[SOUND] [MUSIC] The other aspect, and I've mentioned that the composition of the lava, specifically the proportion of silica in the lava, is a fundamental control on it viscosity and therefore a fundamental control on the nature of the eruption. The other aspect that determines how an eruption behaves is the amount of gas, or volatile elements that are within the lava. Now deep in the earth under the high pressures that occur deep in the earth, volatile elements, specifically water, and carbon dioxide, and some other gases, are dissolved in the lava. It's sort of like if you have a bottle of soda you don't see any gas bubbles in here, because their dissolved within a soda. But if I shake this up a little bit and release the pressure, suddenly you see bubbles rising from depth. So, my point is that at conditions deeper in the Earth, when the lava is still really magma, it contains dissolved volatiles. When that magma rises close to the surface into the throat of a volcano and the pressure is less, those volatiles come out of solution and form bubbles. That is the gas part of a volcanic eruption. Now, sometimes there's more gas, sometimes there's less gas. When you have a lot of gas that changes the behavior of the lava quite a bit. It can make it seem less viscous. In any case, when you have a lot of gas present, under certain conditions, pressure can build up in that gas, because it may be trapped inside the throat of the volcano by some solid lava above it. And that pressure can build up until it becomes greater than the strength of the overlying cap and can trigger a sudden catastrophic explosion. One other point about the presence of gas is that as gas comes out of solution it makes the lava overall more buoyant and it makes it rise higher and faster in the crust, and that helps to bring it out at the surface. Now, I don't mean to imply that gas only occurs in felsic lavas. Sometimes gas does occur, in fact frequently, gas occurs in mafic lavas as well and basaltic lavas. What happens, though, is that because the lava itself has such low viscosity the gas bubbles, like in a bottle of soda, are able to rise to the surface faster than the lava is rising. When they get to the surface, they burst. And just like when you pour soda into a glass, the bubbles burst and get your mouth wet before you drink the soda. Similarly, the bubbles in a lava burst, and they eject spatters of lava up into the sky. And they can produce enough energy to actually cause the lava to fountain out at the surface. So for example, what we're seeing here is a lava fountain. When you see a lava fountain like this, keep in mind that the magma that is sourcing this lava is rising from below. As it rises the bubbles are also rising with it, and sometimes even faster than the lava. As they expand, they're increasing the pressure within the lava. So when the lava reaches the surface it's under so much pressure that it squirts out and ejects into the sky, sometimes 10 to over 100 meters, in some cases. Also, as the bubbles burst, they send spatters of lava out to the side that, as we will see, cool into little pellets of solid rock. So in effect, this is a vigorous eruption, but it's not quite the same as an explosive eruption that's shooting huge quantities of ash into the sky. Now, let's go back and talk about what happens in the case of a rhyolitic lava. Or rhyolitic magma. Remember the rhyolitic magma is rising from depth it contains a lot of dissolved gas. That dissolved gas is coming out of solution, but because the rhyolitic lava is so viscus those gas bubbles can't escape, but they're trapped in the lava itself. So you can imagine, if we look at this little cross sectional sketch of a volcano, that somewhere at depth, here's the rising magma, and bubbles are beginning to form within that magma. But they can't escape, they can't pop out like they do from a basaltic lava, or perhaps more in your everyday experience, like the do out of a can of carbonated water. Now, look at one of these bubbles. At depth, remember there's magma rising over the top of this so that this is now at depth beneath the volcano. That bubble is constrained. As it rises, still further, that bubble's going to try to expand, so in other words, this is moving upwards to here. That bubble's going to try to expand, but it can't because it's surrounded by solid rock. So the pressure in that bubble becomes extreme. And now we've got lots of bubbles around it. You can imagine that all of these bubbles are under extreme pressure. Now what happens during an explosive eruption is that, finally, the pressure in these bubbles gets to be great enough that it fractures the little grid work of solid rock around it. And when that happens the whole system collapses. And suddenly it's like a shotgun explosion. Suddenly, all that gas plus the pulverized rock ejects out of the volcano. And you end up with an absolutely immense explosion. So, it sort of happens in stages. Bubbles that form at depth, bubbles rise to shallower level and become under greater pressure. Finally, the material containing the bubble's fragments, and with all that pressure, like a shotgun blast, it sends all the material out the throat of the volcano, into a giant eruption of ash. We'll get back to the details of the nature of this ash cloud a little bit later. So in sum, we see that the behavior of lava depends, first of all, on the composition of the lava and second of all on the gas content of a lava. [MUSIC]
