At transform boundaries one plate slips and grinds against another. It is a place where earthquakes occur as pressure builds, and then releases with the shifting crust. The San Andreas Fault , which extends from Northern California to the Gulf of California, is one of the most famous transform faults. It is along this fault that Baja California and part of Southern California are shifting northwestward. Quicktime video of animation from the film.
Hands represent two oceanic plates, and thumbs represent magma.
As the magma pushes up, the plates move apart. Move thumbs up, and let hands move apart. Hold one hand at right angles to the other, about two inches away. Let the thumb of your right hand extend down. The gap between thumb and fingers represents a trench.