[SOUND] [MUSIC] What are these conditions that lead to the formation of magma? Why is it we sometimes get melt and sometimes don't? Why is it in the middle of North America or in the middle of South America there are no volcanoes? Whereas there are volcanoes along the west coast of South America or along the cascade chain in North America, or in Japan. What's special about those locations that allow rocks underneath to melt to produce magma, some of which comes up and erupts at volcanoes. Well there are actually a few different reasons and we'll just look at these briefly. The first cause of melting is a phenomena called decompression melting. To understand decompression melting, we have to take a concept from chemistry, which is that whether a material is solid, liquid or gas depends on the pressure and temperature conditions that it's in. So when we take say, liquid water, put it in a freezer and it turns to solid, what we've done is we've changed the temperature without changing the pressure. And under the conditions of pressure and temperature in the freezer it becomes solid. If we take ice out of a freezer and put it on a table, we're changing the temperature but keeping the pressure the same. It undergoes a change of state and becomes liquid. So the more familiar way of causing melting to take place is by changing temperature. But in fact in the earth, quite a bit of melting takes place not by changing the temperature but by changing the pressure. So for example, if we have a block of asthenosphere that's very deep down, it's not only very hot, but it's also under a great amount of pressure. If, for some reason, that hot asthenosphere rises to a shallower level on the crust, even at geologic rates on the order of a few centimeters a year. Rock is a good insulator, so it's not going to lose it's heat very fast. So, it can make that rise and still be very hot. Eventually, it may get to a shallower depth in the Earth where the pressure is less. In fact, the pressure is low enough that that block starts to melt. In other words We can have what's called decompression meting, which melting that takes place when we don't change the temperature but decrease the pressure. So that's one way we have can melting take place. We can represent that phenomenon by a simple graph that shows, with the red line, the conditions of pressure and temperature in the Earth. And with the black line, the conditions at which rock starts to melt. The arrow going from point A to point B shows the path of pressure temperature conditions that the rock takes if it goes from deeper depth up to shallower depth. And what happens is if that rise happens fast enough, it retains its temperature. So it goes off of the red line and eventually touches the black line. And as soon as it touches the black line, it starts to melt. The red line is called the geotherm. The geotherm represents the change in temperature that occurs in the Earth with increasing depth. The black line is called the liquidus, and it represents the conditions of pressure and temperature at which melting begins. So we've seen one reason for melting, and that's decompression. Another way in which melting gets triggered is when volatiles, gassy materials enter solid hot rock. Now, this is a little bit less familiar in everyday experience but again, if you live in a cold climate and have ever gone out and thrown salt on a sidewalk to try to get the ice to melt on the sidewalk, you maybe familiar with the fact that adding certain elements to other materials will change their melting temperature. So when you add salt to ice, it changes its melting temperature so it can begin to melt even if the air temperature around it doesn't change. So in the case of rock If we add water and or carbon dioxide to hot solid rock, those atoms of water, or those molecules of water and carbon dioxide will sort of assist in breaking bonds. We can look at it simplistically. They will start to break or weaken the bonds in the solid and cause the solid to change from solid state into liquid state. We call this phenomenon flux melting. Now another reason for melting is by changing the temperature. And we already talked about this because that's the way in which we cause melting in our everyday experience. We warm up a piece of chocolate, it melts. We warm up a block of ice, it melts. Well, in the Earth warming up rock to cause it to melt happens only in certain cases. For example, let's say that we form a magma down in the mantle. For reasons that are a little bit beyond the scope of our discussion here, I can say that that magma is particularly hot. It's on the order of about 1,200 degrees centigrade. If that magma then rises and enters the crust, the crust melts at temperatures lower than the temperature of that magma. So by adding hot magma into the crust, we will cause the crust to start to melt. And that's called heat transfer melting. We're transferring heat from a hot molten magma at depth Into the crust causing the crust, which melts at a lower temperature, to start to melt. What kind of analogy, well, here's a weird one. Let's imagine we've got a ball of ice cream and we take hot fudge and we pour the hot fudge onto the ice cream. The heat from the hot fudge will cause the underlying ice cream to start to melt, that's heat transfer. Same thing can sometimes happen in the Earth. So in sum, we see that there are three basic phenomena in the earth that will trigger melting. One is decompression. One is the addition of gassy like materials like water and carbon dioxide, which are formally referred to as volatiles. And the third is by heat-transfer melting. When we take a hot magma, place it inside a melt, a rock, that melts at a lower temperature. The heat transfers from the magma to the rock and causes that rock to start to melt. [MUSIC]
