Mechanisms by which materials deform at the Earth’s surface or, more commonly, at depth. These can be: (a) cataclastic, in which individual grains or fragments physically rotate or glide past one another (see cataclasis); (b) dislocation creep, by a gliding motion along crystalline dislocations; (c) grain-boundary sliding; (d) recrystallization; and (e) diffusion of individual atoms. Each process is dependent on the stress, temperature, and duration of the stress.