The fact that the process of convection can occur in the mantle means that this layer is molten. Convection currents occur as a result of the heat from the earth's core. The heat from the core heats the surrounding rocks from the mantle causing plumes of magma to rise. As these rocks rise and reach the base of the crust they become cooler and consequently more dense than the rocks below so they sink. This cycle of sinking and rising from a circular motion and this motion is responsible for the movement of the tectonic plates today.
