SPEAKER 1: So the basic concept is that heat inside the Earth drives motions. We have convection currents, through which heat is dissipated from the interior of the Earth. Those convection currents then have an impact on what happens at the surface. So as the convection currents rise, the surface of the earth is actually fractured, or ruptured, and portions of the Earth's surface spread away from the rising convection currents. That spreading drags the surface of the earth along, and eventually back down into the interior of the Earth, in what we call subduction zones. And this also drives the motion of the continents, what is sometimes referred to as continental drifts. Sediments can accumulate on the continental margins, they get incorporated into mountain belts, and then melting of the slab, or of the mantle above it, can lead to volcanoes such as we find in the Andes in Chile or Mount St. Helens in the west coast of the United States. If we back off and look at what this looks like on a planetary scale, we see that the whole planet can be divided into a series of fragments, which we call plates, and it's the motion of those fragments, those plates, that explain earthquakes, volcanoes, and a host of other features about the planet that we live on.