There's something about introductory textbooks that can be fundamentally misleading. As I plowed through freshman Physical Geology some 36 years ago, I couldn't help but acquire the impression that all of the major problems in the earth sciences had been solved. Yes, there was a chapter about Wegener's conti­ nental drift hypothesis along with a few other ideas concerning earth evolution - expansion, contraction - and including Holmes' convection cell idea. Once I began to take the field trips and participate in junior-year field camp, I began to suspect that the textbooks had left something out. It didn't take long, however, to begin hearing about the "New Global Tectonics", at which point I felt that I had been misled by what I had encountered in my first formal contact with geology. All of a sudden it was clear that there was much more to geology and geophysics than a widely used book could convey. Plate Tectonics is now into its fifth decade. I haven't looked at current intro­ ductory textbooks, so I don't know whether they still convey the impression that all of the major problems in the earth sciences have been solved. Clearly, there is still much to do with continuing elaboration of plate tectonics, among all of the other challenges of the geosciences. This monograph introduces a number of new observations, techniques, and hypotheses as extensions to Plate Tectonics.