INTRODUCTION

In previous chapters, we have dealt with the specific details of the formation of rocks. In this final chapter, we examine the broader question of their ultimate origin. What conditions in the Earth bring about the formation of rocks, and from where does the material come to form them? These are important questions, the answers to which are critical to interpreting Earth's history, for only in rocks is any record of the geologic past preserved. There is certainly no unanimity among petrologists on answers to all aspects of these questions, but the theory of plate tectonics has provided a unifying paradigm that has eliminated much controversy. Difficulty in answering the questions stems from the inaccessibility of the regions in which the controlling processes operate and our limited experimental and theoretical knowledge of the behavior of material under the pressures and temperatures that exist in such regions. Because of the rapid evolution of ideas on this topic, no attempt is made in this chapter to review all aspects of these questions. Instead, some basic principles are discussed which should be of help in evaluating these ideas.

Ever since the formation of the Earth 4.567 Ga ago, heat generated by accretionary processes, radioactive decay, and gravitative differentiation – in particular of the core – has been transferred to the surface of the planet where it has been radiated into space. Conduction, advection, and radiation have all played roles in this transfer.