Chptr16

• Metamorphism


  Stress, deformation and structures
• An understanding of metamorphic rock textures requires a basic understanding of stress and deformation - the foundation of structural geology
• Stress


  A directed force of some magnitude applied over an area
• Deformation


  A change induced by stress
• Stress and deformation are critically important to sedimentary and igneous rocks, particularly with respect to their resource potential and their role in geohazards
• Vector


  A line whose length is proportional to its magnitude and which has an arrow indicating direction
• Resolving force into components

  Force F can be resolved into normal force Fn and shear force Fs
• Three fundamental ways stress can be applied to a rock body

  • Compression
  • Tension
  • Shear
• Principal stress axes


  Three mutually perpendicular vectors representing normal forces of equal (uniform) or unequal (non-uniform) magnitude
• Principal planes


  Planes parallel to two principal stress axes and normal to the third stress axis
• Uniform (isotropic) stress


  All three principal stress axes are of equal magnitude
• Uniform (isotropic) stress


  • No shear stresses occur
  • No change in shape occurs
  • Volume change can occur
• Non-uniform (anisotropic) stress


  At least one principal stress has a magnitude not equal to the other principal stresses
• Non-uniform (anisotropic) stress


  • Shear stresses can occur on rock bodies, but not on a principal plane
  • Shape changes can occur
  • Volume change can occur with corresponding changes in density
• Non-uniform stress promotes the development of inequant grain growth and foliations
• Deformation


  A physical change in the rock due to an applied stress
• Components of deformation

  • Distortion
  • Dilation
  • Translation
  • Rotation
• Homogeneous strain


  Strain is equal throughout the rock body so that parallel lines remain parallel, perpendicular lines remain perpendicular and circles flatten to become ellipses
• Heterogeneous strain


  Strain intensity varies within a rock body, producing angular changes so that lines that were once parallel or perpendicular are no longer so, and circles do not deform to ellipses
• Dilation


  A change in volume
• Translation


  An object has moved from one point to another point
• Rotation


  An object has moved in a circular arc about an axis
• Principal strain axes


  Imaginary lines that are perpendicular to each other and that intersect planes of zero shear strain, denoted X, Y, Z
• Stress induces strain, and the three principal stress axes (σ1, σ2, σ3) can correspond to the three principal strain axes (X, Y, Z)
• Rotational strain


  • Strain axes rotate through time, so that the principal strain axes do not remain the same during progressive deformation
  • Different principal strain axes occur at each incremental step as a result of axis rotation
• Coaxial strain


  No rotation of the incremental strain axes occurred from an initial to final strain state
• Pure shear (coaxial strain)


  • Uniform elongation occurs in only one direction
  • Uniform contraction occurs in a perpendicular direction
  • Strain axes are parallel to principal stress axes
  • Principal strain axes correspond to principal stress axes throughout deformation
  • No change in volume occurs
• Simple shear (non-coaxial strain)


  • Strain axes do not remain parallel during progressive deformation
  • X, Y and Z strain axes rotate during progressive deformation for a fixed single stress orientation
  • Direction of maximum elongation is not parallel to direction of minimum compressive stress or maximum tensional stress
  • Direction of maximum shortening (minimum extension) is not parallel to direction of minimum tension or maximum compressive stress
• General shear


  Combination of pure shear and simple shear
• Elastic deformation


  Deformation produced by stress is totally and instantaneously reversible or recoverable
• Plastic deformation


  Irreversible strain without visible fractures, although microfracturing can occur
• Rupture deformation


  Creates visible fractures in response to stress, resulting in loss of cohesion of rock particles and permanent, irreversible deformation
• Hooke's law


  Linear relationship between stress and strain in elastic behavior
• Young's modulus of elasticity (E)


  Measure of resistance to elastic distortion, a constant of proportionality that describes the slope of the stress-strain line
• Stiff rocks


  Require high stress values to achieve a given strain value
• Rocks that are not stiff

  Deform more with a given amount of stress
• The stress-strain relationship is not time dependent; no time lag occurs, so that strain begins when stress is first applied; and strain ceases immediately upon the removal of stress
• Young's modulus of elasticity (E)

  The slope of the stress-strain line, a constant of proportionality that describes the slope of the line
• Young's modulus of elasticity


  A measure of resistance to elastic distortion, dependent upon the stiffness or rigidity of the material
• A rigid, stiff rock (high E) such as granite requires greater stress to achieve a given strain than a soft, pliable shale (low E)