[MUSIC] One case study of this was the famous destruction of Pompeii in 79 AD. Pompeii was a prosperous port town not far from Mount Vesuvius, not far from the present site of the city of Naples. In 79 AD, it began to erupt, but the inhabitants didn't really know what the eruption meant. And so, they did not evacuate. They stayed in the town. Well, one night, the volcano exploded. It sent a couple of ash flows down over the town followed by a huge amount of air fall ash, the stuff that comes out of that convecting cloud. In the end, the entire town was buried including most of its inhabitants who had not had time to flee. As a consequence, the town of Pompeii and the town of Herculaneum, a nearby city were both buried completely by ash and hidden from view for centuries. It wasn't until archaeologists excavated them beginning in the 19th century that people realized that the ash had preserved an incredible record of Roman life as it happened back in 79 AD. Now as a result of the burial by the ash, the whole town was pretty much buried intact to only be uncovered in the 19th century and into the 20th century by archaeologists. If you go to Pompeii, it's a beautiful tourist destination. You can still see the remains of pretty much an intact Roman city, with columns, with plazas, with rooms, beautiful mosaics preserved inside the walls. You can even see the streets with the ruts caused by the wagon wheels still carved into them, but one of the spookier things about visiting Pompeii is the fact that you can still see the bodies or the casts of the bodies of the people who died there. This was sort of a fortuitous discovery when the archaeologists began working the area, as they were digging they found that there were open spaces inside the semi solid ash. They didn't know what these were at first, but they filled them with plaster and then dug away the ash around them and what they were left with were the shapes of bodies. People, dogs, cats, children, adults sometimes in terrifying poses. Here's some examples, as they're being excavated and now on display. The actual people are not inside these casts. Their skeletons long since have disintegrated and turned to dust, but these are just the shapes that the people had when they were buried during the final moments of their life. Just so we get a sense of the context of Mount Vesuvius, let's zoom in on Italy sticking out into the Mediterranean Sea and we'll see that Vesuvius can be visible from quite a distance out in space. We're zooming on it right now and you can see, I'm going to stop and you can see, there's Vesuvius sitting right at the edge of the bay of Naples and you can see the whole City of Naples sprawled around it. Pompeii is about right where my hand is at the moment off to the south of the volcano. We'll zoom a little bit closer and I'm going to have to change the view here a little bit. I will zoom in and then get a sense of what the volcano looks like from the side and one of the thing's you'll notice as we go in on the volcano is that the present day cone, this portion is actually only about a century old. The volcano was once much bigger, you can see the edge of the old volcano along the side. Here is the modern day cone building. Once the volcano was way up in here and all this material was blasted out, rushing down the slope and eventually hitting Pompeii. Interesting thing about Pompeii is, it's now within the broad expanse of metropolitan Naples. So they're now, many, many people living around the base of Mount Vesuvius, which you can see very clearly. Just to finish off, let's look inside the present day cone that's building in the interior of the remnants of the old Mount Vesuvius. And what we're seeing here is layer upon layer, upon layer of ash and lava that have accumulated in a little over a century. You'll notice the small specs at the top those are people. You get a sense of how rapid this volcano has been building up, emphasizing that it's still active and still poses a threat to the surrounding City of Naples. Another case study of an explosive eruption was the destruction of the City of St. Pierre on the island of Martinique in the Caribbean in 1902. St. Pierre was again, a prosperous port town with about 40,000 inhabitants at the flank of Mount Pelee, an eruption started in Mount Pelee. And curiously, a spine of solid rhyolite built up in its throat and started projecting skyward like the cork in a bottle of champagne. The local government officials trying to avoid panic told people, don't worry. No need to evacuate. People stayed there and one day, that cork popped and a huge glowing avalanche or ash flow rushed down the side of the volcano. Some people estimated it reached speeds of 300 kilometers an hour and in a matter of moments, completely enveloped the town of St. Pierre and incinerated or suffocated everyone in the town in a matter of moments. There were only two survivors, one of whom was a prisoner that was in the dungeon of a jail and the other was a sailor in the harbor who was below decks at the time of the disaster. Another example was the eruption of Mount St. Helen's in 1980. So at the start, Mount St. Helen's was a beautiful cone-shaped volcano just like Mount Fuji. Inside was a chimney like conduit that was accessing a magma chamber at depth and size of the volcano were the remnants of previous eruptions. So, magma began to rise and start filling this chamber. With time, the magma chamber grew and began to build out sideways. And a result, the surface of the volcano began to bulge to the side. Meanwhile, there's a little bit of steam gas and some ash erupting at the top. So now, these are bulging out like that. That's because this magma was coming in. Some of it was pushing that way, some of it was pushing that way. Now, this bulge here was unstable. So one day, all of a sudden, a giant landslide formed and in the next instant. That landslide carried. And broke up, but it carried away the side of the volcano. That decreased the pressure right there. So suddenly, as soon as that pressure was released that blasted out sideways and it released enough pressure, so it also blast out the top. So, there was a huge eruption of ash out the top and a huge eruption of ash out the side. The force of this lateral blast. This was blasting out this way and this was blasting up that way. The forces of this lateral blast flatten the forest for kilometers around the volcano. It knocked over the entire forests that look like a bunch of toothpicks that were lying on the side and it sent out huge amounts of ash that buried this area, and huge lahars then flowed down valleys to some distance away from the volcano. You can see looking at the map of the volcano that the explosion was to the north. And so, the forest was flattened on the north side of the volcano. If you go to Mount St. Helen's 25 years later, you can still see the remnants of the flattened forest knocked down like toothpicks due to the force of the blast. And in Google Earth, you can see very clearly, even now how the volcano no longer has its beautiful fugi-like cone-shaped symmetry, but its north side was gouged away and spread out over the countryside and only now is some of the flattened area beginning to regrow. [MUSIC]
