Hi, my name is David Schultz. Welcome to Our Earth, Its Climate, History, and Processes. In this lecture, I want to delve into plate tectonics in a little bit more detail. And before I do that, I want to just make it clear on a couple of definitions. The first one is continental drift. This is a limited set of observations that relate to the Peleo motion of continents, basically describing the fact that continents have moved around on the planet. This is Wegener's original hypothesis for explaining the observations that he saw. But this is very clearly different than plate tectonics. Plate tectonics is the theory that encompasses continental drift, but it also encompasses the sea floor spreading, the mantle convection, the subduction zones. The whole theory is wrapped up in this unifying theme called plate tectonics. That the plates are lithospheric plates, crust and upper part of the mantle, and that they are moving on the surface of the Earth due to convection currents within the mantle. Vine and Matthews, in 1963, explained that the magnetic anomalies that they were observing were due to the creation of new crust during periods of different phases of the earth's magnetic field. Either pointing generally northward or pointing generally southward. And that these anomalies then could be dated and determined. Then how quickly the bands on the planet then were being created under the water. So if you can date when these reversals happened through radioactive decay of radioactive isotopes and we know the width of a band on the surface of the earth. We know its dimensions. Then, we can work out the rates of sea floors spreading. And it turns out this is about one to ten centimeters per year. It varies on the face of the earth. But in general, this works out to that order of magnitude. Now from observations of volcanic activity and seismic activity, we can see that the earth is divided into a number of rigid plates. These plates are bounded by mid-ocean ridges and ocean trenches and what are called transformed boundaries. Some plates are topped entirely by ocean crust and others are topped by both ocean and continental crust. These plate are also lithospheric. That means that they encompass not only the crust, but also the rigid part of the upper mantle. They're separated from the lower part of the mantel by the low velocity zone. Remember, this is a region where there's partial melting of the peridotite. It's not much. It's just, say, less than 1%. But it's enough to create this weak zone. And it's that weak zone, this low velocity zone, that we believe separates the lithosphere from above, which has the crust in the upper part of the mantle from the asthenosphere and everything below. So these plates then, these lithospheric plates, ride along on partially molten asthenosphere. But remember, the mantle is a solid. It's convecting over very slow time scales, but it is a solid. I mentioned before that these plates meet at three kinds of boundaries. There's the mid ocean ridge. Which is a divergent or also called constructive boundary. Divergent, because this is where the plates are separating apart from each other on either side of these boundaries. Constructive, because we're adding new material. This is where the new ocean crust forms and the plates are drifting apart. Those are the mid-ocean ridges. The second boundary is called the trench. This is a convergent boundary or destructive boundary. Convergent, because these two plates are coming into collision and one plate is being subducted unto another. That's the destructive part of it. One plate is being sent under the other, back into the mantle. The third kind of plate boundary is the transform boundary. It's a fault where two plates slide past each other. This is a consequence of being on a spherical Earth. And you can see it best along the mid-ocean ridges where there are these transform boundaries that lie perpendicular to the main orientation of the mid-ocean ridge. So what drives the plate motions? Well as we mentioned before, Wegener had no mechanism for the motion of the continents. But it was in the 1920s that Arthur Holmes suggested mantle convection might be the answer. You may remember Arthur Holmes. He was one of the people who worked with Rutherford to help date the Earth and suggested that the Earth had a very old age, billions of years old. We now recognize that what drives plate tectonics is the convection inside the solid mantle. And it's the circulations then, that drive these plates apart and together. And this is what we'll talk about in one of the upcoming lectures.
