via ScienceDaily, plate tectonics may not be a permanent geologic process, it might temporarily "grind to a halt" when the continents group together to form a "supercontinent" such as Pangea 350 million years ago.
"Writing in the January 4 issue of Science, Paul Silver of the Carnegie Institution's Department of Terrestrial Magnetism and former postdoctoral fellow Mark Behn (now at Woods Hole Oceanographic Institution) point out that most of today's subduction zones are located in the Pacific Ocean basin. If the Pacific basin were to close, as it is predicted to do about in 350 million years when the westward-moving Americas collide with Eurasia, then most of the planet's subduction zones would disappear with it.
This would effectively stop plate tectonics unless new subduction zones start up, but subduction initiation is poorly understood. "The collision of India and Africa with Eurasia between 30 and 50 million years ago closed an ocean basin known as Tethys," says Silver. "But no new subduction zones have initiated south of either India or Africa to compensate for the loss of subduction by this ocean closure."
If this theory proves correct, it would go a long way toward explaining why the Earth has not lost as much heat as expected given its age. When the continents form these supercontinents, subduction of the seafloor stops and heat begins to build up underneath the supercontinent, leading to massive volcanic activity explaining the far out of palce igneous rocks found in the middle of continents far from the subduction zones where they are usually found.