The tectonic plates do not randomly drift or wander about the Earth's surface. Rather, they are driven by definite yet unseen forces that originate in the Earth.

From seismic and other geophysical evidence and laboratory experiments, scientists generally agree with Harry Hess's theory that the plate-driving force is the slow movement of hot, softened mantle that lies below the rigid plates. This idea was first considered in the 1930s by Arthur Holmes, the English geologist who later influenced Harry Hess's thinking about seafloor spreading. Holmes speculated that the circular motion of the mantle carried the continents along in much the same way as a conveyor belt. However, at the time that Wegener proposed his theory of continental drift, most scientists still believed the Earth was a solid, motionless body.

We now know better. As J. Tuzo Wilson eloquently stated in 1968, "The earth, instead of appearing as an inert statue, is a living, mobile thing." Both the Earth's surface and its interior are in motion. Below the lithospheric plates lies the asthenosphere, a partially molten part of the mantle that flows, albeit slowly, in response to steady forces applied for long periods of time. Just as a solid metal like steel, when exposed to heat and pressure, can be softened and take different shapes, so too can solid rock in the mantle when subjected to heat and pressure in the Earth's interior over millions of years.



Source: http://pubs.usgs.gov/gip/dynamic/unanswered.html (public domain)