What happens when a volcano erupts?

Volcanic eruptions are hugely dramatic events that can literally tear apart mountains. 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.