[SOUND] [MUSIC] Now let's look at the shapes of the volcanoes that form as the result of different kinds of eruptions. Geologists distinguish between different shapes. Because actually it gives us some information about the nature of the eruption and the nature of the material out of which the volcano forms. One kind of volcano is called a Shield Volcano. These tend to be shaped by broad domed and the name shield comes from the fact that they resemble say, Roman shields that are laying on the ground. How do they form? They're composed primarily of two materials. The basaltic lava, which as you recall from our discussion earlier, has low viscosity and can flow for long distances. And they're composed of cinders and lapilli formed from the spattering of basaltic lava fountains. Another kind of volcano is sometimes called a Stratovolcano. Or it's also called a Composite Volcano. These are more typical of andesitic volcanic realms, intermediate lavas. Where you have alternations between lava flows and explosive eruptions. So you have large quantities of ash and other pyroclastic debris being erupted. And occasionally you'll have layers of lava being erupted. Now the explosive eruptions will build up layers of pyroclastic debris. Which might wash away in heavy rainfalls or during floods, or during landslides. But when they get armored by a layer of lava they get fixed in place so that the volcano as a whole can build up to great heights. Some stratovolcanoes, and the word strato is to emphasize the elevation, can build up to a few kilometers high. Mount Fuji in Japan is a classic example of a stratovolcano. These are the volcanos that you tend to think of when you think of a volcano with a upward steepening cone. If we were to slice through a stratovolcano, we can see why we also refer to them as composite volcanoes. What I've tried to show here are the alternating layers of pyroclastic debris and lava flows, and I've also emphasized that overtime, these are not stable. Typically, chunks of the debris, including some lava flows and pyroclastic debris, will slump down from sort of of a landslide that moves material down slope, and then it will be buried by subsequent material. And then sometimes, as we'll see, these volcanos are subject to explosion. And a large chunk of them will be blown away. As an example, let's look at Mount Saint Helens. This is a composite volcano that formed in the cascade chain of northwestern United States. This volcano exploded catastrophically, in 1980, blowing out one side of the volcano. If we see the volcano from one side, it looks like a nice symmetrical cone. But if we circle around and look at the volcano from the other side, we see a huge gouge that's been blasted away, leaving behind a gaping opening in the side of the volcano. But continued eruption afterwards will build up the volcano again. So, stratovolcanoes tend to have a long and complicated history. One final point when looking at this cross-section is that I want to introduce two additional terms that we'll need. Notice that, at the top of the volcano, where there's a bowl like depression surrounded by rock. That's called the Crater of the Volcano. The crater of the volcano is around the vent or the main vent of the volcano. Sometimes there's some side vents as well, but a lot of times, there's a symmetry with the crater at the top. The other thing is notice that at depth beneath the volcano there is an area where magma accumulates, and that's called the Magma Chamber. Usually, there is a main conduit or chimney that comes out of the magma chamber. That carries the magma up to the surface, up to the base of the crater. But there are also typically injections of magma into the surrounding already existing volcano, and these will appear later on as dikes or sills, they're cutting in across the layers of lava and ash. The final kind that I mentioned, typically these are fairly small are cinder cones, and these are composed almost entirely out of lapilli, and the reason they're called Cinder Cones is because another word for lapilli is cinder. And they build up just by the spattering of lava at a vent for a period of time. The shape of them is controlled by the angle of repose of the cinders. By angle of repose, I mean the slope angle that the debris will naturally assume before it's acted on by slumping or river erosion or things like that. You can think of angle of repose as the angle of a pile of sand that builds up on a table if you spill sand onto a surface. Same thing happens with lapilli when they're erupted out of a volcano. Notice that in this particular example, a famous cinder cone in Arizona, you can see not only the cinder cone but also a lava flow that came out at the base of the cinder cone and spread out over the countryside. [MUSIC]
