In Part 3 we come to the core of the subject of this book. We look at convection as a general phenomenon, at the features and properties of the mantle that make convection in the mantle distinctive, at the observations that enable us to infer the general form of mantle convection, at the two major identified modes of convection in the mantle, and at the current picture as best I can assess it.

Convection involves fluid flow and heat transport, and the way convection works in general is presented in Chapter 8. This includes a relatively simple way to estimate the rate of convection, useful ways to characterise convection quantitatively and ways to understand some basic features of a convecting system. The topography generated by convection is a key observable, and the seafloor subsidence discussed in Chapter 7 is put in a more general context.

Mantle convection is an unusual kind of fluid dynamics because the non-fluid plates seem to be an integral part of the convection process. We therefore need to look more explicitly at the way plates move and evolve, and this is done in Chapter 9. Some of the basic ideas are simple, but their consequences are not so obvious to the uninitiated, so some attention to the material and the Exercises is warranted.

Two main modes of mantle convection can be identified, one associated with the plates and one involving mantle plumes. These are discussed in turn in Chapters 10 and 11. Chapter 12 concludes Part 3 with an assessment of the main conclusions that can be drawn at present about the form of mantle convection, including discussions of the main controversies and questions and of some misconceptions.