The potential implications of the picture of mantle convection developed in Part 3 are many and far-reaching, given that mantle convection is the fundamental tectonic driving mechanism. Some of these implications are already being explored, and presumably many other aspects will be explored in due course. Given my desire that the material of this book does not date too rapidly, there is a risk in including any such material. However, the exploration of two aspects has been under way for some time, and they provide particularly important complements to the focus, so far in this book, on the dynamical processes operating at present in the mantle. Therefore I present summaries of both the chemistry of the mantle and of the thermal evolution of the mantle and its implications for tectonic mechanisms at the earth's surface in past eras. Some aspects of these topics, particularly past tectonic mechanisms, are in a tentative stage of exploration, so you should be alert to the likelihood that the subject may move on rapidly. Nevertheless I hope it is useful to indicate some directions in this work that are apparent in 1998.

I discuss the chemistry of the mantle for two main reasons. First, through radiogenic isotopic compositions mantle chemistry gives us time information, and so constrains the evolution of the system. It is thus an important complement to the discussion of thermal and tectonic evolution. The second reason is that there have been many assertions over the past two decades that geochemical observations established one or another fact about the form of mantle convection.