The San Andreas Fault in California is a transform plate boundary.
Plate boundaries are the places where the Earth's tectonic plates come into contact with each other. There are three types of plate boundaries: convergent boundaries, divergent boundaries, and transform boundaries.
Convergent boundaries occur where two plates are moving towards each other and collide. At these boundaries, one plate is usually subducted (pushed beneath) the other, and this can lead to the formation of mountains and earthquakes.
Divergent boundaries occur where two plates are moving away from each other and new crust is formed by magma rising up from the mantle. These boundaries are often associated with the formation of mid-ocean ridges and earthquakes.
Transform boundaries occur where two plates are sliding past each other horizontally. These boundaries do not involve the creation or destruction of crust, and they are often associated with the formation of fault lines and earthquakes.
The San Andreas Fault is a transform plate boundary because it represents the boundary between the Pacific Plate and the North American Plate. These two plates are sliding past each other horizontally, and the movement along the fault line is responsible for many of the earthquakes that occur in California.