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Structural GeologyDeformation


Deformation such as fracturing and contortion of rock layers is the result of ... Deformation is a term that encompasses all changes in the shape and volume of ... – PowerPoint PPT presentation

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Title: Structural GeologyDeformation

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

  • 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
  • 2) Tension forces acting in opposite directions
    along the same line to lengthen or pull apart.
    Rocks exposed to tension lengthen, thin, and
  • 3) Shear forces act parallel to each other, but
    in opposite directions. Rocks subjected to shear
    stress may be displaced along a fault plane.

  • 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
  • Ductile rocks exhibit plastic strain, Brittle
    rocks deform by fracture

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)

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

  • 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

  • If the hanging wall block moves downward relative
    to the foot wall block, it is known as a normal
  • 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

  • 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

  • A mountain is an area at least 300 m higher than
    the surrounding land area, with a distinct summit
  • 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

  • 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)

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

  • 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
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