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Transform
Boundaries
Places where plates slide past each other are called transform boundaries.
Since the plates on either side of a transform boundary are merely
sliding past each other and not tearing or crunching each other,
transform boundaries lack the spectacular features found at convergent
and divergent boundaries. Instead, transform boundaries are marked
in some places by linear valleys along the boundary where rock has
been ground up by the sliding. In other places, transform boundaries
are marked by features like stream beds that have been split in
half and the two halves have moved in opposite directions.
Perhaps
the most famous transform boundary in the world is the San Andreas
fault, shown in the drawing above. The slice of California to the
west of the fault is slowly moving north relative to the rest of
California. Since motion along the fault is sideways and not vertical,
Los Angeles will not crack off and fall into the ocean as popularly
thought, but it will simply creep towards San Francisco at about
6 centimeters per year. In about ten million years, the two cities
will be side by side!
Although
transform boundaries are not marked by spectacular surface features,
their sliding motion causes lots of earthquakes. The strongest and
most famous earthquake along the San Andreas fault hit San Francisco
in 1906. Many buildings were shaken to pieces by the quake, and
much of the rest of the city was destroyed by the fires that followed.
More than 600 people died as a result of the quake and fires. Recent
large quakes along the San Andreas include the Imperial Valley quake
in 1940 and the Loma Prieta quake in 1989.
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