Michael;

Do a search on "subduction" for more information.

In brief, the plates which make up the surface of the earth are driven by convection cells. Material rises, moves horizontally, cools and sinks back down. This pushes the continents apart.

Where it sinks, it lifts up the edge of the continent. This causes buckling (mountains) and reduced the pressure which allows molten rock to collect, which escapes as volcanos.

That's rough but reasonably accurate.