For two centuries geologists have been content to re-create the history of the earth as a series of snapshots that brought to life the “age of coal forests,” the “time of the dinosaurs”, or “icecaps on the world.” Gradually, the focus has sharpened, the color improved, and more detail has been added, but the result was still a series of vignettes, strung together to show the way the world was rather than the way it became so. Now the plate tectonics revolution has taught us that we can and should study change itself, a major shift in our focus, from product to process, from rocks to what makes rocks.

Unfortunately, except for the Quaternary, the traditional approach is likely to be insufficient for this purpose, as the two previous chapters have made clear. Either we must greatly improve our ability to tell time or we shall find ourselves severely limited in the kinds of processes we can profitably study. What we would like to know is clear; it is everything. What we can know depends on how well we can tell time.

THE DESIRABLE AND THE POSSIBLE

Events on the surface of the earth cover a range of time scales. Continents move and mountains rise taking tens to hundreds of millions of years. By comparison a climate change, the building of the Mississippi delta, or the growth and decay of icecaps are almost instantaneous (Table 12.1).