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Mountains and Mountain
Building: Chapter 11
Rock Deformation
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Deformation is a general term that refers to a change
in size or shape of rocks in the earth's crust.
Deformation occurs when stress, or a force over a
given area occurs.
The amount of deformation that occurs and can be
measured, either by a change in shape or volume of
rocks in the earth's crust is referred to as strain.
The four factors that influence rock deformation are
temperature, pressure, rock type, and time.
Temperature and Pressure
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When rocks are near the earth's surface and
temperatures and pressures are normal, rocks
undergo brittle deformation and fracture.
When rocks are at great depths, the temperatures and
pressures are very high.
Under extreme temperatures and pressures rocks can
behave elastically.
When rocks are elastic the type of deformation that
occurs is referred to as ductile deformation.
Rock Types and Time
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Igneous Rocks like Granite and Basalt usually
deform by brittle deformation. Sometimes they
deform by ductile deformation.
Metamorphic Rocks are more likely to deform by
ductile deformation.
Often times Sedimentary Rocks show that ductile
deformation has occurred. One possible explanation
for the ductile deformation is the sedimentary rocks
folded before they hardened (lithification).
The longer stress is applied to rocks, the more
deformation will occur.
Types of Stress
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The three types of stress can be applied to rocks.
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Tensional Stress – When rocks pull apart.
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Compressional Stress – When rocks push together.
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Shear Stress – When rocks twist or push in opposite
directions.
Stresses
Folds
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Often times when Compressional Stresses and ductile
deformation occurs during mountain building, rocks
will fold or bend much like sheets of paper.
When a fold bends upward in an arching shape, an
anticline forms.
When a fold bends downward in the shape of a trough,
a syncline forms.
When a fold occurs in a step or stair-like fashion a
monocline can occur.
Folds
Faults
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Stresses can also cause a fault or fracture in the
earth's crust to occur due to brittle deformation.
There are four main types of faults.
Normal faults – these are caused by tensional
stresses. When this occurs the head (hanging) wall
moves down relative to the foot wall.
When discussing the relative motion of the earth's
crust along faults, the hanging wall or head wall refers
to the side of the fault that rests on the foot wall.
The foot wall is the side of the fault that appears to
support the head wall or hanging wall.
Faults
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Reverse Faults – faults that are caused by
compressional stress. In the case of a reverse fault,
the hanging wall moves up relative to the footwall.
Thrust Faults – these faults are low angle (less than
a 45 degree angle) reverse faults.
All faults that move up and down (normal, reverse,
and thrust faults) are referred to as dip-slip faults.
Strike-Slip Faults – These faults move side to side.
Faults
Faults
Mountain Building
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The formation of mountains is called orogenesis.
Mountains that are formed by compressional
stresses are called folded mountains.
In folded mountains, compressional stresses can
cause anticlines and synclines to occur, or they can
produce reverse faults and thrust faults.
Examples of folded mountain ranges are the
Appalachian Mountains, The northern Rocky
Mountains, and the Alps in Europe.
Folded Mountains
Folded Mountains
Folded Mountains
Folded Mountains
Mountain Building
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Mountains that are formed by tensional stresses are
called fault-block mountains.
Most fault-block mountains are formed as a result of
normal dip-slip faults.
When a normal fault occurs, a block of the earth's
crust (the hanging wall) collapses, leaving behind a
low-lying valley called a graben.
Graben is a German word for a ditch or trench.
The uplifted or higher elevated foot wall(s) is/are
referred to as a horst(s).
Fault-Block Mountains
Fault-Block Mountains
Fault-Block Mountains
Mountain Building
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Examples of fault-block mountains include the
Teton Range of Wyoming, the Sierra Nevada
Range of California, and the Basin and Range
area of Nevada, Utah, and California.
A circular bulge or upwarping of the earth's
crust is called a dome.
A circular dip or downwarping of the earth's
crust is called a basin.
Fault-Block Mountains
Domes and Basins