One of the more enigmatic features of the Earth's style of mantle convection is strike-slip, or toroidal, motion which is present in the main surface expression of the convective process, i.e., plate tectonics. Toroidal motion is uncharacteristic of basic thermal convection, but necessarily forms through the interaction of convective flow and nonlinear rheological mechanisms. Recent studies have implied that the empirically determined power-law rheologies of mantle silicates are not sufficient to generate the requisite toroidal motion. Such a rheology is derivable from a very simple model of the coupling of viscous heating and temperature-dependent viscosity.
Flow is driven by an elongate, plate-like source and equal but opposite sink.
In essence, the fluid dynamical model develops a strike-slip fault.