[SOUND] [MUSIC] Classically, volcanologists, or geologists who specialize in the study of volcano, have distinguished among many different kinds of eruptions based on case studies, based on examples of those different kinds of eruptions. So as you can see here from this diagram, there are lots of different names. Names aren't really important per se, but I'll use them to illustrate different kinds of eruptions. So let's start by looking at a surtseyan type eruption. This is characteristic of when a volcano is erupting under water. And so there's a lot of water around. The water mixes with the magma and the lava coming out. It super heats and becomes steam. And that provides a lot of energy so the eruption basically explodes out of the water. And you get these very chaotic, explosive burps of debris and steam coming out of the surface of the sea. Another type is a Hawaiian eruption. We've already talked about those. Those are the ones that are dominated by fountains of lava that then feed lava flows that flow down the sides of the volcano. Sometimes these lava flows can go quite fast near the source. They may reach 30 kilometers an hour. Of course, further away, they may only move only a few kilometers an hour. Icelandic eruptions are also lava dominated, or effusive eruptions, but they occur when a crack develops. So you get a curtain of lava erupting along this crack, rather than having a single fountain coming out of a single source. Strombolian eruptions, these are ones in which the character of the eruption is such that it sends out lots of volcanic bombs and glowing cinders into the sky. So if you take a nighttime photograph of these, you'll see these arching like curves of fire coming out of the top of the volcano and then plopping down on the side and glowing and tumbling down the side of the volcano. Now, we'll get to the really, really explosive, eruptions. These are the pelean ones. Mt. Pinatubo in the Philippines was an example. And, you can see again, that there's a huge amount of pyroclastic debris, blasted out of the volcano, in a very short period of time, with huge amounts of energy. These are very dangerous eruptions. These are the kind that are most typically associated with large scale destruction and loss of life. Now let's look at a simplified cross section of one of these pelean eruptions more closely. It allows us to talk a little bit about the nature of this plume of ash that's coming out of the volcano. What happens is that when the eruption takes place, the pressure in the lava drives the lava itself up to some elevation, maybe at most 50, 100, couple hundred meters. But then gravity stops that flow and the material starts tumbling down. In addition, all the very fine fragmental debris, fine pyroclastic debris or ash dominated material starts blasting up into the sky. Now some of that debris falls back down onto the flanks of the volcano and avalanches down the sides of the volcano. These avalanches are usually called ash flows. So we have ash flows going down the sides of the volcano in the wake of one of these explosions. The rest of the debris, the rest of the ash, gets shot up into the air, but it's the force of the eruption that sends it so high, rather it's the fact that it's hot. It warms the air around it. So you have this turbulent mixture of hot air and hot ash. And it turns out that, that mixture is buoyant relative to the surrounding cold air. So just like the plume of smoke coming out of a chimney, it rises convectively up to higher levels in the atmosphere. So it may have enough buoyancy that it carries all the way up to the stratosphere, several kilometers up the surface, where it gets picked up by the jet stream and then carried around the planet. So, in sum, we see that there are really three components to what's coming out of an explosive or pelean type eruption. There's the explosively rising column. There's the ash flows going down the side, come from the collapsing part of the column. And then there's the convecting part of the column that goes up to very high elevations. [MUSIC]