Pressure has important effects on plastic flow. Pressure affects the rate of plastic flow through two distinct mechanisms. First, the height of the potential barrier for atomic motion changes with pressure, which usually makes plastic flow more difficult at higher pressures. This effect is mainly characterized by a parameter called activation volume. Experimental observations and theoretical models of activation volume are summarized including the pressure dependence of activation volume. For a constant stress, the rate of deformation changes with pressure by as much as ten orders of magnitude in Earth's interior. Second, pressure modifies the chemical environment particularly the fugacity of water that controls the concentration of point defects and changes the rate of plastic deformation. The fugacity of water becomes higher at higher pressures, which enhances the rate of plastic deformation by several orders of magnitude. Experimental observations and theoretical models of effects of pressure and water on plastic deformation are reviewed including empirical correlations such as the homologous temperature scaling and some atomistic models of defects.

Key words activation volume, homologous temperature, Keyes model, water fugacity, hydrogen-related defects, FT–IR spectroscopy.