Plumes rising from the top of a combined chemical and thermal boundary layer tend to be cooler and more stable in location than plumes rising from an isochemical thermal boundary layer. Models include basal and internal heating and constant or temperature-dependent viscosity. A dense boundary layer tends to stabilize upwellings and decreases the heat flow across the lower boundary. The thermal structure within the boundary layer depends on the contrast in density, the thermal diffusivity of the dense material, and whether viscosity is temperature-dependent. A dense, stable boundary layer is not strongly deformed by the overall convective motions while a marginally stable layer is thinned by downwellings and swept up by upwellings.
Internal heating reduces the contribution of plumes to the surface heat flow.
Increasing the thermal diffusivity increases the temperature of upwelling plumes and may also increase their stability. The boundary layer convects internally when the layer is thick enough and has a sufficient temperature contrast. Increasing the thermal diffusivity in the layer supresses internal convection by enhancing thermal conduction across the boundary layer. Temperature-dependent viscosity enhances internal circulation within the boundary layer since viscosity is reduced by high temperatures. Convection within the layer creates small wavelength variations in temperature.