Violent Volcanoes

How volcanoes blow their tops, and the after-shocks of an eruption...
05 May 2015
Presented by Chris Smith, Ginny Smith


A volcano in New Zealand


Celebrating two hundred years since the devastating eruption of Indonesia's Mount Tambora, this week, accompanied by music from Michael Levy, we explore the science of volcanoes. We find out what causes volcanoes, we ask whether eruptions can be predicted, how we can keep people safe, and we re-create the physics of an eruption in the laboratory.

In this episode

Mt Fuji Stratovolcano

00:37 - Can volcanic eruptions be predicted?

What are volcanoes more common in certain places, what triggers and eruption, and can volcanic activity be predicted...?

Can volcanic eruptions be predicted?
with Professor Marian Holness, Department of Earth Sciences, Cambridge University.

Before a volcano erupts there are often warning signs such as earthquakes. - until recently, however, people Mt Fujididn't know what these signs meant. But what is going on inside the earth that leads to these dramatic events? Cambridge Earth scientist Marian Holness explains to Chris Smith what causes volcanoes..

 Marian - Volcanoes are places on the Earth's crust where molten rock can escape. So, you often find them actually sitting above major fractures in the crust, major faults that go down for tens of kilometres because that provides a very easy pathway for this molten rock to come up.

Chris - What about the geography though because there are some areas that have lots and lots of volcanoes and others that don't. What's special about those areas?

Marian - If we start from the centre of the Earth, we've got a solid core that's made of iron and then the outer part of the core is also made of iron and that's liquid and that's moving around by convection. Now, if we go further out into the mantle, we're back into solid rock again and that's solid all the way up to the surface in the crust. So, if we want to make a volcano, we have got to melt that solid mantle. So, there are three different places where we can manage to do that. Now, if you look in the centres of the oceans like the Atlantic Ocean, we've got two oceanic plates that are being pulled apart. Because you can't make a hole in the Earth, if you're pulling those oceanic plates apart, you're forcing the solid mantle underlying those two plates upwards. It comes up really quickly and as it moves up fast, it doesn't lose any heat so it starts to melt. Then that melt moves straight up to the surface and you get a continuous line of volcanoes all the way down the mid-ocean ridges. So, that's one way of making volcanoes.

Chris - Apparently, those plates moving away from the mid-ocean ridge are going at roughly the same rate your fingernails grow. Is that true?

Marian - That's exactly true - centimetres a year. The fastest plate movements we have are in the east pacific and they're going at 22 centimetres a year.

Chris - Where else apart from mid-ocean ridges then do we get volcanic activity?

Marian - Well, our own favourite volcanic region is in Iceland. That's the nearest to us and that is there because this solid mantle is solid but it's convecting. It's moving. So, there's heat being taken from the centre of the Earth and it's moving up like boiling water. And if you bring a plume of this hot solid stuff up, it's moving fast enough that it will melt. So, you get a hotspot volcano. That's what we've got under Iceland. So, you'll find a lot of sort of holiday destinations in the oceans are actually sitting above hotspots. So, Hawaii is, Saint Helena where they put Napoleon. These are all volcanic islands sitting on top of one of these hotspots.

Chris - What about Italy where Vesuvius is?

Marian - That's the final way of melting the mantle where we have an oceanic plate moving towards a continental plate. The oceanic plate is quite heavy and dense and it will subduct underneath the continent. So, in the particular example we have in Italy, we've got movement of Africa towards Europe. The African plate has got a bit of oceanic plate stuck to the end which is where the Mediterranean is and that is being subducted underneath Europe. Now, that oceanic plate has been in contact with seawater. So, the rocks that you're putting down into the Earth are wet. You're putting those rocks down into the Earth very fast and the water in those rocks gets released. If you add water to solid mantle, it will melt. So, all you're doing is you're pushing down the Mediterranean underneath Italy, the water is coming off, it's going into the mantle and it's melting. So, Italy is blessed with a great family of volcanoes. You've got Etna, you've got Stromboli, you've got Vesuvius, and you've got other small ones as well. They're all there because of the continental collision between Africa and Europe.

Chris - Where does all the heat come from that's making all these possible?

Marian - There are several different sources of heat. When the Earth was originally formed from lots of different particles in a dust cloud, it all came together and that released potential energy and gravitational energy. So, you started out with a great hot ball. The other source of heat which is still going on is radioactive decay. So, we've got a whole set of elements within the Earth that are breaking down radioactively and every time they do that, they generate heat. So, that's why the Earth is hot. It's really, really hot in the middle and it's losing this heat by convection. The sort of surface manifestation if you like of that convection are the volcanoes.

Chris - Going back to what Pliny experienced, what was the cause of these earthquakes that were happening before the eruption really got going properly?

Marian - That was because some of the molten rock was moving up, ready to erupt. So, the molten rock is moving up because it's got a low density. It's quite buoyant, it wants to go up. There are no holes in the Earth. So, the way it makes space for itself is by pushing everything else out of the way. As it pushes it out of the way, it generates earthquakes. It's breaking the rock to get out. So, that is what he was experiencing.

Chris - If you see all of these magma coming up towards the surface of the Earth, do you see almost an inflation going on? Does the Earth sort of swell or does an area where there's going to be volcano, does it get bigger because of that rock moving into it?

