The three primary causes for tectonic plate movement are the convection of material in the mantle, gravity and the rotation of the planet. These forces cause each of the seven major plates and numerous other microplates to move independently of the others at a rate of a few centimeters per year.Continue Reading
The most studied aspect of plate movement involves the large convection currents in the mantle. As energy is transferred throughout the molten levels of the mantle (asthenosphere), new material is pushed up towards the surface, moving the old rock out of the way. The emergence of this new land causes the plates to move where the material comes out.
The most easily recognizable example of this process occurs at the mid ocean ridges, where the plates on either side of the ridge are being pushed away from each other. The ridges where this new material emerges are higher in elevation than the surrounding areas, and gravity causes the older rock to fall down to the lower points, aiding in the movement of the plates.
According to About.com, the rotation of the Earth is also a contributor to the movement of the tectonic plates but is far less significant than either the convection of the mantle or the force of gravity.Learn more about Plate Tectonics