What makes the plates move?

The theory is that the plates-driving force is the slow movement of hot, softened mantle that lies below the rigid plates. There are speculation that the circular motion of the mantle carried the continents along in much the same way as a conveyor belt. Below the lithospheric plates, at some depth the mantle is partially molten and can flow slowly, in response to steady forces applied for long periods of time. The drawing above assumed convection cells in the mantle. Below a depth of about 700 km, the descending slab begins to soften and flow, losing its form.