Monitoring a volcano in honeymoon paradise

To the whitewashed rooftops and sun-soaked terraces of Santorini, where researchers are probing the threat posed by one of Europe’s most active volcanoes. The Greek island - which attracts holidaymakers and honeymooners from across the world - was born from a geological catastrophe, and experts believe another eruption is probable. Isobel Yeo at the National Oceanography Centre is leading the mission to find out more about this submarine volcano, and, specifically, how the water passing through the volcano - like a circulatory system - operates and actually increases, or decreases the prospect of an eruption…

Isobel - I'm part of a team of scientists that are trying to understand how hydrothermal systems, so these are fluid flow systems, interact with volcanoes. There are loads of different types of volcanoes around the world, but loads and loads of them are found in our oceans. And some of those can be quite dangerous. So we have some shallow explosive volcanoes. Some people might remember there was an eruption in 2022 in Tonga, technically the most explosive eruption this century. That's what's called a caldera volcano. So these are volcanoes that are capable of very big explosive volcanic eruptions. Now Santorini is also a caldera volcano. These are really interesting volcanoes, because when they erupt, they tend to empty their magma chamber underneath and collapse. And so because of that, rather than them being a cone, we have sort of a hole in the ground, and holes in the ground tend to fill with water. So for caldera volcanoes, it's really important the interaction that the water that lies on top of them and flows through them has with the magma chamber, and how that changes the hazard that that volcano might produce. So we think that fluid flow can either make volcanic eruptions more or less explosive, and it's that sort of question we're trying to understand.

Chris Smith - How are you trying to get at that, when it is on the scale it is, and in the geography, i.e. subsurface underwater like this?

Isobel - Yes, there's a whole host of different challenges. The primary one is obviously working underwater. So monitoring subsea volcanoes is really, really difficult. The thing that we were trying to do, our primary initial objective for this expedition was to map the fluid flow system within the volcano in 3D. And so in order to do that, we needed to work somewhere that was already really, really well characterised. And this is why we chose Santorini, not because it's particularly dangerous, but because there's been so much work done there before, we have a really good understanding of the geology. So we're building off the back of science that's been done for decades by lots of other really, really good groups of scientists.

Chris Smith - When you say the fluid flows, is this water? Is this magma? Is it both?

Isobel - It's a combination of meteoric and seawater. So mostly at Santorini, seawater because it's an island. And then it's things that have come out of the magma chamber. So magma is this really interesting mix of different things. So it's melted rock, basically. But within that, we have what we call volatile. So these are things like CO2 and water. And they are in solution, they're part of a fluid, but they can start to exsolve, either as fluids or as bubbles of gas. And they mix with the water that's circulating through the volcano. So the hydrothermal fluids that we get out at the end are a combination of seawater and the things that have come out of the magma chamber. And then also because they're heating up, they tend to dissolve things out of the rocks as they come. So by the time they get to the seafloor, they're a real cocktail mixture of different things that we have to unpick.

Chris Smith - And is that how you unpick what is going on underground and underwater? You're able to use the gases and the compositions as a proxy for what must be happening inside the volcano?

Isobel - Yeah, that's exactly right. So we want to map where the fluid flow is going. So we do that using geophysical techniques. And then we want to measure what's actually in the fluid. So we sample the fluids from all of these different systems. There are lots of places on the seafloor where the fluid's coming out. And so we collect fluid samples from all of those different locations. And then we start looking at what's actually in there. So how much of the fluid is made up of things that have come from the magma chamber? How's that changing in different regions and at different times? And what does that tell us about what's actually going on in the volcano itself? These hydrothermal systems are kind of a window into those magma chambers.

Chris Smith  Is it almost like the heating system of your house where you've got a convection system going on? Cold water gets in, gets heated up inside the volcano in the magma chamber, gets a lot hotter, a lot less dense, comes flying out and draws in behind it cooler water. So it's a circulation going on all the time in a sort of stable way.

Isobel - Yes, pretty much exactly like that. I mean, it's a little bit more complicated in terms of how the fluids are travelling into and out of volcanoes. And we don't always understand that. But yeah, fundamentally, it's exactly the same.

Chris Smith - What's the danger then? Because as long as that stays stable, presumably things stay in the status quo. I suppose if something changes, then we might have a problem.

Isobel - Yeah, so there's this kind of paradox, this question of whether or not fluid flow makes volcanoes more or less dangerous. So the volatiles I mentioned in a magma chamber, so things like CO2 and water, they're also the things that drive volcanic eruptions. So if they start to exsolve in the magma chamber and form bubbles, you're reducing the density of that magma so it can start to rise towards the surface and cause an eruption. Now, if they can get out of that magma chamber really efficiently into your hydrothermal circulation, then you're unlikely to see a volcanic eruption because what you need to do is build the pressure up behind some kind of blockage. So in that way, hydrothermal systems may actually make volcanic eruptions less likely or less explosive, but they can create explosions in other ways. So if you mix fluids or water, all of a sudden with a very, very hot rock, you can generate explosions. So in those cases, you can generate sort of eruptions that are kind of like the eruption at Whakaari that happened in New Zealand. They're not really driven by the magma, they're driven by the explosive flashing of water to steam or a hypercritical fluid that causes an explosion. And then the other thing that can happen with fluid circulation is that those pathways which the bubbles from the magma chamber are getting out through, they can start to sort of solidify and block up. And so in that case, if you've got a hydrothermal system, particularly one that is cooling down or has changed pathways, then you might actually reduce the ability of the volatiles to escape and make sort of pressurisation a bit more easy for the volcano.