Heat transport is an integral part of convection. Heat is transported in two principal ways in the mantle: by conduction and by advection. Advection means that heat is carried along with mass motion. Heat is also generated internally by radioactivity. Here we consider these processes in turn. A key feature of heat conduction is that there is a fundamental relationship between the time scale of conductive cooling (or heating) and the length scale over which the process is occurring. This is demonstrated in several ways and at different mathematical levels.

A key application of heat conduction theory is to the cooling oceanic lithosphere, and a key consequence is the subsidence of the sea floor with age. The lithosphere is a special case of a conductive thermal boundary layer, which is the source of convective motion (Chapter 8). The oceanic lithosphere and its subsidence play a central role in the discussions of Chapters 10 and 12 of what can be inferred about the form of mantle convection from observations. The role of the continents in the earth's thermal regime is considered separately, since continental lithosphere does not partake in subduction like oceanic lithosphere.

The advection of heat is a phenomenon that can be understood in quite simple terms. It is presented first in a simple way, and the idea is then used to derive a general equation that describes heat generation and transport, including both conduction and advection. Finally, thermal properties of materials are briefly considered, including their likely variations with pressure. This leads into the concept of adiabatic gradients of temperature and density.