Week 7: Stress and Strain in Rocks

• Stress and Strain

  Stress and strain on rocks is responsible for the deformation structures of rocks

  • Stress is a force that causes deformation and strain is a measurement of deformation
  • Stress – measured in Pa, shear stress: parallel to surface and normal stress: perpendicular to surface
  • Strain – unitless, permanent and elastic strain
• Deformation Types


  • Rocks deform 3 different ways:
  • Elastically – When stress is removed, rock returns to its original shape. However, if the strain on the rock is great enough, the rock will fail resulting in earthquakes (stress is released)
  • Plastically – Where the rocks bend instead of break due to applied stress. The bonds of the rocks break under large temperatures. Resulting structures: Folds
  • Brittlely – Rocks break instead of bend, most rocks in the uppermost 10km of the surface experience brittle deformation. Resulting structures: Faults and fractures
• Measuring stress and strain

  • Stress cannot be measured directly, but rather can be inferred from strain
  • Strain analysis of rocks measures the orientation, magnitude, and distribution of strain features such as folds to infer the stress regime that caused them.
  • This can then be used to infer the strength of stress on a rock based off of the unit’s elasticity, strength, and ductility
• Stress Regimes

  There are extensional, compressional and shear stress regimes
• Extensional Stress Regimes

  Extensional stress in faults leads to the production of normal faults and rift valleys
• Compressional Stress Regime

  Compressional stress in faults leads to reverse faults and thrust faults
• Shear Fault Regimes

  Where shear stress in faults leads to dextral and sinistral faults
• Well vs Poorly Oriented Faults

  • Within constant stress regimes there can be both well and poorly orientated faults
  • Faults are typically well orientated when they have a dip less than 60˚. When faults have this dip, they are fewer perpendicular forces acting on the fault, and more shear, which results in less friction allowing the fault to slip easier
  • However, when the dip of a fault increases to above 60˚, the normal stress becomes increased, and the shear stress decreases. Resulting in the fault locking up and having less movement
• Factors affecting strain behaviour

  • Temperature
  • Pressure
  • Deformation rate
  • Fluids
  • Competency
• Young's Modulus

  Young’s modulus:

  o   Describes the relationship between stress and strain for an object

  o   Each mineral has a different young’s modulus, therefore he mineral makeup of the rock has a strong control over how that rock will deform

  • If the rock is composed of higher competency minerals, then the rock will be stronger and a higher amount of stress will be required for brittle strain
• Fluids in faults

  • Fluids such as water, or oil can find their way into faults
  • Faults are impacted by the influence of fluids. When fluids become involved on a fault plane, they reduce the amount of friction and allow the shear stress to more easily affect the plane. Making slip easier and more probable
• Temperature, Pressure and Deformation Rate

  • Temperature increases the further you travel below the surface. Every km there is a temperature increase of approx. 20˚-30˚
  • Pressure also increases the further you travel into the crust
  • The rate of deformation depends on the stress regime and tectonic setting of the rocks