The document discusses the theory of plate tectonics. It describes how the Earth's crust is broken into plates that move around on top of the mantle due to convection currents in the mantle. There are three types of plate boundaries: convergent where plates collide, divergent where they move apart, and transform where they slide past each other. Convergent boundaries can result in subduction zones, mountain building, or earthquakes depending on the type of plates involved. Divergent boundaries result in seafloor spreading and volcanic activity at mid-ocean ridges.