elastic deformation Temporary deformation, from which material recovers, caused by an applied stress, such that on release of the stress the body reverts to its former, unstrained condition. In purely elastic materials such deformation is described by a linear stress—strain relationship (see HOOKE'S LAW). In rocks, ideal elastic strain is combined with viscous components.

plastic deformation A deformation process which proceeds elastically at low stress values and becomes viscous when a critical stress value is reached. Plastic deformation occurs in rocks under conditions of high pressure and temperature, producing a permanent alteration of their shape but without failure by rupture.