Introduction

Nothing about the earth is fixed, permanent, unchanging. What is today a great mountain that pierces the sky may tomorrow be nibbled down into a mere hill, while elsewhere an undersea mass of sediments may be thrust upward into a lofty plateau. How do we know that such things can happen? After all, though plenty of geologic activity is taking place around us, for the most part the pace is exceedingly slow. Only after millions of years can the processes now at work yield large-scale changes in the pattern of the continents and in their landscapes. What justifies the belief that the earth's crust never stops evolving is the record of the past, a record that can be read in the rocks of the present.

During the past few decades a major advance has occurred in our understanding of the large-scale forces that shape and reshape the earth's crust. The notion that the continents are slowly drifting relative to one another – a notion going back three-quarters of a century but largely scorned for most of that period – has turned out to be the only way to explain a variety of striking observation. These same observations also provide clues as to what makes the continents drift. So far-reaching are the implications of the new dynamic picture of the crust, and so suddenly did they come to light, that it is legitimate to speak of a revolution in geologic thought.