Man's understanding of how this planet is put together and how it evolved has changed radically during the last 30 years. This great revolution in geology - now usually subsumed under the concept of Plate Tectonics - brought the realization that convection within the Earth is responsible for the origin of today's ocean basins and conti- nents, and that the grand features of the Earth's surface are the product of ongoing large-scale horizontal motions. Some of these notions were put forward earlier in this century (by A. Wegener, in 1912, and by A. Holmes, in 1929), but most of the new ideas...