Solid Earth

The Solid Earth comprises a planet's solid inner core, liquid outer core, viscously deforming mantle, and mechanically stronger crust and lithosphere.

Mantle convection is the slow buoyancy-driven motion of a rocky planetary mantle. On geological timescales the solid mantle behaves as a highly viscous Newtonian fluid to first approximation.

The Rayleigh number compares buoyancy driving with viscous and thermal diffusion. For a layer of thickness $h$, temperature drop $\Delta T$, density $\rho$, thermal expansion coefficient ${\alpha }_T$, gravitational acceleration $g$, thermal diffusivity $\kappa$, and dynamic viscosity $\mu$,Thermal convection begins when this exceeds a geometry-dependent critical value.