[MUSIC] What we're going to do today, is basically introduce the theory of plate tectonics, sort of a mini course on the theory of plate tectonics. We'll see, that when you take Wegener's concept of continental drift, add to it, Hess's concept of seafloor spreading, add to that, all the new data that was discovered during the interval of the 1930s through the 1960s or so, what comes out is a grand unifying theory. Now known as, the theory of plate tectonics which we will see, explains a huge variety of Earth phenomena. But to start this, we want to review a couple key points about earthquakes. Because we'll see that, the nature of earthquakes, was really key to the understanding, or the development of the theory, of plate tectonics in the first place. So, remember that when an earthquake occurs, it usually occurs on a fault, meaning a fracture on which there's sliding, and that fault is usually underground, sometimes faults extend all the way to the surface, sometimes faults don't. The area the fault that slips, and generates the earthquake energy, is called the focus of the earthquake, or sometimes called the hypocenter of the earthquake. But when we look at a map of the planet, or when we look at a globe, we can't actually see those points, because they're underground somewhere. So what geologists, or seismologists tend to do, is plot a point on the surface of the Earth called, the epicenter of the earthquake. So when we see a map, that shows the distribution of earthquakes, we are looking at what's called the epicenter. One of the amazing discoveries, that took place in the middle part of the 20th century, back in the 1940s, and 50s, was the realization that earthquakes don't really happen randomly around the surface of the globe. It's not like they're scatter shot, or buck shot from a shotgun that's spread over a huge area. In fact, earthquakes cluster in distinct zones, or belts, and these are called seismic belts, or seismic zones, and when these are plotted on a map they outline very specific areas of the planet. So we're looking at a map of the Earth here, and we're seeing that Earth quakes occur in a belt along the west coast of South America, they occur along the west coast of Mexico in portions of the West coast of North America along the coast of Alaska. In fact, they occur all the way around the Pacific, as if the Pacific Ocean is seismically active around it's entire edge, but that's not the only place they occur. Many earthquakes that people didn't really realize existed, because they're occurring in the floor of the ocean, out in the middle of the ocean. Once people learned how to recognize, or identify the location of earthquakes, they began to realize that there were belts of seismicity that ran down the interiors of the ocean, for example, there's one belt that runs down the middle of the Atlantic Ocean. There's another belt that cuts across the southeastern corner of the Pacific Ocean, and there's one that runs down the middle of the Indian Ocean. Also of course, there are earthquakes that span the major active mountain ranges on the planet. So for example many earthquakes to curl on the Himalayas, and earthquakes occur in the Mediterranean region, again, the point is, that earthquakes do not occur randomly. The majority of earthquakes occur in distinct belts, called seismic belts, now as, I mentioned, until the 1960s, seismologists had no real idea why this happened. There were some speculations but nothing that really held together as a good idea, but then between 1960, and 1968, not one person, but a group, really, of several geologists, probably about a dozen or so, can really be considered the major contributors of the idea came up with the idea of the theory of plate tectonics. And in a nut shell, what plate tectonics says, is that the outer shell of the Earth, and we'll see that it's a layer called the lithosphere. That this outer shell is broken into pieces along distinct boundaries, and that all the movements that generate earthquakes are by and large concentrated along these boundaries. So to understand this, we need to step back a little bit, and first talk about the internal structure of the Earth, because remember, I just said that there's an outer layer, so if there's an outer layer, is there an inner zone? We have to understand a little bit about, how the Earth is structured to understand what this term lithosphere means, and why it's the place where earthquakes happen. [MUSIC]