Small deformations are elastic, meaning the rock will return to its original shape when the stress is removed. Larger deformations, the deformations that produce mountains and faults, are permanent. They remain after the stress is removed, leaving an enduring record of tectonic activity. Rock Deformation To understand rock deformation we need to first understand stress and strain. Rocks at depth are subject to confining pressure that is the result of the weight of overlying rocks and exactly the same as the pressure a diver feels at depth.
Stress is differential pressure.
It is not applied uniformly in all directions. Compressional stress can be modeled with a piece of clay. Squeeze the clay between your finger and thumb and you will exert a compressional stress that causes the clay to contract along the direction of the stress and expand perpendicular to it. Pulling the clay apart applies tensional stress and results in a lengthening of the clay along the axis of the stress and contraction perpendicular to it. To picture shear stress, imaging placing the clay between the opposed palms of your hands and moving your hands in opposite directions.