Marian - Yes, it does. So, you can watch volcanoes breathing if you like. So, when the magma moves up just before an eruption, the volcano swells. You get an eruption, it collapses again.

Chris - How much does it swell up by?

Marian - Well, there was one volcano in America, Mount St. Helens that erupted in May 1980 and it swelled enormously. You didn't even need special instruments to see it. You could see the side of the mountain bulging up immediately prior to the eruption.

Chris - Good grief! So, that's the danger sign.

Marian - Yes. You get out of the way as fast as you can at that point. You've got a timescale of weeks, days. You've got time to leave.

Chris - Professor Marian Holness, thank you very much. It's time to get experimental now and every month, Ginny cooks up something from the kitchen experimentally for us. You've got a toaster on the floor.

Ginny - We've been talking a lot about heat and convection and hot things trying to get to the surface of the Earth. So, I thought I'd show you a little demonstration where you can see why hot things like to rise and why this can be useful. So, I have with me here a toaster, just a general normal toaster you'd find in any kitchen. I've also got a piece of cardboard which I have rolled into a tube.

Chris - So, we've got a toaster on the floor. It's got a large cardboard tube which is roughly a foot and a half high, acting almost like a chimney off the toaster.

Ginny - Yes, so if you put your hand over the top, not too close but right at the top of the tube, you should be able to feel some heat coming off it.

Chris - Yeah. It was a lot of hot air rising out of there, more than out of the House of Commons at the moment actually.

Ginny - So, what we're going to do is we're going to try and trap that hot air and see what happens to it. So, I've here a bin bag and I've just put some little bits of Gaffer tape at the bottom in four places so roughly symmetrical and that's just going to kind of stabilise it. What I'm going to do is put the bin bag over the top of the tube and you might be able to see something starting to happen to the bin bag. Can anyone see anything happening?

Male - It's filling up with hot air?

Ginny - It is exactly. So, the bin bag is expanding and that's just like how our volcano's expanding as the hot magma enters into it. Now, what do you think might happen to this bin bag if I let go of it?

Male - It would rise like a hot air balloon.

Ginny - Shall we see if it works?

Audience - 5, 4, 3, 2 ,1...

Chris - She does deserve a clap for that. Okay Ginny, so why did the bin bag take off?

Ginny - The reason that it was expanding as it was filling with the hot air is because hot air is less dense than cold air. What do I mean by that? Air is made up of molecules and they're whizzing around and it's a gas, so they can all whizz around quite a lot. But when you heat them up, what you're doing is you're giving those molecules more energy. So, they whizz around even faster. As they do that, they're actually bumping into the bag. That's why you see the bag swelling up because those molecules are rushing around and they're bumping into it. Because they're rushing around so much, they also end up further apart from each other than they are in the cold air. So, you've got the same amount of space but fewer molecules because they're further apart. And because there's fewer molecules in the same amount of space, it's going to be lighter. We call that less dense and light things like to float, just like a cork floats in water, but a rock doesn't because the rock is more dense, it's heavier for its size, so it sinks. Hot air is less dense, lighter for its size so it rises. And the same applies to magma. When it's hot, it rises because it's less dense. It wants to float effectively on the denser, cooler magma. And you also get something called convection. So, as you saw with the hot air balloon, it didn't just keep going up and up forever. It went up and then it came down again. And that's because as it rises, the air inside it starts to cool again and then because it's cooling, it's becoming less dense, it's becoming heavier that it can't support the weight of the bag anymore so it has to sink again. The same thing happens inside the Earth in the magma. It gets hot, it rises, it starts to cool and then it can start to sink again. That means it's doing this big sort of mixing. That's what we call convection.

Chris - But it's not powered by a toaster.

Ginny - No. there's a bit more heat going on inside the Earth than there is in my toaster...

12:56 - What happens when a volcano erupts?

Eruptions are dramatic events that tear apart mountains. But what is actually going on inside an erupting volcano?

What happens when a volcano erupts?
with Professor Clive Oppenheimer, Department of Geography, Cambridge University

Volcanic eruptions are hugely dramatic events that can literally tear apart Volcanomountains. To illustrate their power, Ginny Smith creates some mini-eruptions of her own, using Coke and Mentos. But first, Cambridge volcanologist Clive Oppenheimer explains to Chris Smith what is actually going on inside an erupting volcano...

Clive - Quite a lot is going on. As you might imagine, we've had the magma rising in the crust. We know that there are earthquakes as early as 62 AD which probably signalled the arrival of this magma and it took some further years to reach the surface. It can even take centuries or thousands of years to assemble a big magma chain but that's going to fuel a really large eruption. What we can imagine is that there's something a bit like a balloon of molten rock down there which is inflating as the magma is accumulating in it. It eventually reaches the point where the balloon pops and the magma starts ascending towards the surface. What unfolds is the sequence of events - these ash clouds rising very high into the atmosphere, he describes it looking like a pine tree with what we now recognise and call an umbrella cloud.

Chris - What's pushing that ash out of the volcano and up into the air? What's coming out to drive that process when this eruption like this begins?

Clive - One of the things that makes volcanology very complex and describing volcanic eruptions and magmatic processes challenging is that the molten rock is composed of the three phases of matter. There's the liquid part which is obvious that's the molten rock. There are solid crystals in it and there are bubbles of gas. That makes it a very, very complex fluid and it behaves in very complex ways as it reaches the surface. But it's the gas bubbles that really play the critical role in whether an eruption is very violent and explosive or whether it's a more peaceful effusion of lava running down the flanks of the volcano.

