Plate Tectonics

Abstract

Earthquakes, volcanoes, active faults, and mountains tend to occur in long, linear belts on the Earth's surface, bounding large regions that are relatively quiescent. The theory of plate tectonics views the quiescent regions as rigid "plates" and these deformational features, located at the boundaries between the plates, as the inevitable consequences of relative motion among the plates. Since the plates are confined to the surface of a spherical Earth, their relative motion is geometrically constrained. The great power of plate-tectonic theory lies in its ability to provide testable kinematic predictions regarding the expected direction and rate of motion at the boundaries between the plates. In addition, it provides a framework for interpreting past geological events in terms of plate-tectonic processes observable today. Section 1 of this article describes the geometrical aspects of motion of rigid plates on a sphere, and presents rates and directions of motion between major plates, as determined from the relevant data. Section 2 describes the differences between continental and oceanic parts of plates, constraints on plate thickness, and the vertical variations of temperature and strength within the plates. Section 3 describes the elastic behavior of plates, isostasy, and rebound due to viscous flow beneath the plates. Section 4 discusses plate motions as an indication of processes at greater depths in the earth. Finally, Sec. 5 discusses some of the limitations of plate-tectonic theory.