The San Andreas Fault is a continental transform fault that extends roughly 810 miles (1,300 km) through California in the United States. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). The fault divides into three segments, each with different characteristics, and a different degree of earthquake risk. Although the most significant (Southern) segment only dates back about 5 million years, the oldest sections were formed by the subduction of a spreading ridge 30 million years ago.