The answer is convection. The convection force is originated inside the fluid bodies (inside the Earth, in this case) and is due to the difference of densities generated by the difference in temperatures. This is the same force that affects the current of waters and the winds. The mechanism proposed is that the very hot rocks deep in the Earth have a lower density than upper rocks, so those hot rocks (not liquid due to the high pressures) tend to move upward leaving a void. The void will be filled by the next rocks close to the void and that generate a circular pattern that press the rocks to move.
