Thermal Convection in the Mantle: Modeling Aspects

When a layer of fluid is heated from below by external heat sources and/or from within by internal heat sources and cooled from the top, the density of the fluid becomes less in the regions of heating because of thermal expansion and temperature dependence of density. The temperature induced density heterogeneities lead to gravitational instability as the cool dense fluid in the upper part of the layer tries to sink and hot less dense fluid tries to rise upward due to its buoyancy. Once the buoyancy becomes strong enough to overcome the viscous resistance, the convection starts. This process is called thermal convection. In the earth’s mantle, a region lying between 100 and 2900 km depth, this situation exists because of significant temperature difference between the base of lithosphere (100 km depth) and core-mantle boundary (2900 km) and the thickness of the mantle. Convective circulations in the mantle induced by thermal heating is modeled by using fluid dynamical modeling approach. The mantle convects in solid state, meaning it behaves as a fluid in geological times but is solid on short time scales. To understand the nature and conditions for onset of thermal convection one needs to solve the Navier-Stokes equation along with the continuity and energy balance equations. An important aspect of mantle convection is that inertial terms in the Navier-Stokes equation can be ignored because of very high viscosity (~10 21 Pa.s) of the mantle.