Looking into a volcano

Richard - Erupting into the studio now from the University of Cambridge is volcanologist, Marie Edmonds. Hello Marie.

Marie - Hello Richard

Richard - First off, most people know what volcanoes are but what's actually going on inside one?

Marie - What we see at the surface with volcanoes is really only part of the story. Part of my work is looking at magma de-gassing and looking at the gases that are emitted by volcanoes at the surface can tell you what's going on deep in the crust.

Chris - What sorts of gases are coming out of volcanoes?

Marie - Volcanoes emit lots and lots of water and then lots of carbon dioxide, sulphur dioxide, halogen gases and various minor trace components. Water, CO2 and SO2 are the main gases.

Richard - How can we actually analyse these gases and what can they tell us?

Marie - I go with the spectrometer to the volcanoes which is a really good way pf doing it safely. I can go a kilometre away from the volcano, I don't have to go right up to the vents and fill little glass bottles with gases. I can use spectrometers to remotely look at the composition of gases and work out from that what's going on beneath the surface. Gases are, if you like messengers for me to the crust.

Chris - What's a spectrometer? How does it work?

Marie - Gases absorb radiation in characteristic wavelengths so each gas is associated with its own unique signature. If we collect infrared radiation, it could be from the sun, a lava flow or a lava dome. The infrared radiation passes through volcanic gases so what we're looking at is a spectrum which contains all of the unique absorptions due to all the different gases. We can quantify their concentration.

Chris - So this is a neat way of monitoring the volcano without having to put yourself in quite a lot of danger, by getting really close. You can see what the gas composition is remotely.

Marie - That's absolutely right. Volcano monitoring has come a long way since the development of techniques like this. In the last decade or so these techniques have become more and more sophisticated.

Chris - What does this analysis of the gases tell you about the behaviour of different volcanoes?

The Soufriere Hills Volcano on Monserrat © EvaK @ Wikipedia

Marie - The idea is that each volatile species: water, CO2 and so on - they dissolve at different pressures. As magma is coming up from the base of the crust first of all carbon dioxide will exsolve. When is gets to very shallow levels gases like water and SO2 exsolve and form bubbles in the magma. If we measure gases which have very high CO2 content, for example, we know that the gases separated from the magma at depths of a few tens of kilometres. If the gases are rich water then we know that de-gassing is occurring at a shallower depth. We can infer something about the eruption. We can to an extent forecast a volcanic eruption this way.

Chris - Because most people think a volcano just blows up catastrophically like Mt St Helens which is in every geography textbook as the massive, explosive volcano but others are sort of more oozy and splurge out lava without really doing much. Does one turn into the other or is there a risk that one might be one of those kinds and then suddenly explode? What do we know about volcanoes' behaviour?

Marie - A whole different range of volcanoes and different styles of activity. I have actually measured the composition of giant bubbles that have come up very low viscosity magma columns at Kilauea volcano in Hawaii. If you go to a completely different type of volcano such as Soufriere Hills volcano in Montserrat which is an explosive type of volcano and crucially it has very high viscosity magma. This type is the most unpredictable and in some ways the most interesting because it can transform from very effusive dome building activity to explosive, ash-producing eruption columns over the space of a few minutes. That's what makes this type of eruption very interesting.

Chris - Because you're going to back to Montserrat, aren't you?

Marie - Yeah, I've already done a trip there this year to measure CO2 fluxes from the flanks of the volcano. We still know very little about what goes on at depth and what's actually driving the eruption. I'm due to go back in a few weeks' time to continue those measurements. We're going to install a gas sensor close in to the volcano to try and measure the proportion of CO2 to SO2 in the hope that this will give us some idea of whether the eruption is waning or whether it's accelerating: what we can expect in the next few month in terms of activity.