Stress is defined as a force applied over an area, F/A.
Stress may be uniform and equal from all directions:
Alternatively, stress may be unequal from different directions (differential):
Causes of stress include
When rocks deform in response to imposed stress they exhibit strain, which is the differential change in size, shape, or volume of a material. Materials differ in their responses to stress, depending upon composition, conditions of temperature and confining pressure, and strain rate. However, regardless of intrinsic degrees of brittle or ductile qualities, all strained materials pass through 3 successive stages of deformation: elastic, ductile, and fracture (failure, or brittle deformation). Provided that the strain rate is sufficiently slow to allow minerals to accommodate structurally, minerals can adjust to applied stresses by a variety of mechanisms.
Forms of deformation include: