Introduction

Plate tectonics is a model in which the outer shell of the Earth is divided into a number of thin, rigid plates that are in relative motion with respect to one another. The relative velocities of the plates are of the order of a few tens of millimeters per year. A large fraction of all earthquakes, volcanic eruptions, and mountain building occurs at plate boundaries. The distribution of the major surface plates is illustrated in Figure 1–1.

The plates are made up of relatively cool rocks and have an average thickness of about 100 km. The plates are being continually created and consumed. At ocean ridges adjacent plates diverge from each other in a process known as seafloor spreading. As the adjacent plates diverge, hot mantle rock ascends to fill the gap. The hot, solid mantle rock behaves like a fluid because of solid-state creep processes. As the hot mantle rock cools, it becomes rigid and accretes to the plates, creating new plate area. For this reason ocean ridges are also known as accreting plate boundaries. The accretionary process is symmetric to a first approximation so that the rates of plate formation on the two sides of a ridge are approximately equal. The rate of plate formation on one side of an ocean ridge defines a half–spreading velocity of u. The two plates spread with a relative velocity of 2u. The global system of ocean ridges is denoted by the heavy dark lines in Figure 1–1.