The thermal state of the earth's interior is a topic of fundamental importance in many branches of geophysics. Investigations ranging from the dynamo theory of the origin of the earth's magnetic field to the driving mechanism for plate motions are all ultimately concerned with the question of how heat is transferred in various regions of the earth and what are the resulting flow patterns and temperature at depth.
Present ideas on this problem are in a state of flux. New insights are being brought to bear on old problems, and additional questions are being raised. For example, analysis of postglacial rebound data from the Canadian shield has led to the interpretation that the effective viscosity of the lower mantle may not be significantly larger than that of the upper mantle, as had been previously thought. As a consequence, there has been a revival of interest in mantle-wide convection as the dominant mode of heat transfer within the mantle. A major uncertainty which remains, however, is whether mantle-wide convection would consist of a single flow system extending from the base of the lithosphere to the core-mantle boundary or whether there are, for example, separate flow systems in the upper and lower mantle, separated by a thermal boundary layer at about the depth of the 670-km phase transition.