Harry Hammond HessBorn 24 May 1906; died 25 Aug 1969 at age 63. American geologist who made the first comprehensive attempt at explaining the phenomenon of seafloor spreading (1960). This revived Alfred Wegener's earlier theory of continental drift. Together, these provided an interpretation of the earth's crust in terms of plate tectonics. The surface of the globe is not continuous. Rather, it is broken into a number of huge plates that float on the molten rock under the crust, moved over eons of geologic time by convective currents driven by earth's internal heat. With this motion these plates rub against, collide with, or separate from other plates. Thus the nature of earthquakes and volcanoes could be explained, plus the existence of ridges of young rock mapped around the globe under the ocean where the sea floor was spreading.