Chris - Can you explain a bit more about that and why those bubbles are there in the first place and why some volcanoes have them and others don't?

Clive - The Earth is composed of many, many chemical species. There's a lot of carbon, a lot of sulphur, a lot of hydrogen, and these are light elements that given the chance, want to be gases rather than being dissolved in molten rock. As these magmas are ascending through the crust up towards the surface, those chemical species are forming bubbles. As they rise even closer to the surface, the bubbles are expanding very, very dramatically and taking up a lot more space. So, the whole process can accelerate, leading to a very violent eruption. Ultimately, those bubbles can rupture the molten rock and shower the atmosphere with particles of ash and pumice in a really violent explosive eruption.

Chris - Which is what Pliny was seeing.

Clive - Very much so and the eruption goes on for a couple of days or so 

Chris - What I'm holding in my hand Clive actually is a thing which is the size of I think my probably my head and a bit more. If that were granite, I'll never be able to do that. I'm holding it in one hand. It's a huge block of stone, but it's extremely light. This is pumice, isn't it?

Clive - Yes, this is pumice. Maybe you have a pumice stone in your bath and scrape off all of your callouses with it on your feet, but it's really a foam. It's a volcanic rock made of the same kinds of materials that you might see in a more familiar, very dense lava, but it's full of tiny bubbles where the volcanic gases used to be. So, this was frothed up at the surface and then because it's so light, it can be shot very high up into the atmosphere. One of the other things about pumice is it will float on water until all the little air holes get saturated. It's possible to find the pumice from eruptions that has travelled thousands of kilometres across the ocean.

Chris - What about this piece then? This is something very, very different. This is ridiculously heavy and very, very small for its weight. What is that?

Clive - This piece certainly hasn't come from Vesuvius. This is a chunk of black rock that's twisted in to form a little bit like a piece of rope. This is a piece of a lava flow and it's a texture that we call 'pahoehoe' from the Hawaiian word. It's where you have relatively running lava that, as it's being erupted and flowing across the ground, forms these coils and rope-like forms like this.

Chris - But because it doesn't have all that gas in it like the sample that the pumice we were just seeing, it wouldn't be explosive like Vesuvius was. That's why Hawaii just sort of oozes and is very pretty to look at and much safer I suppose relatively speaking compared with these massive explosive eruptions like Vesuvius and more recently, Mount St. Helens.

Clive - That's right. I mean there are certainly still a number of hazards that volcanoes like Hawaii pose. One is that the lava is so runny that they can flow very quickly and reach quite long distances from the volcano. If they reached settled areas, that's a problem. But certainly, you don't get the same violence that you do in one of these eruptions that we now call a Plinian eruption as described by Pliny the Younger.

Chris - From Cambridge University, Clive Oppenheimer. Thank you very much. So Ginny, Clive has been just talking about different types of eruption and I understand that you're going to do a Kitchen Science eruption for us.

Ginny - Yes, so I'm going to do an experiment now that a few of you might have heard of. Who here has heard of the Coke and Mentos experiment?

Audience - Yeah!

Ginny - Quite a lot of you. Okay, so we're going to do this twice and hopefully, this might illustrate some of the differences we see between different volcanoes. So, the first bottle of Coke I've got is full sugar Coke and I opened it a few hours ago, and I've given it a little bit of a shake. So hopefully, that will have got rid of some of the gas in it. I've got a rolled up tube of paper and a piece of card, just like a postcard. I'm going to put the piece of card over the top of the bottle, the rolled up tube on top of that and then I'm going to take about 6 Mentos. I'm going to line the tube up. I've dropped the Mentos into the tube which is lined up with the top of the Coke bottle. So, all that's preventing them going in is my postcard. And then what I'm going to do is pull the postcard out and run away as quickly as I can.

Chris - Do you want to count it in?

Audience - 3, 2, 1...

Chris - Okay. We've now got quite a large mess. What did you see?

Male - I saw a load of cola that's exploding like a volcano.

Ginny - So, what was happening was that the liquid Coke was fizzing and bubbling. We've got lots of gas produced and that was then coming out of the top of the bottle because it was too big to fit in the bottle.

Chris - And that's what pushes it out because the Mentos sink to the bottom. They make all these gas get produced or come out of solution all around them and that takes up loads of space and there's now not enough space in the bottle for the gas plus the cola.

Ginny - Exactly. So, Coke already has gas dissolved in it. That's why it's fizzy. It's got carbon dioxide. But what gas needs in order to form bubbles is something called a nucleation site. Basically, it's very difficult to make a bubble unless there's something to make it on and that's something can be a bit of dirt, it can be a little scratch in the glass. It can be all sorts of things. so, if you had a perfect glass and you poured your champagne or your beer into it, you actually wouldn't get any bubbles which should be a bit rubbish. It's the imperfections that allow the bubbles to form. What's special about Mentos is they have that sort of crispy coating on the outside and that's made by spraying sugar onto them. In doing that, it makes a very rough layer and that layer is brilliant at nucleating bubbles because it's so rough. It's got all these little crevices that the bubbles can grow on. So, when I put the Mentos in and they sink through the liquid, as they're sinking, loads and loads of bubbles are forming on the outside of them and those bubbles expand and the pressure caused by them drives the Coke out of the top of the bottle.

