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where ocean crust dives under a continental margin, melting (and creating a
line of volcanoes on the surface) in the process? Clearly there were many
volcanoes along the west coast of North Ametica, and as the movements of
the oceanic crust was deciphered in the early 1960s, it hecame clear that
many of the mountains of North America were indeed produced hy subduc-
tion. The high Cascade volcanoes of the Pacific Northwest are an excellent
example. Equally clear, however, is the fact that this type of plate-to-plate
contact, creating subduction zones, was not the only type of plate interac-
tion going on. Other processes of mountain building have been taking place
as well. Perhaps nowhere are these latter types of plate interaction better
studied than in California.
California is earthquake country. Earthquakes occur when rocks move.
Most of California's earthquakes can be attributed to the type of plate-to-
plate motion known as strike slip. This type of movement, wherein the edges
of plates scrape by each other, is causing the westernmost portions of the
Golden State to move northward. This motion will continue until the vari-
ous fragments, carrying San Francisco and Los Angeles on their backs, run
into something, such as another part of the west coast of North America.
It now seems probable that much of western North America was cre-
ated by collisions with smaller, subcontinent- or island-sized land masses,
forming mountains in the process. We are witnessing the start of one such
voyage; its destination is still unknown. San Francisco and the long sliver of
land it sits on will, over the next few millions of years, leave North America's
embrace to become a long borderland west of the continent, thereby creat-
ing an inland sea. Sometime in the far future, it will probably collide with
North America again. This seems to have been the tectonic style for west-
ernmost North America for several hundreds of millions of years: Land
masses large and small have been moving northward and colliding with
western North America, becoming part of the continents in the process, or
sometimes continuing their northwatd journey. Geologists have deciphered
this history not only by studying the San Andres fault complex, but also,
where possible, by conducting paleomagnetic analyses for various bits of
North America.
