Deformation of Rocks- When rocks are put under directed pressures, they may respond to the applied pressure in several ways:.
1. Folding or bending.
2. Faulting of fracturing .
Stress.
Stress = Force area.
Concepts of Force and Stress.
Force : An action exerted by one body on another.
Force= mass x acceleration.
Stress: A force acting upon a surface is referred to as pressure or stress.
Rocks are affected by four kinds of stress which differ in their directional properties.
1. Compressional Stress: Occurs when a rock is squeezed and the opposing forces are directed toward one another.
2. Tensional Stress: Develops when a rock is pulled apart, with the forces directed away from each other.
3. Shear Stress: Generated along planes parallel to the deforming forces. Planes are being pushed adjacent to each other. Ex. Cali.
4. Hydrostatic Stress: Stress is applied equally on all surfaces of a body is referred to as hydrostatic stress or confining pressure. Ex: Diamonds, earth's core.
Concept of Strain.
Strain: The change in size and shape of a body resulting from the action of an applied stress field. Expressed as Dilation or distortion or a combination of both.
Dilation "Delta = V1-Vo/Vo----- Hydrostatic stress .
Distortional strains are associated with shape change can be studied into two kinds:.
A. Longitudinal Strain--- e= L1-Lo/Lo.
B. Shear Strain--- psi= --- Shear stress.
Behavior of Rocks during Deformation.
When rocks deform they do so in three different ways, depending on the relationship between the stresses applied and the resulting strains.
1. Elastic Deformation- If a deformed body recovers its original shape after the stress is released, the strain is said to be elastic. Ex. Sponge, golf ball, etc No permanent strain.
2. Plastic Deformation- If a deformed body does not recover its original form after the stress is released the strain is said to be plastic.
3. Brittle Deformation- During brittle deformation deforming stresses overcome a material's cohesive strength and the material breaks or fractures.