Chris - What is the relationship between this and what we're hearing about what Pliny saw?

Ginny - Inside magma, there is dissolved gas, just like there is inside our Coke. As that starts to rise, the pressure is released and that allows it to nucleate and form a foam much like that and that's what pumice is.

Chris - What about the other second experiment you got sitting there?

Ginny - Okay, so my second bottle is unopened. So, there is no chance that any of the gas has escaped yet. It's also Diet Coke. No one is entirely sure why, but apparently, Diet Coke is supposed to work a little bit better than full sugar. One theory is that it's slightly less viscous so it should bubble out more easily. The other idea is actually that the sweetener used in Diet Coke lowers the surface tension of water. So, water has this thing where it likes to hang on to itself and that's why, you can actually fill up your glass and make it slightly domed because of this thing called surface tension. The sweetener in Diet Coke disrupts that a bit. So, it makes it easier for the water molecules to fly apart and hopefully, we'll get a more dramatic eruption. Who wants to see that?

Chris - That's sort of like pulling the pin on a hand grenade.

Ginny - So, what you could hear there was actually some of the gas escaping. That's not ideal because we want to use the gas to make our eruption but hopefully, it was only a little bit of gas so it won't matter. So, I'm going to get my paper tube and my postcard lined up again.

Chris - Okay, so we get the Mentos out of the packet. Is 6 the optimum number then?

Ginny - Six is what I tried it with in the garden the other day. Okay, let's drop the Mentos in the tube.

Chris - Do you want to count this down?

Audience - 3, 2, 1 go...

Ginny - So, who thought that that looked better than the first one?

Audience - Me.

Ginny - That went about a foot high out of the top of the bottle. The first one, it just sort of bubbled over the top gently but that one, we actually got a kind of jet about a foot high. Because there was more gas in there, there was just more potential for that gas to nucleate and form bubbles and erupt out. But if you guys have a look at the bottles now, what can you see about the Coke that's left in the bottles?

Girl - More stayed from the one with the sugar and more went out in the Diet Coke.

Ginny - Exactly. So, if I look at the bottle of sugar Coke now, it's still about 2/3 full. We actually haven't lost all that much whereas if I look at the sugar-free Coke, there's only about a third left in there. So, because you've got more gas, you end up with this more dramatic reaction that drives more magma out. But in both cases, there is still magma left behind and that's important. That's why a volcano never ends up completely empty and you can actually get multiple eruptions from the same volcano.

Calbuco Volcano by Philip Oyarzo Calisto

27:11 - The impact of volcanic ash

Falling ash and scorching pyroclastic flows are the most dangerous elements of an eruption. How do they work?

The impact of volcanic ash
with Professor Andy Woods, BP Institute for Multiphase Flow in Cambridge

Despite its spectacular appearance, it isn't actually the lava that comes out of a volcanoCalbuco volcano eruption that has the most wide-spread and destructive effects. Volcanoes often spew out tonnes of ash, which can travel long distances both in the air and along the ground as deadly pyroclastic flows. Andy Woods shows us how he models these ash flows using a fish tank and some coloured water. But first, he explains what Pliny the younger would be able to see while watching the eruption of Vesuvius in AD 79... 

Andy - At this point, what we're hearing in the description is that the ash emitted from the volcano was rising high into the atmosphere and then spreading out in the atmosphere as a cloud. And as that spread out, it would've formed a very thick cloud, kilometres in vertical extent. And so, no light would be able to come through that so it will get darker and darker. As that cloud spread out radially, the ash falling out would then land on the ground and form a big blanket of ash on the floor.

Chris - Really? Kilometres thick?

Andy - Yeah. So, the typical height of rise of these large plumes is up to 20 - 25 kilometres in these big eruptions. So, the ash is extremely hot. It's got temperatures up to about a thousand degrees when it comes out of the ground. That enormous amount of thermal energy can be converted to potential energy which is the energy you need to lift material high into the atmosphere. In fact, I have a little experiment I brought along to demonstrate how this process works. Behind me, I have a tank which is about a metre high. It's 20 centimetres by 20 centimetres in cross section and it's full of salty water. And the amount of salt in this water decreases as we go up through the tank. So, the water at the bottom of the tank is very salty and as we move to the top of the tank, the water gets much less salty.

Chris - Why is that?

Andy - So, that's the way we set the experiment up in order to model the atmosphere. When you rise up through the atmosphere, the temperature actually gets warmer and warmer effectively. And so, the density essentially gets lower and lower as you rise up in the atmosphere. So, when this ash comes out of the volcano, it rises up at very high speeds of 200 metres per second and it mixes with air, low down in the atmosphere and heats up that air. It's that heating of the air lower in the atmosphere that generates this low density mixture of air plus the particles. As we saw before with the bag lifting off the toaster, once we get the density of that air plus the ash to fall below the density of the surrounding air, that mixture can rise into the atmosphere. As it rises, it continues mixing and it'll continue mixing and continue rising until the thermal energy becomes exhausted. And that thermal energy becomes exhausted when it's mixed so much of the cold air in the lower atmosphere that essentially, the bulk temperature of that mixture becomes similar to the much higher temperatures higher than the atmosphere.

