Download Earth`s crust, the surface layer of the planet, is

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Earth's crust, the surface layer of the planet, is not solid
and unbroken. The forces that rage inside the planet have
fractured this brittle layer. Some of these fractures, called
faults, lie beneath the surface of the crust. Other faults,
however, have ruptured the surface, cracking the crust into various-sized
blocks of rock. These blocks dip and rise along faults in
response to pressure underground. One block may move
up while the other moves down. Sometimes the movement is
enough to form valleys or mountains. Other times that
movement is not vertical but horizontal, as one block slips
along the fault relative to the block on the other side.
Movement of crustal blocks along faults may be regular and
slow or sporadic and sudden. When two blocks are forced
to move against each other but are locked into position,
stress builds up. When that stress becomes greater than the forces holding the blocks
together, the blocks are forced to move suddenly and violently. The ground vibrations
accompanying that release of energy are better known as an earthquake. There are more
than one million earthquakes a year on Earth, though more than 60 percent of those are
too faint to be felt. Crustal movements along faults are occurring continuously across
most of the planet's surface.
A fault is defined as a crack or fracture in Earth's crust along
which rock on one side has moved relative to rock on the
other. (When no movement has occurred, the fracture is
known as a joint). When a fault breaks the planet's surface,
it may range in length from a few inches to thousands of
miles. The line on Earth's surface defining the fault is known
as the fault line or fault trace.
Forces and changes:
Construction and destruction
Aerial view of the San Andreas Fault
slicing through the Carrizo Plain just
east of San Luis Obispo,
Any rock subjected to intense stress or pressure over time will deform. At higher
temperatures and pressures, rock will soften and bend.. At lower temperatures and
pressures, however, rock will break or fracture instead of bending. This type of
deformation happens to rock in the upper part of Earth's crust. Faults are a clear example
of brittle deformation.
The stress that is continually acting on and deforming Earth's surface may be in different
forms: tensional stress, which stretches or pulls rock; compressional stress, which
squeezes and squashes rock; and shear stress, which changes the shape of rock by
causing adjacent parts to slide past one another. All of these stresses are directly related
to events occurring deep within the planet. Earth's internal processes, from the core to
the crust, have put the surface of the planet in motion, constantly changing its shape