A displacement in the plates that make up the earth's crust, often responsible for the occurrence of raised beaches and for seismic and volcanic activity. Plate tectonics is a theory dealing with the dynamics of the Earth's outer shell, the lithosphere. The theory states that the lithosphere consists of about a dozen large plates and several small ones. These plates move relative to each other and interact at their boundaries, where they diverge, converge, or slip harmlessly past one another.