It is this spreading motion which is separating Baja California from mainland Mexico, and has led to the formation of the Imperial Valley and Salton Sea in southernmost California. The San Andreas fault is simply one of these transform faults which offset a spreading center. On the south it joins with the East Pacific Rise where it comes out of the Gulf of California, and the north end runs off the coast at Cape Mendicino on the northern California coast. North of Cape Mendicino, the sense of motion reverts back to spreading, and there is actually a relatively short segment of the original East Pacific Rise still lurking beneath the waters of the Pacific Northwest - making seafloor basalt which then moves to the east until it runs into northern California, Oregon, and Washington. At that point the seafloor is subducted along the Cascadia Subduction Zone, giving rise to the volcanic mountains of the Cascade Range.
Volcanic activity is common at both spreading centers and subduction zones.
But, because there is no ripping apart or subduction taking place along a transform fault, there isn't any magma formation to lead to volcanoes. However, the intensity of the faulting easily makes up for this apparent lack of natural disasters.