Chris - What about the eruption column? How are you going to create that?

Andy - For the volcano, we brought a pump along and we have a tank of red water to denote the red hot ash that is coming out of the volcano. If I turn the pump on, we're going to see the red liquid will come up and the red liquid is fresh water. So, this is a model of very low density water. So, let me turn this on.

Chris - We're now going to blow the water in through a hole which is right in the middle at the bottom of the tank.

Andy - Imagine that Coca-Cola frothing up out of the top of the bottle. It's now coming into the tank and we see this red liquid rising up and you see it's a very turbulent flow. It's mixing and engulfing a lot of the water down here as it rises up. What you see is it's stopped rising at this point and it's now beginning to spread out to the walls of the tank. So now, imagine you're standing down here at some distance away from the volcano and above you, when you look up, you see this red cloud over you - this would be the black ash. And the light from the sun can't penetrate through that big cloud that's spreading out. And so, Pliny would've seen it go progressively darker and darker as that ash spreads out and forms what we call a giant umbrella cloud which will spread out tens of kilometres away from the volcano at that height of about 20-25 kilometres above the ground. The ash will gradually start raining out because the ash is heavy, forming a huge blanket of ash.

Chris - But do you know what sort of mass of ash is going to be ejected by a volcano, sort of like the size of Vesuvius?

Andy - With a big eruption like Vesuvius, we're looking at 106 or 107 kilograms of material coming out per second.

Chris - A thousand tons a second.

Andy - Yes. So, if you think about a domestic fridge, that may be 1 ton. So imagine you got a cubic metre - a fridge - and imagine having 10 million fridges coming out every second. That's the amount of material that you'd see coming out on one of these big eruptions. But of course, it's not the size of a big fridge. There's going to be a whole series of ash particles ranging from very, very small particles that are tens of hundreds of microns which is 1/10 of a millimetre. They go up to the size of this piece of pumice we have here which may be up to about a metre in size. Now, as the eruption at Vesuvius continued, what happened after some time was the pipe or the conduit from the reservoir of magma underground up to the surface became eroded. It was essentially sandblasted by all this fast moving rock going past it. As it eroded, the eruption became faster and faster, more and more intense. Eventually, the very dense mixture that came out of the ground couldn't actually rise up and become buoyant. Instead, it collapsed and formed a flow, a pyroclastic flow. This would run along the ground.

Chris - Can you just explain a little bit about why it does go along the ground again because I didn't quite get that? Why does it not want to carry on going upwards though?

Andy - When it comes out of the volcano, it's by mass, it's mostly the solid material. And so, it's actually quite a lot denser than the air. It's only during the first kilometre or two as it rises that it mixes with enough air and heats that air, so its density falls below the density of the surrounding air. But as the flow rate goes to larger and larger values, it can't mix enough air in that lower kilometre or two to actually become less dense in the surrounding air. So, instead of behaving like a hot air balloon, it's more behaving as if it's a hosepipe pointed upwards with water coming out of it and it goes a certain distance and then it collapses back like a fountain back down to the ground. So, that starts spreading out along the ground. It's still very hot and you still have all these very fine ash. It may be hundreds of meters thick. So, if you imagine tall buildings may be tens of meters high and it's going to be travelling at hundreds of metres per second. You couldn't run away from it and it will be actually very hard to drive away. So, even if you think if Usain Bolt who runs 100 metres in 10 seconds, it will be going 10 times faster than Usain Bolt. So, if we perhaps have a look at the experiment, we can see how this flow works.

Chris - This looks like a fish tank but a little bit longer and a little bit narrower. It's a couple of metres long, 20 centimetres deep, 10 centimetres wide, and it's full of a clear liquid. Is that water?

Andy - So, it's full of salt water and at the end of the tank, at one end of the tank, we have what we call a lock gate. It's a vertical piece which actually separates the first 10 centimetres of the tank from the rest of the tank. Behind that lock gate, I'm going to add some particles to the salty water and so, you can think of these particles as being like the rocks and the ash, mixed up in the flow. Because I add these particles, the density of this fluid behind the lock will become greater than the density of the fluid further down. And so, this is an analogue of looking at the dense flow of rocks and air moving into the air with no rocks in it. So let's pour the particles in and I'll stir it up so I've got a suspension of particles. I'll now pull out the lock gate. What you can see is the flow is running along the tank because it's dense and you see it's a very turbulent structure. It's engulfing lots of the water above it as it runs along. It's gradually slowing down and it's slowing down because the particle load is falling out of the flow. And so, there's nothing to drive it further forward.

Chris - So, what we saw, when we took away the sluice gate was that the mixture went zooming along the floor of the tank which will be like the volcanic cloud coming down the side of a mountain I suppose. But the further away it went, the slower it went and that's eventually because there's no particles left to push it along anymore.

Andy - That's right and that's part of the story about how these very powerful ash flows propagate. But there's a really surprising additional effect that occurs because these ash flows are very hot. And so, I want to show you a second experiment where we're going to include the effect of both particles but also, the fact that the ash flow is hot. Now, the fact that it's hot means that the air in the flow is less dense than the surrounding air. And so, as this flow propagates along and it drops out particles, eventually, it'll have dropped out enough particles that what's left becomes less dense than the surrounding air. At that point, it should lift off the floor and rise up into the atmosphere. I've got a red liquid or put some particles in and this is fresh water now. So, to model it, the fact it's less dense, we're using fresh water. we got salty water in the tank. So, the fresh water alone wants to rise to the top. But because we've put particles in, the mixture of the fresh water in the particles starts off being more dense. It's only when the particles fall out that it can lift off.

