Stress and Strain

• STRESS is a force acting on a rock. In stress, the direction in which the forces are acting affects how rock responds to the stress.
• As slabs of lithosphere move, rock layers experience unequal magnitude of stress from different directions. This is referred to as differential stress.
• Types of Differential Stress

  • Compressional Stress: Squeezes rocks together, causing folding or breaking. Associated with convergent plate boundaries.
  • Tensional Stress: Pulls rocks apart, causing lengthening or breaking. Associated with divergent plate boundaries.
  • Shear Stress: Forces acting parallel to a plane but in opposite directions. Associated with transform plate boundaries.
• • Strain: A change in the shape or volume of a rock in response to stress.
• • Elastic Deformation: Temporary deformation that is reversible - the rock returns to its original shape and size when stress is removed.
  • Brittle Deformation: Permanent deformation that leads to the breaking of the rock, occurring when stress exceeds the rock's strength. More common at lower temperatures and shallower depths.
  • Ductile Deformation: Permanent deformation that leads to folding or bending of the rock, occurring at higher temperatures and greater depths where the rock is softer and less brittle.
• Factors Influencing Deformation

  • Pressure and temperature (increase with depth)
  • Mineral composition of the rock
• Stress-Strain Relationship

  • As stress on a rock increases, it undergoes successive deformations from elastic, to ductile, to brittle.
  • The type of deformation depends on the balance between the rock's strength and the applied stress
• Hard, brittle rock (e.g. granite):More likely to undergo brittle deformation

  Soft, ductile rock (e.g. shale):More likely to undergo ductile deformation
• • Mt. Everest: Constantly undergoing compressional stress due to the collision of the Indian and Eurasian plates, causing the mountain to grow taller over time.
  • East African Rift System: An example of tensional stress causing the continental crust to pull apart.
  • San Andreas Fault: An example of shear stress where the North American plate slides past the Pacific plate.
• • Rocks experience differential stress from various directions, which can lead to compressional, tensional, or shear stress.
  • Rocks respond to stress through strain, which can be elastic, brittle, or ductile deformation.
  • The type of deformation depends on factors like pressure, temperature, and mineral composition of the rock.
  • Understanding stress and strain is crucial for explaining the formation of various geological features and structures.