[MUSIC] So now let's look at a map of the Earth again, and this time we're going to take the earthquake belts, marked by these clusters of little dots, each of which is the epicenter of an earthquake. So that this is not a snapshot at a given time. Just to clarify, this map shows all the earthquakes that have happened say over about 30 years or so. If we look at that map, we see that the earthquakes define distinct boundaries. And geologists now recognize that those boundaries are boundaries of blocks or pieces of the lithosphere. So in other words, this outer shell, the lithosphere is not one coherent piece, like a complete shell of an egg. Rather, it's broken along discrete boundaries into separate pieces. And we refer to those separate pieces as plates, or more formally, as lithosphere plates. We define the positions of those plates by the locations of the major earthquake belts. So if we take this map and draw lines where the major seismic belts are, we can outline effectively the discreet plates of the Earth. Now there aren't a huge number of them. Many of them are very big. Some of them are fairly small. Depending on how you want to count them, they're about 15 to 20 plates on the surface of the Earth. About 15 of them are quite large, the rest are fairly small. And depending on how you want to split things, you can divide microplates and find various other smaller pieces as well. But we won't worry about those for now. Now if you look at this map, you'll see that each different plate has a name. Some of the names are fairly obvious, some of the names are a little bit more obscure. If you pull apart the plates and just artificially create a map that you've taken apart each block and just created some space in between to see the shapes of the plate, or to emphasize the shapes of the plates, you see that they're different sizes, they're different shapes. It's not like the Earth is cut into a grid, rather it just happens that these are the various plates that exist now. And as we'll see, the configuration and the geometry of plates changes over time. So 100 million years ago, 200 million years ago, things looked quite different on the surface of the Earth. If we study the nature of these earthquakes, we can distinguish between different kinds of motions. And this gets back to our thinking about the nature of the types of faults. So for example, we saw that some faults result in one block going down. Some faults result in a block going up. Some blocks result in a block slipping sideways. Remember these are called normal faults, these are called reverse faults, and these are called strike slip faults. Simplistically, we can look at plate boundaries in the same way and recognize three distinct kinds of plate boundaries. So the first type, divergent plate boundaries. They're moving apart. They're marked usually by a geologic feature called a mid-ocean ridge. These are submarine mountain ranges that we will explore a little bit more later. The second type, convergent plate boundaries are marked by the presence of a trench. Where one plate slides beneath another. And the third is called a transform plate boundary, and those are marked by strikes of faults, big strikes of faults. So let's now look at each of these types of plate boundaries individually, starting with mid-ocean ridges or divergent plate boundaries. So in a mid-ocean ridge, two plates are moving apart. But as I mentioned, you don't get a gap or a hole in between. Rather what happens is that the soft plastic asthenosphere starts to rise as the plates move apart and effectively create space. And as that asthenosphere rises, it gets into a region where the pressures are a little bit lower, and that, for reasons we'll discuss later, creates the conditions where rock of the mantle melts. And that generates the molten rock that eventually becomes the basalt of the sea floor. So effectively, at a mid ocean ridge new oceanic plate is produced as the plates are moving apart and that process is called seafloor spreading. It was first recognized and identified around 1960. That means that if you look at a map of the ocean floor such as the Atlantic Ocean floor, the mid ocean ridge, what's called the Mid-Atlantic Ridge, runs down the middle of the ocean. And on this map we recognize different ages of seafloor and represent them by different colors. So the youngest seafloor, the stuff that's being formed most recently is red along the ridge. So there's a red stripe down the middle of the ocean. The next oldest seafloor is further out from there. Represented by the green stripe. And the oldest ocean floor of the Atlantic is along the coast lines on either side. Some on the East Coast United States, some along the West Coast of Africa, some along the East Coast of South America. So in other words, the age of the ocean floor in the Atlantic Ocean, the age of the Oceanic plate or the lithosphere plate underneath the Atlantic Ocean floor gets progressively older as you go from the axis of the mid-ocean ridge, the mountain range running down the middle, to the margins or towards the coastlines. So as the plates break apart, the upper or brittle portion cracks and splits and there are normal faults that are generated. And the slip on these normal faults generates many of the earthquakes at mid-ocean ridges. [MUSIC]