Chris - This business about it getting to a certain point and then stopping, are there any sort of contemporary examples because didn't Mount St. Helens do that where you saw trees wiped out, wiped out, and suddenly, you got to a point where all the trees were standing again?

Andy - Yes. So, Mount St. Helens is really the first eruption where this phenomenon was understood because there were very large Douglas fir trees that were very, very big trees and they were all just knocked over like matchsticks by the big pyroclastic flow. But about 15 kilometres from the volcano, there was a line where beyond there, all the Douglas fir trees were still standing. At that point, the flow had mixed with enough air and it dropped out enough particles that the rest of it was less dense and it rose up into the air. And we've now seen a number of examples of this at different volcanoes with flows travelling tens of kilometres and then lifting off and rising into the air.

Chris - Okay, let's give it a go.

Andy - So, what I'm going to do is I'm going to put in my fresh water.

Chris - So what you're doing Andy, you drag the slider back so that we've pushed some of the salt water out of the way and created a space at the end of the tank that you're now filling with the fresh red dyed liquid. And we're now adding some particles in, there you go.

Andy - And then I pull out my sluice gate again. We'll see it runs along the bottom just as before. But as it runs along and drops out the particles, it's now beginning to lift up.

Chris - It suddenly stopped at one point and went straight up in the air. Now, it's all gone to the top.

Andy - So now, it's running along the top surface. You can think of this as being 20 kilometres above the ground and again, it will become very, very dark underneath that cloud. And so, it may be that in a big volcanic eruption, the material doesn't go straight up into the air, but may run along the ground a certain distance before it lifts up. I think now, you can see very nicely all the particles dropping out and this is what Pliny will be describing about - being in a rain of ash particles. Because the lower part of the atmosphere has lots of moisture, all our rain cloud system in it, that moisture is also carried up in this convective structure and it makes the volcanic particles often makes them very wet, and you can get hailstones and rainstorms associated with these big clouds. And so that's why you often see lightning and other effects because of the charge on the particles and also because of the presence of the vapour in the atmosphere. So, they're very dramatic events!

One of the lives claimed by the eruption of Vesuvius

41:06 - How volcanoes affect human health

Millions of people live close to volcanoes, but what danger are they in, and are there any long term health effects caused by volcanoes?

How volcanoes affect human health
with Dr Peter Baxter, Cambridge Volcanology Group

Volcanoes that erupted in the past, like Vesuvius, Tambora and KrakatoaPompeii ash person had a massive impact on those living nearby, claiming many lives. But they can also have the surprising effect, which Ginny Smith demonstrates, of causing beautiful sunsets all over the world. But first, Cambridge volcanologist Peter Baxter explains to Chris Smith why some eruptions are so deadly, and how people can be killed... 

Peter - Well, that was the big question which was asked at the eruption of Mount St. Helens, immediately afterwards when 57 people were killed in the area near the volcano. A very large amount of ash fell downwind into populated areas. I was in a team working in the United States at that time and we were contacted to go and investigate and find the reasons why people can be injured or killed in eruptions. After that point, we had no knowledge at all. To fast forward to Vesuvius, after several eruptions and being able to look and obtain more data, what we see in retrospect now in Pliny's account, what could've happened, and the scientific work that followed on the eruption at Mount St. Helens, on Vesuvius and the AD 79 eruption, for the first few hours when the eruption was occurring, and the pumice was falling from the sky, people in Pompeii were under a rain of pumice which was building up on the roofs of their houses and taking shelter inside, resulted in deaths caused by the roofs of the houses collapsing in on the people. Out of over a thousand bodies which have been unearthed in the excavations of Pompeii, about 400 of those were excavated from buildings where the roofs had collapsed in on the people. You can imagine that there were two or more metres of pumice and ash on top of the roofs. And so, when the roofs caved in, people would be buried in the ash and would suffocate.

Chris - So, that's obviously one very dramatic and physical reason. What about the fact that this stuff is in the air and people are breathing in this gas and dust? Does that make a difference?

Peter - Yes and when Mount St. Helens erupted, there were people living 200 kilometres away who were in an area where the ash had fallen heavily. There was no rainfall. There was very dry area and for a whole week, the ash was being resuspended and although it wasn't pitch dark, people couldn't move, they couldn't drive or take any transport because they couldn't see where to go. We thought at that time that people would actually die potentially under this huge amount of air pollution. But in fact, it wasn't that bad. And so, in most of these ash falls, we don't expect a lot of problems except that people who've got lung diseases like asthma or chronic lung problems, people who have been smoking heavily during their lives, and so forth, these people will be badly affected and need to take shelter, away from ash. But in the eruption of Vesuvius, the greatest danger was the ash fall and the pumice falling on the roofs and then caving in with people inside sheltering. After about 12 hours of this eruption with ash fall, the pyroclastic flows and surges began and then we moved into a different problem for the people who were still there.

Chris - I mean, they were quite literally being cooked - these people - weren't they because you've got gas temperatures of maybe 300 degrees, maybe up as high as we've just heard, a thousand degrees around where there are people?

