This document provides an overview of the theory of plate tectonics. It explains that the Earth's crust and upper mantle are broken into plates that move around on top of the mantle. There are three main types of plate boundaries: convergent where plates collide, divergent where they move apart, and transform where they slide past each other. Convection currents in the Earth's mantle cause the plates to move by heating and cooling material in the mantle. As plates interact at boundaries, it causes volcanic activity and mountain building at convergent boundaries and new seafloor at divergent ones.