Abstract
This is a brief description of the theory of Plate Tectonics, a new method of looking at Earth's geological evolution developed in the 1960s. The observations that led to the concepts of plate tectonics, sea floor spreading and continental drift are reviewed, with emphasis on studies of the Earth's magnetic field and its reversal properties. Data on seismology, topography of the ocean floor, the nature of the oceanic crust, and the loss of heat from the ocean floor are also discussed.