Tectonic plates sometimes collide. The world's great mountain ranges were formed when plates collided and the rock had nowhere to go but up. Tectonic plates are moving slowly, rubbing against each other or even in opposite directions, which is what causes the majority of the earth's seismic and volcanic activity. If you study where the tectonic plates are located, you'll notice that the vast majority of mountain ranges, volcanoes, and earthquakes occur along the edges of these plates, while the centers of the plates remain relatively stable, in geologic terms.
