Written by Alan Barker, Freelance Writer, British Science Festival 

Rebecca Williams is a volcanologist and the recipient of this year’s Charles Lyell Award Lecture for Environmental Sciences at the British Science Festival. In her talk, Deadly Clouds and Volcanic Flows, she will explain how her work on pyroclastic density flows might help to save lives when volcanoes erupt. Alan Barker put on his hard hat to learn more. 

Pyroclastic density currents sound pretty serious. What are they?

They’re the currents that swoop down volcanoes during big eruptions. They killed the citizens at Pompeii, and they were responsible for 30,000 deaths on Martinique when Mount Pelée erupted in 1902. In fact, apart from volcanic tsunamis, pyroclastic density flows are the biggest killers in volcanic eruptions.

We’ve known about these currents for a long time - Pliny the Younger wrote about them – yet we don’t actually know very much about how they behave. If you’re close enough to observe them in great detail, then you’re not likely to survive to tell anyone about it. So we have to try to look at them from afar, or by looking at the rock record. But the flows are surrounded by clouds of dust and ash, so we can’t see what’s going on inside them; and the rock record only gives us a static snapshot of these deposits. We need to understand how these flows behave, so that we can estimate danger more effectively.

How does your work contribute to this effort?

I’ve developed a new technique for looking at these deposits. I was able to track, for one particular volcano, how the current moved across the landscape, how it evolved through time and space.

Where was the flow that you were studying?

A beautiful Italian island called Pantelleria – it’s probably one of the least famous Italian volcanoes. I could say you should go there on holiday, but that would ruin it for those of us who want to keep it a secret!

And why did you choose this volcano?

Two reasons. First, the currents that formed this deposit were super-hot: when the ash and the material was deposited on the landscape, it welded back together to form volcanic glass. So the deposit was really hard and really well preserved, all over the landscape, including slopes up to 90 degrees. They’re among the best exposure of this kind of deposit anywhere.

And the second reason is that the eruption was tapping a magma reservoir that was chemically zoned. Magma – the stuff that forms lava – has a particular chemical signature. In this eruption, the chemical signature changed from bottom to top of the magma chamber. When the eruption evacuated the magma chamber, the chemical signature of the deposit changed over time. And that’s what I used to track the current as it evolved across the landscape.

And what did you discover about those changes?

In its early stages, the current was really feeble. It got to a 15-metre hill and was easily deflected. But then, through time, the current grew in size and force; it was able to swoop over hills several hundred metres high. So these currents don’t just go everywhere all at once, which is what we used to think; they can change their flow paths very quickly. We can feed that understanding into future models of eruptions, so that – hopefully – we can predict their behaviour better.

But how typical was the eruption that you studied?

That’s very much part of the problem. Every volcano has its own heartbeat. We’re trying to look at fundamental behaviours in these currents, to see whether they’re common to other flows. I’ve also moved into analogue modelling, running experiments in the lab to see whether we can replicate the same behaviours that I’ve seen in the field. That helps you find more generic behaviours, which you can then put into the computer models and simulations to generate better predictive tools.

Deadly Clouds and Volcanic Flows is on Thursday 7 September at 14.30.  Book tickets on the British Science Festival website.