Preamble

Convection is an underlying theme for this chapter, the following four and Chapters 12 and 13. With the probable exception of the inner core, the entire Earth is convecting and this must always have been so. Many of the topics that geophysicists study, including earthquakes, tectonics and the geomagnetic field, are consequences of convection, thermally driven in the mantle but at least partly driven by a process of compositional separation in the core. The Earth is a thermodynamic engine, generating mechanical energy in the process of transferring heat from the hot interior to the surface. The sources of heat are considered in Chapter 21 and the thermodynamic efficiency and resulting mechanical power in Chapter 22. The calculations rely on estimates of the thermal properties considered in this chapter, especially the Grüneisen parameter.

There must be a general correspondence between local high temperatures and low seismic wave speeds in the mantle and this is sought by the technique of seismic tomography (Section 17.7), but the pattern of convection is not so simple as to make this straightforward, except for the observation of high wave speeds in the cool subducting slabs. Superimposed compositional variations confuse the picture. One approach is to compare the P- and S-wave speeds and for this purpose we need to know the temperature dependences of the wave speeds. It appears that, at least in the deepest part of the mantle, temperature and composition are correlated.