1. Stress and Strain
Stress is a force applied over an area. One type of stress that we are all used to is a uniform stress, called pressure. A uniform stress is a stress wherein the forces act equally from all directions. In the Earth the pressure due to the weight of overlying rocks is a uniform stress, and is sometimes referred to as confining stress.
If stress is not equal from all directions then we say that the stress is a differential stress. Three kinds of differential stress occur.
· Tensional stress (or extensional stress), which stretches rock;
· Compressional stress, which squeezes rock; and
· Shear stress, which result in slippage and translation.
When rocks deform they are said to strain. A strain is a change in size, shape, or volume of a material. Continue reading