A quarter century ago, few geologists gave the oceans of the past much if any thought. The emphasis on the ocean basins by the plate-tectonic revolution has changed that and we have come to realize how important ancient oceans are for our understanding of the earth as a whole. As yet we cannot discuss with confidence any oceans older than those of the late Cenozoic, and next to nothing is known about those of the Precambrian. Still, in two decades a rich harvest of new information has been reaped, due above all to a major program of ocean drilling that began in 1968. New concepts are worth testing, and as regards chronology we have advanced considerably. If what follows is hedged with doubts and cautions, another decade or two should change that.

HOW THE OCEAN WORKS

The great ocean rivers, the surface currents, are driven by the planetary winds, by the trade-winds on either side of the equator and by the westerlies at 45–60° N and S. A simple ocean bordered by land would have two equatorial currents, pushed westward by the trades (Figure 10.1). At the western barrier, some of the water would be reflected back along the equator, while the remainder would form large gyres in the northern and southern hemispheres, eventually returning east under the influence of the westerlies. Another gyre, flowing counterclockwise, would exist in each subpolar zone.