If you look closely in the lower right-hand portion of the illustration you can see the East Pacific Rise, a portion of the Mid-Ocean Ridge that defines the eastern edge of the Pacific Plate. As the giant Pacific Plate encounters the North American Plate the two grind against each other with such force that they have caused a network of fractures, or faults, across the state of California. The San Andreas Fault is part of this fracture zone that extends for 700 miles from Mexico to just north of San Francisco, where it dives into the Pacific Ocean.

The stresses created along this fault are enormous and are responsible for the infamous San Francisco earthquake of 1906. Over 20 million people live along this fault, which continues to periodically rock the state. Actually, western California is considered part of the Pacific Plate and, over the next several million years, will break away from the mainland and drift steadily northward.
       
Thanks to Bruce Heezen and Marie Tharpe.

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