The goal of this book has been to present mantle convection as a topic accessible to all geoscientists so that it can become a routine part of their thinking. This is necessary, because it is so fundamental to so many geological processes. It is also possible, as I hope this presentation has demonstrated.

The understanding of mantle convection that has been developed in the book is fairly well established. There remains some debate about a possible layer in the deep mantle, apart from D″, though the debates usually seem to be conducted in ignorance of the heat flow constraint given in Chapter 8. There is also some debate about the existence of mantle plumes, though this persists mainly in circles that are apparently ignorant of the physics of mantle upwellings given in Chapter 7. In any case readers should be able to follow the essence of such continuing debates, which are a normal part of any scientific topic. Hopefully they will be armed with a better-informed perspective.

Two kinds of implication have also been discussed in this book: for the tectonic evolution of the Earth, and for the chemistry of the mantle. The early tectonic evolution of the Earth is still unclear. Observational constraints are so sparse and possibilities still so diverse that progress will require a continuing conversation among all those involved, and a grasp of the essence of mantle evolution models will be important.