Re: How do tectonic plates move?

The simple answer is that the gravitational pull of the earth is the ultimate driving force behind all plate motions. How this occurs, however, is still a matter of active research and debate.

Here is a website that diagrams the three main ways that have been proposed:

http://www.windows.ucar.edu/tour/link=/earth/interior/how_plates_move.html

The three chief mechanisms that have been proposed are convection drag, ridge push, or slab pull.

Convection drag is the idea that the convecting mantle drags the lithosphere along. The lithosphere can even be seen as the cold outer "scum" that forms on the top, cold, side of the convection cell. The convection is driven by differences in gravitational pull on rocks with different densities. Each small cube of hot, less dense, mantle experiences less pull than the same-sized cube of cold, more dense, mantle. As a result, the less dense cubes tend to rise and the more dense cubes tend to sink. Since the interior of the earth is hot and the exterior is cold, this produces an unstable system and produces convection. The hot, less dense, rising mantle cools off as it nears the earth's exterior. It becomes more dense as that cooling occurs. As it becomes more dense, it experiences increasing pull and it begins to sink. As it sinks, it enters hotter regions and heats up, becomes less dense, experiences less pull and starts to rise and the cycle repeats itself. This repeating cycle then drags the upper "scum" of the lithosphere along.

Ridge push is a driving force that originates from the topography of ridges. As new lithosphere (and its underlying mantle) moves away from a ridge, it cools, contracts, and becomes more dense. As a result, it drops in elevation, creating a topography that slopes away from the ridge. For any mass sitting on a slope, the downward pull of earth's gravitation tends to move things down the slope. As the mass moves down hill, it also is pushed to the side by the material of the slope. So, on the slope of the ridge, gravitational forces pull the lithosphere down the slope of the ridge and that crust tries to move sideways away from the ridge and pushes on the rest of the plate.

Slab pull is a driving force that comes from the sinking of the lithosphere in subduction zones. Subduction is driven by downward gravitational pull on the subducting lithosphere. Soon after the lithosphere forms, it has cooled and contracted to the point where it is denser than the underlying asthenosphere. The same sort of density contrast that drives convection also drives subduction. As the sudbucting crust moves downward into the mantle, it may exert a tension on the rest of the material of the plate dragging it along.

In reality, all three of these forces probably contribute to the motion of plates. For an interesting example of the relative contributions, look at the figures in this paper (you may get a message about Japanese fonts, which you can ignore and still see the figures):

http://www.eri.u-tokyo.ac.jp/seno/phs.driving_abst.html

David Smith, Ph.D., Da Vinci Discovery Center, Bethlehem, PA