Week 7: Stress and Strain in Rocks

Cards (12)

  • 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
    • Strainunitless, 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