Science Interviews


Sun, 22nd Jan 2006

How Volcanoes Work

Dr Janet Sumner, The Open University

Part of the show Geology of Natural Disasters, Volcanoes and Earthquakes

Chris - You work at the Open University and you're a volcanologist, but what does that actually involve?

Janet - I do field work, analytical work and monitoring work on active volcanoes. I started out life on volcanoes in the UK, which you'll be pleased to know aren't active anymore. I've now moved on to active volcanoes. What I do is look at volcanoes, but then come back and do stuff in the lab.

Chris - So do you go and get a sample of lava and then find out what's in it?

Janet - Yes, and I've brought along one of the things I collect to show you. Can you guess what it might be?

Chris - Well it actually looks like a coconut shell, as it's about the right size, but it's made of rock.

Janet - This is actually a volcanic bomb, and let me tell you how hard it is to get one of these on a plane nowadays, especially in your hand luggage!

Kat - It's like an enormous rock sherbet lemon and it's really heavy. I've got it here on my desk. But what is it?

Janet - It's a piece of molten rock that got thrown out of a volcano when it exploded. As it travels through the air, it gets shaped into an aerodynamic form, and hence it looks likes a sherbet lemon sweet. It's got a very chilled rind on the outside and then the inside would have been filled with what was once molten rock. I can't use that stuff in the lab because it's about 2000 degrees centigrade and much too hot, so I use something called an analogue. I've also bought my analogue in to show you, and Kat's got it in her hand.

Kat - It's a Cadbury's Cream Egg!

Janet - I hope it's still complete because it has to be complete for my experiments! It's basically the same kind of thing as a bomb, because it has a chilled, brittle outer rind and a viscous, runny interior. What happens is that when these things come falling out of the sky, they land on the ground and burst open. The runny molten rock comes out, runs together and forms lava flows. I'm working with cream eggs and golden syrup to work out how far these lava flows can go.

Chris - OK, well tell us a bit about what a volcano is, because everyone's seen the pictures and seen eruptions, but what's actually going on? If you were writing a geology textbook, what would you say was happening?

Janet - Well it's linked into plate tectonics. Most volcanoes occur where you either have spreading plate boundaries or one plate is being forced down under another. What happens is either hot molten material from the mantle or nearer to the Earth's core is rising up to the surface. This is because it becomes very buoyant when it's warm, or cold heavy material is being squashed down and forced under another tectonic plate. That then goes down and melts. So it's like a constant recycling motion. You can think of it as a pot of custard that you've allowed a skin to form on. If you then heat that up on the stove, the runny custard underneath will start to convect and boil. It will eventually split open the surface skin and then new runny custard will boil out onto the skin surface.

Chris - Why do we have plate tectonics? If you look at Venus, it doesn't have any plates.

Janet - Well it doesn't really have plate tectonics, but it does experience the same kind of over-turning that the Earth experiences. It's just that Venus over-turning happens on a global scale within a few tens to hundreds of thousands of years, or at least this is what we think happens. Rather than individual plates jostling around, what happens with Venus is that the whole surface resurfaces itself within a short space of time. This gives Venus a new planetary surface. Earth resurfaces itself on a slower scale than Venus.

Kat - Other things you've brought in here are a pumice stone that looks a bit like it's been used on someone's feet, and also a big chunk of black glass. What have these got to do with volcanoes?

Janet - I brought along the pumice stone because I thought it was the one bit of a volcano that most people will have in their bathroom and might actually have touched. I brought in the glass because it was also formed by a volcano. It's incredibly sharp and used to be shaped and used as cutting blades. It's basically a form of silica, and because it cools so incredibly quickly, it forms glass. So these are just a few examples of what a volcano can produce. It's tremendously varied.

Kat - It's a beautiful colour. Is this obsidian?

Janet - It is obsidian.

Kat - It's absolutely jet black. A fantastic colour.

Chris - When you look in a text book, you will often see this beautiful picture of planet Earth with layers peeled away almost like an onion, and they'll say that this is the crust, and then we're into the mantle, the outer core and the core. How do we actually know what's in there and what that structure is? It's obviously not practical to drill a hole down to the centre of the Earth.

Janet - Well no, but I think some people have tried and gone some considerable distance, but we're never going to get that far. It's a probably a question that my colleague sitting next to me could answer, because a lot of it has been done through seismic studies. People have fired seismic waves through the Earth and basically worked out whether it's molten or solid rock from the delay times.


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