Peter - Yes. We know from the investigations that we've done since Mount St. Helens that the temperatures at Pompeii were about 300 degrees centigrade. And so, you can imagine how hot that is and if you put your hand in boiling water for example which is 100 degrees centigrade and then you'd turn up the heat, and if this is impacting on your body, then it's a very high temperature and people die almost instantly if they're exposed to it.

Chris - I'm lucky enough to have visited Herculaneum and there are quite a few people who were taking shelter in the boatsheds which are now paradoxically many miles from the sea, owing to the fact that so much material rain down and move the coastline back, you know kilometres, didn't it? But there are all these people sheltering in these boatsheds and they've all got things like skulls fractures and things like that which people couldn't explain for a while until someone said, "Could it be that actually, these people were cooking so well and so fast that their brain quite literally exploded?"

Peter - I mentioned that the temperature in Pompeii was 300 degrees centigrade which people were exposed to as the surges came down from the volcano. At Herculaneum, it was even hotter and we go up to 400 degrees and even 500 degrees centigrade where people were sheltering in the caves by the beach. Sheltering from the eruption but also potentially, waiting to leave by the boats. The very first search from the volcano came down into Herculaneum where the people were sheltering and the cloud went straight into the caves where they were on the beach. They were instantly killed by this intense heat which probably completely toasted the flesh away from their bodies and they were left with just skeletons and this could've happened almost instantaneously.

Chris - What about other things that can come out of volcanoes? We've heard a lot about gases and things like that. Are there any noxious gases that can literally knock people out, not just because of thermal effects but because they're just very, very poisonous?

Peter - It wasn't a major factor in the Vesuvius eruption but in other instances, it can be important. And particularly in volcanoes which can just emit gas only and the gases like sulphur dioxide from a volcano can be very irritating to a population living downwind. We had an interesting eruption in Iceland which began in August last year, the Holuhraun eruption where the fissure opened and this was a lava eruption, so not like the Vesuvius eruption we're talking about which is an explosive one, and here, we have very fluid lava running out of a fissure in the centre of Iceland, at the same time, a very high discharge of gas, mainly sulphur dioxide. The levels of sulphur dioxide in the air as the plume swept around Iceland depending upon the wind direction were high enough really to trigger asthma attacks and people who suffered from asthma or really cause a lot of problems in people with chronic lung disease. But the Icelanders have this very good resistant houses to weather wind and cold because obviously, much of the year, they live in very cold conditions. They were given warnings where the wind was blowing, where the plume would go, and people with these problems could stay inside and be absolutely protected.

Chris - It's good to hear. Peter Baxter, thank you very much.

Ginny - Now in this show, we've looked at the devastating effects volcanoes can have on those living nearby. But large eruptions like Tambora can even cause changes on the other side of the world. 1816 is known as the year without a summer because of the huge volumes of ash produced by Tambora. This formed a layer in the atmosphere, reflecting away the sun's energy and causing a reduction in global temperatures for around 1 degree which is enough to cause crop failures and widespread famine. But this eruption also caught some strange and beautiful effects which I'm going to demonstrate to you now. So, we're going to do a little experiment here and I have two versions of it. I have one that's going to show you guys really well today and I have another version that you could try at home. So, I'm going to need someone to come in and help me out with the one that you can try at home. What's your name?

Jillen - Jillen .

Ginny - Okay, so what I've got here is I've got a pint glass. I've got a bowl of water and I've got a jug and I've got in this bottle some milk. So, what I'm going to do is just pour the water into the jug for you because it might spill. And then all you need to do is fill up the pint glass with the water from the jug for me. Can you do that? Now, I'm going to add a dash of milk and it really doesn't need to be very much, just a tiny amount.

Chris - Wow! When you said tiny, that really is tiny, Ginny. That's literally a few drips.

Ginny - Yeah, it really is, just a few drips. You can see that white kind of swirling around. So, what's that done to the liquid?

Jillen - The water is sort of white because of the milk.

Ginny - Great, exactly. So, we've only added a tiny bit but it's gone all kind of cloudy, hasn't it? Now, what we're going to do is we're going to take this torch. I'm going to hold up the glass and shine the light from the bottom. What colour does the milk look?

Jillen - White.

Ginny - Can you see any other colours in there at all?

Female - Purple.

Ginny - Can anyone see anything?

Male - Blue.

Ginny - Okay, so when I shine the light from the bottom, it makes the milk look sort of bluey, slightly purpley. If you look down the top, what colour does the light look?

Jillen - Orange.

Ginny - It looks orange, exactly. Now, that's interesting. If I pour some of this milky water out, have a look again. What colour does the light look now?

Jillen - Yellow?

Ginny - Yellow, so it's changed colour. All I've done is poured out a bit of the milky water so you're looking through less of the water. I'll pour out a bit more and what colour does it look now?

Jillen - White.

Ginny - White. So, by changing the amount of milky water we're shining the light through, we're changing the colour of the light.

Chris - Why did that work?

