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.