Seismology:Understanding Extreme Earth
Chris - I'm Chris Smith and we are talking extreme geology this week, as we're joined by three esteemed guests from the Department of Earth Sciences at Cambridge University - James Jackson, Tehnuka Ilanko, and Arwen Deuss, who - you're a seismologist, Arwen. So, let's shake up the world of geology a bit and tell us what you do.
Arwen - Well, we've heard about earthquakes and volcanoes from the other two people here. That's the main thing you'll see, the main thing you'll hear about on the news. But all these things that happen at the surface of the Earth, they're driven by processes deeper in our planet. If you want to understand them then we need to know what's happening in the deeper parts of the Earth.
The way we look at that is by using seismic waves. So, we can look at the earthquake itself, but these earthquakes, they generate waves. They travel all the way through the Earth. They don't only travel along the surface. They go right through its centre. So, an earthquake that would happen in New Zealand, we can pick that up here in the UK. We wouldn't feel it as human beings, but our seismometers, they're very sensitive instruments that can feel these waves. They make recordings of those, so we can make pictures of what the deeper parts of our planet looks like. This is a very, very nice way to understand the tectonics, the movement of plates, the generation of earthquakes, the reasons why there are volcanoes in some places. We can link those to the deeper parts of our planet and we make pictures, just like making a brain scan and our pictures look very similar. Give them colours, red and blue, they relate to regions in the Earth where the velocity is slightly or slightly lower, and we think we can link those to regions in the Earth where the temperature is slightly higher or slightly lower.
Now, volcanoes, they need higher temperatures and we would see them in our models, places where the temperature's probably higher that might lead to volcano at the surface of the earth. What we can also see is places where the velocity is slightly faster and the temperature is slightly lower. That's probably places where plates - we just heard about how we have in the middle of the oceans, different plates, new plates being generated. We have volcanic eruptions and we generate new parts at the surface of the Earth. If we generate new parts, we need a place where that's being compensated, we need to destroy these new plates and those are called subduction zones. A very important place where we have a subduction zone under Japan, probably being the cause of the big earthquake that happened and caused the tsunami.
Chris - Arwen, I've got an email here from Tony who says, "If we were to cut the Earth across and look inside and draw a sort of map, what is the structure of the Earth?"
Arwen - The Earth mainly consists of a few big layers. It's like the different layers of an onion. The top layer is about 3,000km thick. We call that the mantle. That's where all the excitement is happening that will relate to the tectonics there. Then we go below that. We get the core and that's a very, very hot region made of fluid, iron and nickel. It's fluid and inside there, there is a very small region about a thousand kilometres thick which is the solid inner core. So, there are mainly three big layers - the solid mantle, a fluid outer core and then a solid inner core again.
Chris - How do we know what's in those things because you've said there's this iron in there, but no one could go there, so how do we know there's iron in there?
Arwen - It's a very good question. We cannot go there, but we have an idea of what the average composition of a planet is. We can look at meteorites and we have a feeling of what the average composition is of things that get formed in the solar system. And we know those must contain a lot of silicon, but also a lot of iron. When you form a planet, the iron is very heavy and it will sink towards the centre of the planet forming a core and the lighter elements like silicon will stay in the outer part, the mantle.
Chris - Because I've got an email here from someone saying, "What would happen at the core of the Earth because if all the heavy stuff sinks and we know that gravity is attracting everything to itself then at the very centre, there should be no net gravitational effect, so wouldn't the core actually be hollow?"
Arwen - Well, there's also a pressure effect. So, we have incredible pressures deeper in the Earth, so it wouldn't be possible to sustain a hollow anywhere deeper in the Earth. So no, we don't have a hollow at the centre. We have a proper solid inner core and no hollow part there.
Chris - And Jan says by email, "What keeps the Earth's core hot?"
Arwen - A lot of the heat has just been that since the generation of the Earth. It's heat that still remaining from when we generated our planet. The planet is very hot and it's just still slowly cooling down. On top of that, there's probably some radioactive decay which will also generate heat. So, there are different processes happening, but the main heat that's coming out is also - because I said there is a fluid outer core and a solid inner core. That solid inner core is slowly solidifying, so it's growing and growing, getting bigger over time. When it solidifies, that releases more of the heat which makes the rest of the core very hot.
Chris - And on the surface of the Earth, it's carved up into plates and we have continents. If we look on any other planet in the solar system, do they also have tectonics like that or are we unique?
Arwen - We have other planets that are a little bit similar to the Earth and we can see some evidence of tectonics on those other planets although the real way to find out would actually be to go there and see, can we measure any quakes on those different planets?
Chris - After you.
Arwen - But if you look at Mars for example, we do think there might have been or there might still be plate tectonics there, and another planet that people have been looking at is Venus. They're very different to Earth though an the things that are happening or the process that might be happening deeper in those planets are probably quite different. The interesting thing will be, to send seismometers to these planets. That would be very impossible probably for Venus but it will be possible for Mars and there are some plans to send seismometers there and hopefully then we'll be able to study better if there's any plate tectonics there.
Chris - Let's take some questions.
Cris - Hi. I'm Cris from Cambridge. The information that you collect following an earthquake, does that help you build up a picture of where is the best place on the earth that you can harness geothermal energy, like they do in Iceland?