Ginny - What happened there is the milk is doing something really clever. It's doing something we call Rayleigh scattering. What that means is that when the light hits it, it bounces off in all sorts of directions. So, light is a wave and if you imagine a wave in water, if it hits something, it can bounce back. And sometimes it can bounce back at different angles. The milk in the water is doing that to the light. It's scattering it. Now, because light is a wave, it comes in different wavelengths. So, red light has a long wavelength and blue light has a short wavelength that's the kind of scale that goes all the way through the rainbow. Short wavelengths scatter better. So, when you're looking at the milk with the light underneath and you're looking at it from the side, what you're seeing is the light that gets scattered out and that looks blue. And that's why the milky water looked blue. But when you're looking down the top, you're seeing the light that's travelled through the water and that is whatever is left once the blue has been scattered out.

Chris - Why does the blue get thrown out the side then?

Ginny - Because it's bumping into these particles of milk and because short wavelengths scatter more, that's the one that bounces back.

Chris - The red comes right through because it's less scattered.

Ginny - Exactly, yes. So, when the light has to travel through all the milk, there's a really good chance that most of the blue will have scattered out. So, what you get left with is red. When there's only a little bit of milk to travel through, it's less likely that the blue has been scattered out. So, you get a mixture of blue and red which gives you orange then yellow. And then when there's hardly any milk at all, you get white.

Chris - So, with that really big long tube you've now got here, you're able to effectively do this many, many times over what you've got in the glass. So, you should get maximum scattering and therefore, you should see very red at the top and very blue at the bottom.

Ginny - Hopefully, we're going to effectively see a sunset. So, the Rayleigh scattering is the same reason that we see blue in the sky. Because the sunlight is hitting the atmosphere,it's scattering and we see what's being scattered and that's blue. The colours that you see when looking directly at the light, they relate to a sunset. So, I've got some more milky water here.    This one, I actually use milk powder and that's going to behave more like the particles of ash in our volcano. So, what we had in the glass is effectively a normal sunset. When the sun is high in the sky, the light from it has to pass through a bit of atmosphere to get to us but not huge amounts. So, some of the blue is scattered and what you get left with is a kind of yellow orange sun. At sunset, the sun is low on the horizon so the light has to pass through more atmosphere before it gets to our eyes, more blue is scattered out, and we're left with a beautiful glowing red sun. What lots of people noted is that in years with big volcanoes like Tambora, sunsets became more vibrant, beautiful colours. There are some beautiful paintings and poems written in the year 1816 that scientists now think are based on this effect. So, I've got my water with milk powder which is going to represent ash and I'm going to pour it into this tall tube. I'm going to turn on the light at the bottom of the tube. What can you see?

Jillen - It kind of like changing shades as you go up the cylinder. So, it starts off as a sort of whitey colour and then it changes as you look up the tube so the yellowy colour then an orangey colour.

Ginny - So, when you're looking through all of that atmosphere with all those particles in, you get a really dramatic effect of this scattering and you see some really beautiful sunsets.

Chris - Ginny Smith, thank you very much. Any questions?

Male - Which eruption was the biggest?

Male - It's actually not an easy thing to answer because volcanoes that erupted billions of years ago, we don't have much evidence left to know how big they were. The biggest one that you'll find is an eruption known as the Fish Canyon Tuff eruption 4 or 5 million years ago. We have a magnitude scale which is a bit like the Richter scale for earthquakes and that's about a magnitude 9. Compare that to the Vesuvius eruption that we've been talking about, that's about a thousand times bigger. The last really big eruption in more recent times was that of the Toba volcano in Sumatra that had a magnitude of about 8.8 which is equivalent to something like 3,000 or 4,000 cubic kilometres of magma. And that was only about 74,000 years ago at a time when our human ancestors were around and there's been a lot of debate as to what effect that eruption might have had on our ancestors at the time.

Amelia - My name is Amelia and I would like to know where were that tectonic plates on the Earth?

Chris - Marian, tectonics.

Marian - Tectonic plates, there's an awful lot of them. Along the centre of the Atlantic, it's splitting apart. So, you've got one plate on the west side which is the American plate and then you've got the European and African plates on the east. So, Africa has got its own plate. Europe is made of a mishmash of lots of small plates that are all sort of sliding past each other. The biggest plate of all is probably that of the Pacific. The Pacific Ocean, almost all of it is a single plate that's moving towards Japan at the moment and away from America.

Graihagh - Given today that so many people still live on the mountainside of Vesuvius, what happens if there is an eruption? Is there some sort of evacuation process that's in place today?

Chris - Peter...

Peter - So, there is a national plan, but if we knew when it was going to erupt, everything would be dead easy because you could just get people out of the way. But we have this huge uncertainty of even if it's showing very serious activity, whether in fact it is going to move into full eruption or not, but the danger is so great that there will be a tendency to evacuate many, many people in advance. But that's the crucial thing, is knowing whether you can get people out in time, if you delay the evacuation call. Of course, hundreds of thousands of people won't like being moved to another part of Italy if the eruption doesn't actually follow.

Jillen - Do we always have an earthquake before an eruption starts like Pliny described?

Marian Holness [Earth Sciences, Cambridge] - Yes, I think you do. The Holuhraun eruption in Iceland was preceded by a great swarm of earthquakes. Iceland has got an enormous number seismometers scattered around it. And as soon as the first one recorded the first sort of set of earthquakes, all the geophysicists at Cambridge University got terribly excited and jumped on a plane with even more seismometers and they scattered them all around the area and they could actually see where this magma was moving by tracking where the earthquakes are in real time, and that was terribly exciting. So, they could tell pretty much when that eruption was going to happen.


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