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.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 temperatures rise about 1degF for every 50 feet.At this rate of the temperature 40 miles below the earth's surface should be over 4 000deg.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. According to the passage,  _   prevents most rock from melting at its usual melting points.
the pressure of the rock itself the pressure of the rock above the pressure of the earth's outer layers the heat given out by some radioactive elements