The deformation of minerals under mantle conditions generally follows a flow law of the form where is the deformation rate, is the deviatoric stress, is the shear modulus, d is the grain size of the rock, Q is the activation energy for the deformation mechanism, T is the temperature in Kelvin, R is the gas constant and A is a constant.
Viscosity is defined as therefore, deformation is directly related to viscosity.
Other factors, such as partial pressure of oxygen and water content may also have important effects. A rheology with a linear stress, deformation-rate creep mechanism, such as diffusional flow, is referred to as a Newtonian rheology. The question of which mechanism dominates in the mantle depends on the average grain size of the mantle minerals. This suggests that grain-size reduction occurs as a result of the phase transformation in the analog system. Karato and Li further suggest that the lower mantle may be controlled by diffusion creep.