Mountains and Mountain Building: Chapter 11

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Mountains and Mountain Building Chapter 11
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Rock Deformation

• 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.

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Temperature and Pressure

• 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.

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Rock Types and Time

• 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.

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Types of Stress

• 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.

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Stresses
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Folds

• 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.

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Folds
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Faults

• 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.

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Faults

• 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.

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Faults
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Faults
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Mountain Building

• 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.

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Folded Mountains
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Folded Mountains
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Folded Mountains
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Folded Mountains
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Mountain Building

• 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).

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Fault-Block Mountains
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Fault-Block Mountains
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Fault-Block Mountains
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Mountain Building

• 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.

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Fault-Block Mountains
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Domes and Basins