Because of convective motion in the Earth mantle, the temperature in the convective mantle shows relatively little variation. Convection acts as an efficient mixing process. For instance, in the lithosphere, temperature increases from ca. 0ºC at the surface to 1300ºC at depth of 100-200 km. In contrast, from the top of the convective mantle (i.e. the base of the lithosphere) to the core/mantle boundary (i.e. a distance of ca. 2700-2800km) temperature increases by only ~3000ºC. Nevertheless, temperature gradient in the lithospheric mantle, and therefore mantle heat flow entering the crust shows a relatively larger variability, which impacts on the lithospheric geotherm. This variation of temperature gradient is largely due to variation of lithospheric thickness.

The graph on the right illustrates the sensitivity of the geotherm to the mantle heat flow, also called basal heat flow. It shows three geotherms calculated assuming same rate of radiogenic heat production, same conductivity and same crustal thickness. Only the mantle heat flow varies. A basal heat flow of 12 x 10^-3 W.m^-2 (yellow geotherm) is characteristic of cold and thick cratonic lithospheres, whereas a basal heat of 36 10^-3 W.m^-2 is representative of thinned lithosphere. Increasing the mantle heat flow from 12 to 36 W.m^-2 double the temperature at the Moho.

The graph on the right illustrates the sensitivity of the geotherm to the thermal conductivity (W.m^-1.K^-1). It shows three geotherms calculated assuming same rate of radiogenic heat production, same same crustal thickness, and same mantle heat flow. The conductivity is known to vary with temperature. Here however, we assume that k is constant through depth. k is proportional to the ability of material to conduct heat away. Therefore the larger the conductivity of rocks the lesser heat they can store and the cooler the geotherm.