When two sections of the Earth’s crust move relative to each
other, the zone in which they do is known as a fault.
The two sections can move towards each other (in relative
terms), apart from each other, or laterally (i.e. sideways to each other).
Large areas of land can be pushed up or drop down as the result of a fault.
If two faults occur in parallel, the land between the faults
can move downwards to form a rift valley, the best example of this being the
Great Rift Valley in East Africa.
Fault movements occur suddenly, as pressures that have built
up over tens or hundreds of years are released. This is a major cause of
earthquakes.
Faults can allow material from deep in the Earth’s crust to
come closer to the surface. This can include valuable minerals and deposits
such as gold and silver. There is clearly an economic benefit to mankind in
exploiting these resources, but there is also a downside in that zones that are
mineral rich may also be prone to frequent earthquakes.
A prime example of this is California, where the discovery
of gold led to the region attracting huge numbers of people to settle there,
but the local geology made it subject to earthquakes. The whole area is riven
with faults, the most prominent being the lateral tear fault known as the San
Andreas (see picture).
© John Welford
Geology is never really wrong.
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