Convection currents inside the Earth’s Mantle cause a cycle of heating, rising, cooling, & sinking down of material.
The transfer of heat inside the Earth provides the energy to move plates.
http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html and http://www.seismology.harvard.edu/Projects.html Blue blobs show where colder, denser material is sinking into the mantle. Near the surface, most of the colder material is in the ancient roots of continents. Subducting slabs of oceanic lithosphere appear, recycled into the mantle from oceanic trenches.
http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html and http://www.seismology.harvard.edu/Projects.html Red blobs are warmer plumes of less dense material, rising principally into the ocean- ridge spreading centers. A huge plume seems to be feeding spreading at the East Pacific Rise directly from the core. Most of the heat being released from the earth's interior emerges at the fast-spreading East Pacific Rise.