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In geology, a fault is a planar fracture or discontinuity in
a volume of rock, across which there has been significant
displacement along the fractures as a result of earth
Large faults within the Earth's crust result from the
action of plate tectonic forces, with the largest forming
the boundaries between the plates, such as subduction
zones or transform faults.
Mechanisms of faulting
Because of friction and the rigidity of the
rock, the rocks cannot glide or flow past each
Stress builds up in rocks and when it reaches
a level that exceeds the strain threshold, the
accumulated potential energy is dissipated by
the release of strain, which is focused into a
plane along which relative motion is
The process by which the shape of a
rock changes because of stress .
There are two types of stress that occur
Compression occurs when rock is
Compression happens when two
plates collide at a convergent
Compression occurs and forms
large mountain ranges.
The Himalayas, for example, were raised by the compression that
accompanied collision of the Indian plate with the Eurasian plate. Another
example is Europe's Alps and Jura mountains which were also formed by
horizontal compression, generated in their case by collision with the African
plate and the Eurasian plate.
Tension occurs when an object is stretched.
Tension occurs when plates move away from
each other at plate boundaries.
At the Mid-Atlantic ridge the seafloor is
spreading at a rate of about 3cm per year. The
frequency of earthquakes at a mid-ocean ridge
will depend on how much tension is happening
at that point. The more tension means the
more seafloor spreading, resulting in a higher
frequency of earthquakes at a particular midocean ridge.
Some rock layers break when stress is
applied to them.
The surface along which rocks break is
called a fault.
The blocks of crust on each side of the
fault are called fault blocks.
A fault has a foot wall and a hanging
You could walk up this face of the fault, on foot,
hence the name footwall.
You could hang from this wall.
This is a
Notice how the
rock layers are
the same at
the red lines.
Hanging wall and
3 Types of Faults
Rocks are pulled apart because of
In a normal Fault, the hanging wall slides down the
In a reverse fault the hanging wall is
pushed upward due to compression.
Opposing forces cause rocks to
move horizontally. Earthquakes
occur along these faults.
San Andreas Fault, California
3 Most Common
Types of Mountains
Mountains exist because tectonic
plates are continually moving
around and colliding with each
There are 3 types of mountains,
named for how they form:
Folding is a process in which the Earth's plates are pushed together
in a roller coaster like series of high points and low points. Folding
bends many layers of rocks without breaking them. The
Appalachian Mountains and Rocky Mountains of the United States,
and the Alps of Europe are examples of mountain ranges that were
formed by folding.
Folded Strata (Layers)
Fault- Block Mountains
Mountains sometimes form when many layers of the Earth's crust are
moved vertically upward at fault lines by pressures caused by plates
colliding. Fault lines are great cracks in the crust. The mountains that are
formed in this way are called fault-block mountains. The Sierra Nevada
mountains in California and Nevada, and the Grand Teton range of
Wyoming are examples of fault-block mountains.
Fault block mountains form when
tension causes large blocks of
crust to drop down relative to other
Volcanic mountains are forms when
magma erupts from a divergent
boundary and hardens.
Many volcanic mountains exist under the
Some volcanic mountains rise above the
surface of the ocean to from islands,
Hawaii for example.
Faulting has a major influence on the way
the earth looks. Mountains form and
disappear over time, as well as large rift
valleys and other features. This has an
impact on where and how we live.