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 didn'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 Earthís 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 donít. What's special about those areas?
Marian - If we start from the centre of the Earth, weíve got a solid core thatís made of iron and then the outer part of the core is also made of iron and thatís liquid and thatís moving around by convection. Now, if we go further out into the mantle, weíre back into solid rock again and thatís 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, weíve 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 doesnít 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, thatís 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 - Thatís exactly true Ė 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. Thatís the nearest to us and that is there because this solid mantle is solid but itís convecting. Itís moving. So, there's heat being taken from the centre of the Earth and itís moving up like boiling water. And if you bring a plume of this hot solid stuff up, itís moving fast enough that it will melt. So, you get a hotspot volcano. Thatís what weíve 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 - Thatís 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, weíve 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, itís going into the mantle and itís melting. So, Italy is blessed with a great family of volcanoes. Youíve got Etna, youíve got Stromboli, youíve got Vesuvius, and youíve 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 thatís 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, weíve got a whole set of elements within the Earth that are breaking down radioactively and every time they do that, they generate heat. So, thatís why the Earth is hot. Itís really, really hot in the middle and itís 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 itís got a low density. Itís 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. Itís 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 didnít 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, thatís the danger sign.
Marian - Yes. You get out of the way as fast as you can at that point. Youíve got a timescale of weeks, days. Youíve got time to leave.
Chris - Professor Marian Holness, thank you very much. Itís time to get experimental now and every month, Ginny cooks up something from the kitchen experimentally for us. Youíve got a toaster on the floor.
Ginny - Weíve been talking a lot about heat and convection and hot things trying to get to the surface of the Earth. So, I thought Iíd 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, weíve got a toaster on the floor. Itís 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 weíre going to do is weíre 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 thatís 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 - Itís filling up with hot air?
Ginny - It is exactly. So, the bin bag is expanding and thatís just like how our volcanoís 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 ,1Ö
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 itís 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. Thatís 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, youíve 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, itís 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 doesnít because the rock is more dense, itís 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 itís hot, it rises because itís 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 didnít just keep going up and up forever. It went up and then it came down again. And thatís because as it rises, the air inside it starts to cool again and then because itís cooling, itís becoming less dense, itís 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 itís doing this big sort of mixing. Thatís what we call convection.
Chris - But itís not powered by a toaster.
Ginny - No. there's a bit more heat going on inside the Earth than there is in my toaster...