[SOUND] [MUSIC] I'd like to finish our discussion of volcanic eruptions by focusing on a type of eruption that people have never actually seen, but have seen the consequences of. These eruptions are the eruptions of what are commonly called these days, supervolcanoes. A super volcanic eruption is simply an eruption that is so huge that it dwarfs anything that we've ever seen or anything that's recorded in human history. Now there's several different types of super eruptions. One of them are called flood basalt eruptions. During a flood basalt eruption, it's an effusive eruption, it's a lava dominated eruption, erupting basaltic lava, but the volume of lava that's erupted is absolutely immense and typically the lava is a little bit hotter, so it's even less viscous and can flow even further. In fact in some of these flood basalt eruptions, it's as if the lava's flooding in some areas sometimes tens to hundreds of kilometers across. If we look at this map, we can see that there are several places on the surface of the earth today that record the remains of these volcanic eruptions. For example, the Columbia River Plateau in Northwestern United States. The Parana Basin, in Brazil. The Kuru, in Southern Africa. The Deken, in India, and a region called the Siberian Traps, in, Siberia. Traps is a word that's sometimes used for sills of volcanic eruptions in the older literature. In all of these places, there are layer upon layer, upon layer of basalt. And often, this construction of basalt is so thick that it causes the area to be a plateau. So sometimes these were in the old literature called plateau basalts. Here's some example of the region in the Columbia River that has long since been inactive. So there's no volcanic eruption there today, but the lava flows are left over. And these are now being cut down into by rivers, so we can see the cross-sections of the lava flows along the sides of valleys and that's what we're looking at here. Here are hills that are carved out of layer upon layer of flood basalts from the Deken traps of India. Another kind of supervolcanic eruption produces gigantic calderas. We haven't talked about calderas yet, but simply speaking you can imagine a large strata volcano that's erupting potentially andesite and rhyolite and so forth. And a large magma chamber forms underneath that volcano. The way I'm depicting magma chambers is just schematically. It's not really clear what their geometry is, they may just be interconnected networks of cracks, they may be crystal mashes, it's not clear but that's a little beyond our scope. And here's the relevant, the relics of the older volcano. Now let's imagine that there's an explosive eruption, and all of this magma blasts out, as well as chunks of the volcano itself and what's left collapses back down into the now drained magma chamber. The result will be A low depression that is collapsed inward into what had been the relict of the magma chamber. And there will be broken blocks and debris and so forth down in this area. In other words, this, when it drains, empties, and the rest of the volcano falls down into it. Sometimes, this'll get covered by a layer of ash, but nevertheless, we're left with this big depression. Later on, who knows, you may have a lake fill that depression. But in any case, you end up with a broad, circular depression that can be hundreds of meters across to kilometers across, in some cases tens of kilometers across. A famous example of one these calderas is Crater Lake in Oregon, where the caldera has filled with water. So, you see this somewhat circular depression filled with a deep pressure. Now a much bigger example, formed in what is now Yellowstone Park, about 600,000 years ago. Again, before human history record. This caldera is so big that you might not even realize when you're inside of it, unless you know what you're looking for. In fact, you can see that almost a third of Yellowstone Park is within the area of this caldera. Old Faithful, other geysers and hot springs are within that area. When this erupted, it ejected on the order of 1,000 cubic kilometers of debris into the sky. But by comparison, the Mount St. Helens eruption the we've talked about, that was one cubic kilometer. The Mount Pinatubo eruption of 1991, which is, probably the largest one that's been recorded visually, was about ten cubic kilometers. Tambora, which erupted in 1815 as an explosive eruption, that was about, 100 cubic kilometers. So this eruption of Yellowstone was about ten times that, 1,000 cubic kilometers of debris was sent up into the atmosphere. Now that was so much that a significant layer of ash covered most of the area that's now the United States. By comparison, you can see that the Mount St. Helens ash covered a relatively small area. Now, imagine what would happen if an eruption that size were to happen today. We don't know, because we've never seen anything like it in human history. So, we'll close this discussion by reminding you of one final point. Don't forget the importance of plate tectonics in understanding the distribution and also the character of volcanic eruptions. With plate tectonics in mind, we can remember that different kinds of lavas form in different environments. The basaltic lavas tend to form along mid-ocean ridges and tend to form during certain stages of rifting and during certain stages of volcanic arcs. Andesitic and Rhyolitic lavas tend to form dominantly in conversion plate boundary volcanoes on continents. We don't have time to go into lots of detail about this but if you think through the implications of plate tectonics theory and think through the different settings and ways in which lavas or magmas form at different plate settings, you can begin to understand why different kinds of eruptions have the character they do in the places that they do. [MUSIC]
