San Andreas fault. Folded rock strata exposed in a roadside cliff cutting through the San Andreas fault. The twisting and folding is caused by tectonic forces. The San Andreas fault is one of the world's great seismic faults, forming the boundary between the Pacific and North American tectonic plates. These plates move in opposing directions, slowly warping the rock around the plate margins as they rub against each other. Earthquakes result from sudden movements of the plates that release built-up pressure. The San Andreas fault extends almost the full length of California. Photographed on Highway 14 near Palmdale, California, USA.