Structural GeologyDeformation

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Structural Geology-Deformation

• Deformation such as fracturing and contortion of
  rock layers is the result of dynamic forces that
  occur in areas of seismic activity, volcanism,
  and mountain building
• Deformation is a term that encompasses all
  changes in the shape and volume of rocks as a
  result of stress
• Stress is the term for any force applied to a
  given area of rock
• If the intensity of stress is greater than the
  internal strength of a rock, it will undergo
  strain, which is deformation caused by stress

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Stress

• Stress comes in three varieties, all measured in
  force per unit area
• 1) Compression squeezing or compression by
  external forces directed toward one another.
  Rocks exposed to compression stress are shortened
  in the direction of stress by folding or
  faulting.
• 2) Tension forces acting in opposite directions
  along the same line to lengthen or pull apart.
  Rocks exposed to tension lengthen, thin, and
  fault.
• 3) Shear forces act parallel to each other, but
  in opposite directions. Rocks subjected to shear
  stress may be displaced along a fault plane.

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Strain

• Three types of strain can result as rocks are
  subjected to stress
• 1) Elastic strain deformed rocks return to their
  original shape when stress is relaxed
• 2) Plastic strain rocks are deformed permanently
  without breaking, as in folding
• 3) Fracture rocks are permanently deformed by
  breaking
• Ductile rocks exhibit plastic strain, Brittle
  rocks deform by fracture

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Geologic structures

• Folds rumpled or bent planar rocks, created by
  compression stresses occur deep within the crust
  where rocks are ductile
• Three basic types of folds
• 1) Monocline one simple bend in otherwise
  horizontal rocks
• 2) Anticline upward arched or convex upward fold
  (oldest rocks in its core)
• 3) Syncline downward arched or concave downward
  fold (youngest rocks in its core)

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Joints, Fractures, and Faults

• Stress and weathering of rocks often leads to
  cracks in rocks known as fractures
• Fractures that show no movement along the
  fracture plane are joints
• Fractures with displacement of rock along one or
  both sides of the fracture plane are faults
• The fracture plane in a fault is know as a fault
  plane

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Faults

• The two sides of a fault can be named the hanging
  wall block and the foot wall block
• The hanging wall block is the block which hangs
  over the foot wall block
• The foot wall block is the block on which your
  feet would stand if you were in a cave dissected
  by the fault
• Movement in a fall may occur on only one side of
  the fault, or both, and this is often difficult
  to determine total movement between the blocks
  is know as relative movement

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Faults

• If the hanging wall block moves downward relative
  to the foot wall block, it is known as a normal
  fault
• If the hanging wall moves up in relation to the
  foot wall, it is a reverse fault
• A reverse fault with an dip angle less than 45
  degrees is known as a thrust fault
• A fault that involves horizontal movement along
  the fault plane is known a strike-slip fault

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Faults

• Strike-slip faults are characterized by movement
  along the fault relative to the observer if the
  movement on the block across the fault from the
  observer (the block you are not standing on) is
  to the left, it is a left-lateral fault, if the
  block across the fault from the observer is to
  the right, it is a right-lateral fault
• A fault that combines dip and strike movement is
  known as an oblique fault

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Mountains

• A mountain is an area at least 300 m higher than
  the surrounding land area, with a distinct summit
  area
• Linear associations of mountains are termed
  mountain ranges
• A mountain system is a complex linear zone of
  deformation and crustal thickening that may
  consist of several or many mountain ranges
• Mountains may form in many different ways
  differential erosion creates mesas and buttes,
  example Monument Valley, Utah

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Mountains

• Mountains can be created by volcanism at plate
  boudaries or at hot spots
• Mountains can form from erosion of country rock
  that exposes underlying intrusive batholiths,
  example Stone Mountain, GA
• Block faulting creates mountains as blocks are
  dropped and uplifted along faults in regions of
  tensional stresses, example Basin and Range of
  western U.S. (uplifted blocks horsts, down
  dropped blocks grabens)

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Mountain building Orogeny

• Orogeny is an episode of mountain building during
  which intense deformation occurs, as at
  convergent plate boundaries
• Orogenic zones along convergent margins are
  termed orogenic belts, the circum-Pacific
  orogenic belt (or Pacific Ring of Fire) is the
  largest orogenic belt on Earth
• Accretion of new material at convergent margins
  produces complexes of often unrelated rocks
  called terranes, or exotic terranes

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Isostasy

• The principle of isotasy refers to the floating
  equilibrium of the Earths crust with the
  underlying denser mantle
• Isostatic loading of the crust will cause it to
  sink further into the mantle, displacing mantle
  material, much as an iceberg displaces water as
  it floats
• Erosion of mountains (or melting of glaciers)
  removes the load on the crust and allows
  isostatic rebound, a gradual return of the crust
  to its original position