Thermally activated processes in the Earth: Creep and seismic

Non-elastic behaviour of crystalline material results from the thermally activated motion of crystal imperfections; perfect crystals are perfectly elastic. The mobility of crystal defects is determined by an activation energy, E*, an activation volume, V*, and an activation entropy, S*, which to a good approximation are independent of temperature and pressure. Consequently the mobility of a crystal imperfection depends on depth in the Earth in a characteristic way, increasing rapidly at first due to the steep thermal gradient near the surface and then decreasing due to the effect of pressure. Thermally activated movement of imperfections controls diffusion, phase transformation kinetics, high temperature creep, recrystallization and the damping of elastic waves in solid mantle material. Diffusion creep, in which the motion of dislocations is not required, results from the diffusion of atoms in a stress gradient and produces viscous flow at geologically significant rates in a polycrystalline solid. The viscosity of the mantle resulting from diffusion creep passes through a minimum and is strongly depth dependent. A characteristic depth dependence of the damping of elastic waves is also found but the viscosity cannot be deduced from the observed internal friction