Some Definitions

Stress: a force tends to change size and shape of the material (crustal rocks).

Forces
Compressive: shorten the body    ---> <---

Tensional: elongate, pull the body, stretch it      <----   ------>

Sheer stress:   <------

                          ------->

Body types:
Brittle: under stress stretches and then cracks, fractures (example, crustal surface rocks)
Ductile: under the stress body behaves elastically

Strain: adjustment, response of the rock unit to applied stress = DEFORMATION

Plastic strain (deformation): The material bends under stress doe not return to its original shape after the stress released.

Elastic strain:  body recovers its original shape after stress released "bouncing back".


Fault: a fracture in bed rock (brittle body) along the movement of the earth (earthquake).
Fold: bending in layered rock