains steam and other gases under great pressure. If the rock above often 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. 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. In this way, volcanic ash, cinders and lava build up the cone-shaped mountains that we call volcanoes.

The subject of this passage is the ______.

A．formulation of volcanoes

B．results of volcanic actions

C．work of geologists

D．interior of the earth