Question

what processes drive motion of the plates?

Answer 1

It is a topic that geologists argue over due to the energy required. Have you looked into subduction zones and what that replaced/augmented as a form of motive power?

Answer 2

There are three major ways that plates interact along boundaries: (1) they can move away from each other (diverge), (2) they can move toward each other (converge), or (3) they can move past each other, parallel to the boundary (transform). Each of these interactions produces a different and characteristic pattern of earthquakes, volcanic activity, and topography. The results of these interactions also depend on the type of crust involved, and there are two types of crust: oceanic and continental. Continental crust is thick and buoyant; oceanic crust is thin, dense, and forms at mid-ocean ridges.
for more reading http://www.visionlearning.com/en/library/Earth-Science/6/Plates-Plate-Boundaries-and-Driving-Forces/66