You may recall from earlier courses in which folds, faults, and metamorphic foliations were discussed that when stresses are applied to rocks, they experience changes in shape and/or volume. These changes in shape and/or volume that occur in response to stress are called strains. They are analogous to the strains that occur in bones and muscles when they change shape in response to stress. Non‐elastic strains are subdivided into those in which rocks break along fractures such as faults or joints and those in which shape changes are accomplished without macroscopic fracturing. Irreversible strains that involve visible fracturing are called rupture; those that do not are called plastic strains and accommodate plastic deformation. Rupture is favored by rapid strain rates (think how fast the bone changes shape as it fractures), low confining pressures, and low temperatures. On the other hand, plastic strain is favored by very low strain rates, high confining pressures, and high temperatures (ssss1a). Under such conditions, deep below the surface, rocks respond very slowly to stress in a manner more like Playdough® or modeling clay than like the rigid rocks we see at Earth's surface. How can rocks undergo significant strain without rupturing? A major key lies in the large number of defects that the minerals in rocks contain.