[MUSIC] Now, as a consequence of these plate motions, the map of the earth is constantly changing. Too slowly to make a difference in our lifetime or even in in human history, but enough so that over geologic time, which spans millions, billions of years, the map of the planet has changed radically. Here are three interpretations of what the Earth's map might have looked at in time in the past. In the present, you see a map that looks like the present day planet. And because of the way the bathymetry and topography is shown, you can actually detect where the plate boundaries are. A hundred million years ago, the Atlantic Ocean still existed but it was much narrower. 250, 260 million years ago, the Atlantic Ocean did not exist. And you could have walked from central Illinois to South Africa without ever getting your feet wet. Before that, the map of the planet looked different. There were other oceans. There were other continents, things that didn't even necessarily look the same as the continents that we see today. And before that, things were even more different. The continents effectively danced very slowly around the surface of the Earth over geologic time as the oceans between them formed or are consumed. All these a consequence of plate tectonics and the map of the planet constantly changes. This now explains a number of the ideas that Alfred Vegner suggested back at the beginning of the 20th century. That leaves one question, and it's still somewhat of an open question. And that is why do plates move? Why does plate tectonics take place? Well, there are a number of phenomena that are probably involved. We'll discuss three. First, mantle convection. Then, slab pull and finally, ridge push. One, is that the interior of the Earth, the mantle, is convecting. Convection occurs any time you have a material that is able to flow, and you have areas that are hotter and therefore less dense, and areas that are colder, and therefore more dense. Because in a gravitational field, the less dense areas will try to rise like a balloon, whereas the denser areas will try to sink. Now, we said that in the asthenosphere of the Earth, motions can take place. That's because in the context of geologic times, with motions on the order of two to ten centimeters a year or so, the Asthenosphere is able to flow. So because the Asthenosphere is able to flow, plates are able to subduct. They're able to sink down into the Asthenosphere. Because the Asthenosphere can flow, Asthenosphere can rise and generate the melts that ultimately become new sea floor at mid-ocean ridges. And because the Asthenosphere can flow, continents can move. Some probably have deep roots but they're able to move relative to even deeper portions of the earth. Well, part of plate motion may actually be a consequence of convective motions that take place. That's probably not the whole story and maybe not even the most important part of the story. Another part of the story maybe that when plates sink, it's kind of like an anchor that pulling on an anchor cable. The sinking plate is denser and it pulls the rest of the plate with it. And when plates form a mid ocean ridge the area around the ridge as we've seen from these bathymetric maps is relatively high. And as a consequence it's got gravitational potential energy that can push to the side and cause the plates to move apart. As an analogy you can think of a glass that was filled with honey at the bottom and water at the top. If you tilt the glass to the side, the honey will flow up the side of the glass. And then if you quickly put it back to horizontal, you'll have a sloping surface of honey. Obviously the honey, which is denser than the water above, is going to gradually try to push itself away, until it eventually becomes horizontal again. During this time where there's a sloping interface between the sticky, dense honey and the less sticky water, there's going to be a push to the side. In geology, in the case of mid-ocean ridges, that force is called the ridge push force. So, part of the story of how plates drive, or why plates move, has to do with convection in the mantle. Part of it has to do with what's called ridge-push force due to the elevated areas along mid-ocean ridges. And part of it has to do with what so called slab-pull force due to the sinking of plates. Combination of those forces is enough to cause vast areas of the outer shell of the Earth to move over time. In summary, we started out by posing the question why do earthquakes occur? We saw that that opened the door into an understanding of a hugely important theory, the theory of plate tectonics. And we then explored that theory of plate tectonics recognizing that the outermost layer of the lithosphere is broken into plates, that those plates move relative to one another, that we can describe three different kinds of plate boundaries, whether the plates move apart, together or sideways. And that we can understand that earthquakes happen at all those plate boundaries and more, that some of are places where continents are beginning to break apart. Some are where continents were squeezing together. And some so-called interplate earthquakes happen away from plate boundaries, probably because of motions on long-lived weaknesses. Finally, we've seen that we can describe these plate motions, and realized that when we are talking about these plate motions, we're talking about a few centimeters a year. A very small distance in the context of human experience, but when added up over geologic time can account for large displacements, displacements of plates that are on the order of thousands of kilometers. In fact, during the past 200 million years, actually during the past 180 million years ago, the plate tectonic motions have resulted in the opening of the entire North Atlantic Ocean. And finally, again just to reiterate, because of plate motions over time the map of our planet has changed radically. And so what we see today is the continents and oceans did not exist in the same configuration in the past. [MUSIC]
