Plate tectonic help??????????

In the deep interior of the Earth, temperatures are high, lowering the viscosity of the constituent rocks. Decreasing viscosity causes an increase in the Rayleigh number, resulting in convection. Thus the main mode of heat transport in the deep interior is through convection. This convection exerts forces on the base of plates moving them away from regions of warm up-welling and towards regions of cold down-welling.

Cold subducting lithosphere thermally contracts increasing its density. As well, subducted basaltic crust may transform into eclogite (a denser rock) with increasing depth. Once the density of the subducted lithosphere is greater than the surrounding mantle, the slab is negatively buoyant and gravity aids in its descent.

It is fairly well established (although your teacher may not be aware of this) that the dense sinking of subducting slabs is far more important to moving plates than the forces exerted on the base of plates by convection currents.