[MUSIC] Now lets digress a little bit and focus on the specific nature of the materials that are erupted during a volcanic eruption. Let's start with the products of the efffusive basaltic eruption. In other words, let's start by look at lava flows that come out of a basalt volcano. A good example of these is, as I mention, on the island of Hawaii. If you go there, you will see that there are two very distinct types of lava flows. For example, here we see what's called a Pahoehoe lava flow, which is a lava flow that has a very smooth surface. And if we look more closely at another location on that same flow, we see that the lava's surface has wrinkled into these coils that really resemble ropes. It's obviously gotten stuck by some obstruction there, and so these ropes have curved into a very, very interesting abstract geometry. In contrast to that, if we go to another place on Hawaii, we'll see what's called an Aa flow. In which the surface is very, very rubbly and composed of angular, jagged, sharp fragments. These terms Pahoehoe, for the smooth ropey like lava, and Aa for the rough broken up lava, are from the Hawaiian language. In the old days when people didn't have roads to drive on the native Hawaiians would use the Pahoehoe surfaces as the ones that their foot paths would be on, because they were certainly a lot more comfortable on bare feet than the rough Aa surfaces. We talked earlier about lava freezing to form different kinds of igneous rocks base on the chemical composition. And we mentioned briefly that lava also cools to form volcanic glass. Let's add to that by pointing out that, simplistically, there are two different occurrences of volcanic glass. In some cases you get solid masses of coherent glass. That produces a rock called obsidian. Sometimes it's black, sometimes it's red, usually it's fairly dark. Obsidian, because it has a glass-like character, fractures like glass. And so if you take a chunk of it and hit it with a hammer, it will form scallop shaped curving fractures. And if you do it just right you can make very sharp edges that way, which is why obsidian was a popular material among indigenous people for making arrowheads, and axes, and other kinds of tools before the metal ages. In some cases, the volcanic glass freezes with the bubbles still in it. Remember, we talked about how as felsic lava reaches the surface, it sometimes becomes a froth of solid frozen volcanic glass and bubbles. When that happens you get a material called pumice. Pumice sometimes contains so many gas bubbles that it actually can float on the surface of water. Now, let's look at the nature of volcanic ash. What is ash? Well, much of the ash that comes out of a volcano is composed of tiny glass shards, tiny little glass specks. And these form when the lava explodes and the recently formed solid rock, that was in that bubbly and broken up lava mixture in the throat of the volcano, gets shot into the air and, or, when some of the liquid lava gets sprayed into the air as a consequence of the explosion. Sometimes, the explosion is strong enough that it blasts apart the preexisting rock that composed the volcano. So in addition to recently flash frozen droplets of lava, it also contains fragments of pre-existing volcanic rock that made up the volcano. This combination of tiny fragmental material shoots into the air. We can some examples of this from this explosive eruption of a volcano in Japan. You can see it sort of making the air seem smokey during the eruption of Mount Saint Helens. In fact, if you hold a hand full of it you'll see that it doesn't look that different from the ash that comes from burning wood, but it's a lot different. Burning wood is composed of soot made out of carbon, whereas volcanic ash are tiny little fragments of silicic glass. When the ash goes into the air some of it falls down like snow and blankets the landscape, and some of it avalanches down the side of the volcano. But in the end, it will accumulate in layers, and when it solidifies it forms a kind of volcanic rock called tuff. We didn't talk about that yet when we’re talking about the kinds of igneous rock earlier because we're focus on compositional classes. Tuff is a term that's used generally for felsic volcanic ash that accumulates in layers and then becomes solid. Here's some examples of tuff that are exposed near Los Alamos, New Mexico. It's colored by iron staining, due to some of the iron component within the rock, but nevertheless, it's overall, almost like sedimentary layers, just layers of snow, that accumulated volcanic ash. And so here's a close up of the ash from near Los Alamos. And you can see it's composed of very tiny fragments of ash as well as some larger chunks of pumice. [MUSIC]
