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.
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Deformation occurs when stress , or a force over a given area occurs.
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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 .
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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.
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When rocks are at great depths, the temperatures and pressures are very high.
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Under extreme temperatures and pressures rocks can behave elastically.
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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.
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Metamorphic Rocks are more likely to deform by ductile deformation.
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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).
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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.
Tensional Stress – When rocks pull apart.
Compressional Stress – When rocks push together.
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.
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When a fold bends upward in an arching shape, an anticline forms.
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When a fold bends downward in the shape of a trough, a syncline forms.
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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.
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There are four main types of faults.
Normal faults – these are caused by tensional stresses. When this occurs the head (hanging) wal l moves down relative to the foot wall .
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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.
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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.
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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 .
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Mountains that are formed by compressional stresses are called folded mountains .
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In folded mountains, compressional stresses can cause anticlines and synclines to occur, or they can produce reverse faults and thrust faults.
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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 .
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Most fault-block mountains are formed as a result of normal dip-slip faults.
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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 .
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Graben is a German word for a ditch or trench.
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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.
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A circular bulge or upwarping of the earth's crust is called a
.
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A circular dip or downwarping of the earth's crust is called a
.

Fault-Block Mountains

Domes and Basins