Arwen - I think we know pretty well which are good places to harness geothermal energy and it's a beautiful thing. When you go to Iceland, yes, there is hot water and we don't need to burn any fossil fuels for that. I think we know the geology at the surface well enough to find those places, but if, for whatever reason we wouldn't, yes, you might probably find that there are places with small vibrations which would be related to the volcanic activity and would be places to harness geothermal energy.
David - Hi, David from Bury St. Edmunds. What sort of resolution does your seismographic mapping technique give?
Arwen - It depends on the seismic waves you use for looking at the data. It also depends on how deep you want to look into the Earth. Near the surface, we can have a very good resolution and we look at data that travels really fast and the waves oscillate really, really, really, fast as well. They will oscillate maybe at a second, that means that the wave going up and down only takes a second. Then we can have resolution of a few kilometres. The deeper you go in the Earth, the less and less resolution you have and you're looking at hundreds of kilometres and at the deepest part, it's almost a thousand kilometres. So, you have a wide range of resolutions.
Chris - I've got two emails on almost the same question. So, John Stenson and also Delisia. They would like to know, what provides the energy that keeps the plates moving and why do tectonic plates drift around because I think the statistic is, they move at roughly the same rate my fingernails grow. Is that right or faster?
Arwen - Yes, they do. Now, most of the heat comes from the core, the latent heat of the core. So, that needs to go somewhere. So, it's in the core and it needs to get into - it gets into the mantle which is cooling down and at the top, you get a surface which is like a crust you would have if you're cooling down some milk. You get a crust on top of it as well and that's moving, adjusting to it. We don't really think that actually moves the plates. That's probably a separate system, but they are moving at speeds similar to the speed your fingernails grow with. We know it keeps itself going, so we can determine what the powers are of the forces that will be building the new plates and how they're being dragged down. But how we would actually generate the plate tectonics from the motions deeper in the planet is something we just don't fully understand yet.
Pete - Hi. It's Pete from Cambridge. Can you explain what causes the Earth to have a magnetic field and why it sort of switches over geological time?
Arwen - Yes, it's a very good question. You're right. The Earth has a magnetic field and when you look at it from the outside, it looks like a bar magnet which you've been playing with as a kid probably. However, in the Earth it's not a bar magnet. It's actually generated not by the solid inner core but by the fluid outer core. There are motions and because it's at such high temperature, these motions, they generate the magnetic field and we call it the geodynamite. We don't fully understand how to do that. We try to do this with computer calculations, but it's such an extreme environment that we cannot do it yet with computer calculations. Some people try to do it in the lab as well and again, you cannot get iron at such high pressures and temperatures in a lab. So, people are looking at sulphur experiments. So, it's the outer core that's generating the magnetic field. We can understand that it will flip at irregular intervals. Again, how that's all exactly happening - It's something we're still trying to learn about.
Chris - And James, the fact that we've got a magnetic field and that that field gets written into rocks when they're deposited on the ocean floor is actually one of the reasons why we now know about the movement of tectonic plates. Isn't it? Wasn't that some American geological surveys that showed that?
James Jackson - No, it wasn't. It was people in Cambridge who showed that, famously. In 1964, two people in our department.
Chris - Wasn't that some august Cambridge scientists?
James - Yes, you're quite right. It was some people in my department who discovered that in 1964. It was very clearly their discovery. What actually happens is because of the magnetic field as we've heard, magma comes out of volcanoes in the middle of the oceans and as that magma cools and makes rock, it has a lot of iron in and the iron gets magnetised in the direction of the Earth's magnetic field at that time. So, if you can imagine a conveyor belt coming out in the middle of the ocean, it's like a tape recorder. It gets magnetised in stripes as it comes out. These stripes are either in the orientation of today's field or as someone said in the audience, the field flips and it looks like the reverse. But basically, the ocean is just a mass of stripes of magnetised rock.
And, if you can see that that's how it's formed, you can wind it back the other way if you like and see how the continents move around. The continents are just passengers on this conveyor belt. So, if you can rewind the conveyor belt, you take the continents with you and that's how you can make jigsaw maps of how the continents all fit together.
Chris - Vanessa Penman got in touch by email and says, "What's actually the consequence of the Earth's magnetic field reversing?"
Arwen - It wouldn't be very nice to experience it. Especially nowadays, we really depend on our magnetic field for a lot of different things. It flips and it doesn't flip instantaneously. It can take up to a few hundred, maybe even a thousand years. What first happens is that the magnetic field gets less and less, and less strong. That wouldn't be so nice because we need the magnetic field, it protects us against solar radiation. We use it for navigation, birds use it. So, I think it would be quite difficult to imagine what will happen when the magnetic field flips. The problem is, we don't really know when it will happen. It happens at irregular intervals. It can be millions of years before we get one and it can be just tomorrow.
Simon - Hi. I'm Simon from Cambridge. We know that the inner core is a solid structure, but with all that heat surrounding it, how do we know that it's solid and why is it solid?
Arwen - Well, we know it's solid because we've used these really, really big oscillations. If you have a big earthquake, the whole Earth will ring like a bell. The whole Earth will slowly expand and contract and this will take days to continue. Now, we can listen to all, or look at all the different tones as you would say, if you see this as a musical instrument and all these different measurements we can make. If you want to explain them, there's only one way to do it and that's with a calculation in which we let the inner core be solid. If we allow the inner core to be fluid, we cannot explain the data that we measure after these big earthquakes.