Michael - do a little more research on Plate Tectonics in general - The continental plates are made of a different material than the oceanic plates - The continents are higher in silica (felsic) and tend to float higher than the heavy basaltic/mafic ocean plates.

Its not really a simple question - when the two continents come up against eachother one has to give - the heavier one (oceanic crust) sinks beneath the lighter one (continent) and also pushes up the lighter one.

And theoretically, its all driven by convection in the mantle.
Canie