 This sketch shows much closer to what our conception of how plate tectonics actually works. And I make a distinction between plate tectonics and continental drift, because I think continental drift specifically refers to Wegener's hypothesis, which didn't have a mechanism, and therefore you can't really call it a valid theory because it wasn't accepted by others. And I did have some things wrong with it. Plate tectonics is a theory, and it works to explain most of the rest of the phenomena that we're going to talk about in this course. So in this sketch, the mantle's down here. Here's a couple of continents, and the sea floor. But now you can notice that sea floor spreading is the mechanism by which new crust is created. So there's a big sea floor spreading center in the center of the Atlantic Ocean, for example, gradually pushing the continent side of it away as the ocean gets bigger. Now the earth remains the same size, and that means that if you create new crust, then crust has to be consumed somewhere else. And we take care of this with a subduction zone, like the one shown here, where sea floor goes down and gets recycled into the mantle. So this is just a sketch, and we'll flesh out more about how this looks and how this works as this course progresses.