Simple scaling analysis reveals that the largest force acting in a typical plate tectonic cycle (spreading ridge to subduction) is the slab pull due to the negative buoyancy of the cold oceanic lithosphere, along with the dense eclogite of oceanic crust being subducted. However, one should also consider the heat budget of the mantle. If the mantle heat is mainly sustained by internal heating, then downwellings, i.e. subducting slabs, will drive convection. If there is a considerable component of bottom heating by a superheated core, then thermal upwelling would add an important driving mechanism to the mantle-lithosphere convection system as well. I think the true situation is found somewhere in between, with subduction zones certainly playing a big role.