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Plates Sliding Past each other cause Earthquakes.

There is a famous example of plates sliding past each other
where the Pacific Plate moves past the North American Plate.
Most of this boundary is under the sea,
but part of it is on land on the West Coast of North America.
This boundary is called the San Andreas Fault,
and the city of San Francisco is built right on top of it.

The plates do not slide smoothly past each other.
They get stuck at the boundary (there is very high friction)
and then very large forces build up as the plates keep
trying to slide. Eventually the forces are great enough
to overcome the friction and one plate moves rapidly
against the other for a few seconds. This is an earthquake,
and the shock can send out waves through the Earth's
which are strong enough to cause buildings to collapse.

It is not possible for scientists to predict accurately
when an earthquake will happen. The enormous forces
can cause the plates to slide past each other at any time.
It is also not possible to
predict when a volcano will erupt for the same reason.

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