What happens when plates collide? It depends how the plates are moving when they meet:

• When two plates collide head-on, they push each other up and form mountains. That's how the Himalayas and other great mountain ranges (including the Rockies, long ago) were created.

• When one plate dives below another plate, it creates a subduction zone as the diving plate is crushed and melted. This process often creates volcanoes as the magma (molten rock) rises up to the surface.

• When two plates slide past each other, they create a transform fault, like the San Andreas fault.

Earthquakes can happen in any of these situations. Despite the powerful forces driving plate movement, the plates themselves spend much of the time locked in place by the friction of the plates rubbing against each other. Eventually, however, they build up so much pressure that the plates abruptly snap forward. Then the ground can shift a few feet—or a few dozen! Shock waves from that sudden motion shoot out iin all directions, creating an earthquake.

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