We began by taking stock of the different sources of internal heat in the Earth and the main modes of heat release: conduction and convection. We introduced boundary layer theory for convection, and the Rayleigh number, and discussed how we came to realize the presence of convection in the rocky mantle. We presented the global distribution of heat flux at the Earth's surface, and illustrated, in particular, that oceanic heat flux verifies a square-root dependence on plate age, up to around 80 Ma, as predicted for the conductive cooling of a half-space. We described the temperature curve as a function of depth in the Earth (the geotherm) and introduced the notion of potential temperature.
Next, we described the influence of viscosity on the convective regime and presented our knowledge of the viscosity profile in the Earth's mantle and the different types of measurements that can be used to constrain it. Finally, we introduced the notion of dynamic topography and how geoid, post-glacial rebound and global tomography data can be used to clarify this viscosity profile. We concluded by presenting the various definitions of the lithosphere/asthenosphere boundary: mechanical, thermal, seismic or "electrical".
