Forming Volcanoes - A Geological Controversy
Chris - We are talking all about the science of volcanoes this week, and to help us forecast an eruption, we need to understand as much as possible about the processes that cause a volcano to form in the first place, and we're joined now by Professor Gillian Foulger who works on understanding how volcanoes form in places like Hawaii. Gillian Welcome.
Gillian - Hello, Chris.
Chris - Welcome the Naked Scientists. Tell us if you would. First of all, how do volcanoes form and what are the areas of contention?
Gillian - Well, most volcanoes in the world occur along the edges of tectonic plates. So, most people are probably familiar with the concept that the Earth's surface is broken into lots of plates like the shell of a tortoise, and along their edges where they move past one another, there are huge cracks going right through the lithosphere layer down into the Asthenosphere (outer mantle) below and molten material can leak up through these cracks and form volcanoes. So, the volcanoes such as Mount St. Helens and the cascades volcanoes, the volcanoes of Japan, they all occur along plate boundaries. There isn't too much controversy about how they form, but the real controversy lies with the volcanoes that form smack in the middle of plates...
Chris - Can you give us some example of those?
Gillian - Well, an example is Tristan da Cunha, Reunion Island, people also interestingly enough put Iceland in that category, and of course, Hawaii.
Chris - Okay, so these are volcanoes that pop up in the middle of a plate. So what goes at the margin of a plate or plate boundary cannot go for what's going on in the middle. So what do scientists think is driving the emergence of a volcano at that point then?
Gillian - Well, there are two competing hypothesis. There's the plate hypothesis which says, "Hang on, plates are not completely rigid. That's just like a cartoon world." Geology is much more complicated than that, and huge plates do have cracks, they do pull apart in their middles, and we can have the occasional volcano that comes up through the crack in the middle of a plate.
The competing hypothesis is the traditional plume hypothesis which suggests that you have a hot diapir, a hot thermal coming out from the Earth's core, rising 3,000 kilometres through the Earth's mantle and punching its way through the plate at the surface, not caring whether it's in the middle of a plate or not.
Chris - Have we any idea as to why such a pulse of energy should be unleashed by the core in that way? Why should that happen and why don't we see this more extensively then?
Gillian - Well that's a very good question, Chris, because you often hear people say - Hawaii is a typical example of a plume, but in fact, Hawaii is completely unique on the Earth's surface. There isn't anything else like it anywhere. But coming back to your original question - why should this happen - people have suggested, people have pointed out that the earth's core is about 1,000 degrees hotter than the material immediately above. So they have this model like a kettle on a stove with a hotplate underneath and the hotplate is heating water in the kettle, and causing diapirs to rise up. So, that's the fundamental concept behind that theory.
Chris - So it's almost like a weak spot and the energy gets focused somewhere, and as soon as something gives, then it gets channelled up through whatever that weakness is, and that just happens to be say, Hawaii. There must be other examples of these plumes, like the ones you gave where this energy is unleashed, but how can we resolve that question? How can we study this? It's not trivial to get into a planet that's got a radius of 6,000 kilometres and find out what's going on in the middle. How can we address this problem?
Gillian - Well one of the primary methods used is using earthquake waves to CAT scan the Earth. So, when earthquakes occur, rays go through all parts of the Earth and we have seismometers on the surface, so this is like taking a person into a hospital and CAT scanning them. We can look at the structure inside the Earth. And what we're really looking for is to see if under places like Hawaii and Iceland, if we see some kind of a structure going all the way from the surface right down the Earth's core, and if we saw that, that would pretty much be strong evidence in favour of the plume hypothesis.
Chris - So what have you seen? You presumably haven't seen that yet. So what have you seen?
Gillian - No. What we tend to see almost everywhere is structures which can go down several hundred kilometres, but they don't go down the full 3,000 kilometres that they would have to do to reach the core.
Chris - So do you think it's just a question of making more observations or do you think we need to rethink this model and in fact, you don't need to go all the way down to the core? Perhaps it can act as a sort of vent for pressure slightly outside the Earth's core.
Gillian - Yes. I think we don't need to go down to the Earth's core. I think everything is happening just in the upper 3, 4, 5, 600 kilometres. It's not going all the way down 3,000 kilometres to the core. But regarding how do we address this problem, well, part of the reason why this controversy is so exciting is because a lot of things - a lot of human aspects are weaving in which almost stand outside the science. The plume hypothesis has been popular for a very, very long time and there's great reluctance to let it go partly because it can sort of be trotted in to explain everything. Whatever you see or you don't see, you can find some way of turning the plume hypothesis around to explain that. But the plate hypothesis on the other hand makes specific predictions which we should be able to go out and test. So, there's great excitement in the geological community at the moment and some people say this is the most exciting and fundamental controversy that's developed since plate tectonics.
Chris - So obviously, we will be in a position at some point in the future to make reasonable predictions about volcanic activity at plate margins where one plate is either subducting or overriding another, but if we've got this problem with plume volcanoes, how can we predict those? If we don't understand what's causing them in the first place, how can we predict their activity?
Gillian - This subject is really looking at big scale and long term volcanic behaviour. So, unlike
what Hazel (Rymer) was describing, Hazel is looking at the activity of specific volcanoes on the kind of timescales that are relevant to human beings a few years or a few decades. But we're looking at the big scale of things and we're looking on timescales of millions of years. So the sort of problem we would be interested in that we could contribute to would be when Kilauea in Hawaii becomes extinct, where is the next volcano going to form.
Chris - And presumably also, you're in a position to inform the climate change debate because we know that volcanoes and volcanism have had a big contribution to the Earth's atmospheric composition over many, many years, and understanding this important contributor must therefore also feature quite heavily in the argument.
Gillian - Yes. It would certainly give relevant data to that subject indeed.
Chris - We must leave it there. Thank you, Gillian for joining us. That was Professor Gillian Foulger, who is at Durham University.