Stress is a force that can change the shape or volume of a rock. The types of stress causing rocks to undergo strain or deformation include compression, tension and shear.
Stress occurs in the rock when pressure is applied over a certain area to cause changes in volume or shape, known as strain. Compressional stress is the result of forces that squeeze the material, while tensional stress is the result of forces that stretch the rock. Shear stress occurs when parallel oppositional forces act on the rock.
As rock is subjected to increasing stress, it undergoes three stages of deformation. During the earliest stage, elastic deformation, the stress is still reversible. Ductile deformation follows, where the strain is irreversible. Rock that continues to undergo stress may fracture where the material breaks, which often depends on the composition of the rock and type of stress. The precise degree of deformation is also dependent on several other factors, including temperature, confining pressure, stress rate and mineral composition.
Stress can cause strain and deformation at large scales, such as plate tectonics, or on small scales, such as local structural geology. Geologists are trained to observe and study such changes to better understand the movement and forces that affect the earth's crust.