The magma on which the continental plates "float" has currents which appear to remain relatively stable in terms of direction and velocity for long periods of time. A bit like the Gulf Stream, I guess. The force of these currents is great enough to force the plates together for what appears to us to an amazing amount of time. The Pacific plate subducts under the Australian plate, from memory and New Zealand kind of trembles on the brink, so to speak. Incidentally, the Himalayas and the Tibetan plateau are all the result of the plate on which India sits pushing up at the southerly end of the Asian plate. Some force, huh?
It's driven by magma being forced up between the plates in the centre of the major oceans - the Pacific and the Atlantic. The mid-Atlantic ridge is highly volcanic. New plate material is formed by the magma and is pushed away from the scene of its creation. Or something like that, anyway.
It's all been proved by the study of changes in magnetic polarity in the material on the seabed. Turns out that the patterns of magnetic change are mirror images of each other on each side of the mid-Atlantic ridge, showing that the material on both sides was created at the same time.
I dunno, one too many Discovery Channel programs, I guess!
