[MUSIC] [MUSIC] Now let's get back to what happens when rock starts to melt. Now, if I took water, and I stuck this bottle in a freezer, and it turned into ice, the ice would have the same composition as the liquid. If I took that ice out of the freezer, put it on the table, it would turn into water, and the water would be exactly the same as the ice. So in our everyday experience, we're familiar with melting relatively simple substances, so that they go from being a solid to a liquid of the same composition. When we're dealing with rocks however, the process of melting is really quite different. And that's because rock is a very complicated material that contains atoms of many different elements, structured in minerals of many different kinds. So if we take a rock, and let's for the sake of our discussion, say that we heat it up by heat transfer. What happens is that the liquid that forms does not have the same composition as the original rock. Within that rock, and again, the chemistry is a little bit more complicated than what we can talk about here. But within that rock only certain materials, or certain components go into the molten form. So when rock starts to melt, it actually doesn't completely melt. It undergoes a process that geologists refer to as partial melting, only part of it melts. Further, the liquid or the melt or the magma that is formed by partial melting has a different composition than the original rock. When we take a rock and start to melt it, the liquid that forms, or the magma, does not have the same composition as the original rock. Now for the sake of our discussion right now, we're going to simplify things and just say that by and large, much of the solid rock is composed of silicon dioxide, iron oxide, and magnesium oxide. Those are three kinds of molecules that occur in rock. We refer to the silicon dioxide as silica. So it turns out that when we start melting something, the magma that forms will be richer in silica, relative to the magnesium in iron oxides than the original rock. So if we start out with a rock that has lots of magnesium and iron oxide and a little bit of silica when the melt forms, the melt will be richer in silica. So we've talked about rock, we've talked about molten rock. These are somewhat easy terms to discuss because everybody's familiar with rock. Most people are familiar with the concept of something melting. Now let's embellish this discussion by introducing some adjectives to refer to different kinds of molten rock, based on their chemical composition. Not all melts have the same chemical composition. You may have seen photographs of volcanoes and seen the molten lava, the red lava coming down the side, never given a thought to what's it's made out of. But in fact, it's made out of a bunch of chemicals. And in some cases there is more silica. In some cases there's more magnesium and iron oxide. In some cases there are other elements, as well. So for the purposes of our discussion, let's introduce four terms that geologists use to describe different compositions of molten rock. Molten rock that is particularly rich in iron and magnesium oxide is called ultramafic. It has a little bit silica, but relatively little. Then we have mafic rock. Mafic rock has a little bit less iron and magnesium oxide in proportion to silica. Then we go to intermediate rock. Once again, it has a little bit less iron and magnesium oxide relative to silica. Then we get to the far end, where we have felsic magmas, or sometimes called silicic magmas, which have relatively high proportion of silica relative to the magnesium and iron oxide. So we've got these four names, ultramafic, mafic, intermediate, and felsic. That term intermediate, it's kind of a funny term. It's just known as intermediate because it's part way between mafic and felsic. So we've just finished a discussion of what a volcano is, in a general sense. That led us to explore the concept of what molten rock is. And that led us to explore the concept of what states of matter are in the first place, solid, liquid, and gas. And then finally, we began to understand the mechanisms that lead to melting. Physically what causes melting to take place in the earth and generate magma? And we concluded by talking about the fact that there are different kinds of magma, based on the chemicals that are within the magma. [MUSIC]
