[SOUND] [MUSIC] So rock in our everyday experience is clearly a solid. You make gravestones out of it, you make sculptures out of it, you make table tops out of it. And when you go out into nature, you can see large outcrops of solid rock that stand there for seemingly forever. Here's an example from Joshua Tree National Monument, where we see big boulders of granite just sitting on the landscape. They're clearly maintaining their shape for a long period of time. Ice, familiar water ice is a solid too. So if you live in a cold part of the world where snow forms in the winter and ponds or lakes freeze over, you know that ice is a solid. You can even walk on it. But when the spring comes along and temperatures warm up, that solid ice begins to melt. So for example, here's a mountain lake in the spring time. A lot of the lake is still covered by solid ice, but there are also puddles of molten ice or water. Notice that even though in every day experience we talk about ice and water, in reality what we're talking about is solid ice and molten ice. They're the same stuff, but one's in solid form and one's in liquid form. Now, the same concept applies to rock. We're familiar with solid rock, we can also have molten rock, except it's probably not an everyday experience for you to see that, unless you happen to live next to a volcano. But here's a photograph of molten rock on the island of Hawaii. You can walk up to this area, take a stick and poke this molten area that's colored red because it's so hot that it glows red, and it will give. And in a movie of this, you will actually see this body of molten material spreading out over the landscape, spreading out and covering up the material that's below. So this is an example of molten rock. Now I'm going to introduce two terms, maybe familiar to you already, but these are the geological terms that we use to refer to molten rock. Geologists use the word magma to refer to molten rock that is still underground. It has not yet come up and erupted at the Earth's surface. We use the term lava to refer to molten rock that has come out onto the Earth's surface. So in this photograph here, we're actually looking at lava, because this is molten rock that's flowing out over the Earth's surface. Now, once we understand more about volcanoes, we'll know that this rock came from underground. This lava came from underground. So all lava starts out as magma somewhere down in the earth. But if that magma makes it to the surface, then we call it lava. Again, just a little bit of a spoiler for the future, we'll see that some magma solidifies and becomes solid rock under ground. Some doesn't solidify until it turns into lava or becomes lava, flows out on the surface and then solidifies. So some rocks form from magma, some rocks form from lava. But we'll get back to that. So the next question is, where does molten rock come from? In other words, where does magma come from? Now, it's a common mis-impression that many people have is that the crust of the Earth, and remember we use the term crust to refer to the outermost shell of the planet, the part of the planet that we live on. Many people have the impression from popular media that the crust floats on a giant underground sea of molten rock. In fact the shell that we stand on is sitting on top of molten rock, and in that context, they may think that volcanoes occur where somehow a crack or a conduit develops down to that molten rock that allows molten rock to come up to the surface. But that's wrong, it's not like that at all. In fact, the interior of the Earth is solid. So going back to our understanding of the layering in the outermost part of the Earth, remember that the crust and the rigid, uppermost part of the mantle comprise the lithosphere. Overall, that is a rigid layer, the outer rigid layer of the Earth. Beneath the lithosphere is the asthenosphere. And we introduce the asthenosphere by saying that it is more plastic, it is able to flow. But let's modify that statement a little bit, or emphasize that when we're talking about the flow of the asthenosphere, we're talking about flow over geologic time. The motions are maybe on the order of centimeters a year, the kind of rates that we saw, the rates at which plates move. At centimeters a year, basically a block of asthenosphere sitting on the table will pretty much retain its shape. So we consider the asthenosphere to be solid. It's not like the asthenosphere can flow up a crack, come out at the surface, and become lava. So what that means, geologists have come to recognize, is that molten rock, magma only forms under special circumstances in special locations. It's not like there's an entire molten layer underneath the crust. Rather, there are special places down in the interior of the Earth, some of them at the top of the mantle, some of them deep in the crust, where conditions are right to cause pre-existing solid rock to begin to melt. And that's how magma is produced. [MUSIC]
