This blog is written by Pete Rowley, a volcanologist who pours sand down chutes to see what it does. Sometimes he’s allowed out to play with real volcanoes.
A churning grey cloud is bursting towards you. Just a few short minutes ago the volcano in the distance erupted, a dark plume surging into the sky. The cloud that had been going up soon started coming back down again – billows of ash and gas expanding and bursting through each other – and is now travelling down the side of the volcano at a speed you didn’t think possible. A pyroclastic density current (PDC) is on its way.
We have plenty of video footage and photographs of PDCs. Hollywood have even had some decent attempts at representing them (thank you Dante’s Peak). Countless examples of the rocks formed by these flows are preserved in the historic and geological record. The problem is we have never been able to gather any meaningful information from inside them; PDCs are way too destructive for that.
Pyroclastic density currents are the deadliest of volcanic hazards. 30,000 people were killed by PDCs during the 1902 eruption of Mt Pelee, Martinique; the city of Saint-Pierre was obliterated. In the 1990s the island of Montserrat was changed forever as the risk of PDCs led to evacuation of the capital city Plymouth, after which the reality of PDC devastation rendered it uninhabitable.
PDCs are made up of hot volcanic gas and ash, mixed with fragments of the volcano and bits of fresh pumice, all falling downhill to engulf whatever is below. But for all that we know about what they carry, and how they form, we still don’t really know what’s going on inside the flow. And their behaviour is weird! Unlike a landslide or a lava flow, they don’t always just come to a halt or pond when they reach a flat surface – they can keep going. For really large flows, that can mean travelling tens or even hundreds of kilometres from the volcano. PDCs don’t respect valleys a great deal either – there is sometimes enough speed and momentum for parts of the flow to travel uphill, overcoming tens or hundreds of meters of topography. Trying to understand this behaviour relies on understanding how the flow physically works, and that is not a simple problem.
If the process of understanding PDCs is complicated, interpreting their deposits is even more so. These volcanic sediments are hugely variable and can contain a range of structures and particle sizes. Deposits from a single PDC can change enormously in all three dimensions. What you find in one location will almost certainly change as you move towards or away from the volcano, across the deposit horizontally, or through the deposit vertically. In one place you might find beautiful cross beds – signs of dunes formed by the current as it travelled- while in others there may be nothing but a uniform blanket of pumice and ash. There could be very fine layers of sediment (known as ‘laminations’), or not. You might find that the size of pumice clasts gets bigger as you move up through the deposit. The deposit may get thicker. Or thinner. Or seem to have removed parts of the sediment below. Or thin out and disappear completely as you encounter places where the flow passed over without ever depositing anything.
Dark coarse grained sediment waves inside finer grained deposits from the 1980 Mt St Helens eruption
Making solids behave like liquids
Unlike the underwater currents that PDCs are sometimes compared with, the only fluid volcanic flows contain is gas. Everything else is solid particles of ash, pumice and rock. But that gas is important- it can be enough to make the flow mobile.
Magma has gas dissolved within it. During an explosive eruption hot magma fragments to form ash, and this gas is released. As well as this volcanic gas, cold air from the atmosphere is drawn into PDCs as they travel, where it heats and expands. The combined effect is enough gas pressure to stop solid particles hitting one another, a process we call ‘fluidisation’. And fluidisation is awesome.
Take a pile of sand. Bury your hand in it. You’re probably having a hard time pushing your fingers in. The grains don’t really want to move past each other. As you push your fingers in more they’re just compacting the material below, making it even harder to make progress. You’ve got sand jammed under your nails, and you’re looking at me wondering why I asked you to do this when we’re supposed to be talking volcanoes.
On the other side of you I have a nearly identical box of dry sand, but I’m pumping high-pressure air through it. I’m fluidising it. You lower your hand, and it drops effortlessly through the sand. It’s a bit like water, but you don’t feel the same pressure, and you don’t need a towel after you take your hand out. You put your hand back in and move it side to side, sending waves rippling around the sand box. The sand is behaving just like a liquid. Weird.
By injecting gas into the sand, we are simulating the effect of gas pressure inside a PDC. That means with a little bit of careful design we can start to recreate the behaviour of these odd flows in the lab.
Making it happen in the lab
In a lab, we can build special tanks called ‘flumes’ to experiment with how PDCs move and deposit material. Put simply, we release sand down a slope (as if it were volcanic material travelling down the side of a mountain) to see what it does. Our aim is to better understand processes that are almost impossible so to see in real time, in person. Many landslide experiments in similar setups involve pouring sand down a slope- but how can we recreate the gas-rich, fluidised flow of a PDC?
Making ‘PDC’ deposits in a laboratory flume
In our PDC experiments, we create tiny holes across the entire base of the flume and inject gas all the way along it as the flow passes over. We keep the current supplied with gas in the same way that the ash and pumice particles do in a real flow. And suddenly, we can see what is happening inside (something approximating) a PDC.
Even better – if we allow the gas pressure to disperse as the flow travels along the flume, the flow starts to deposit material. And now we can start to link the processes going on inside the flow to what these deposits look like. Understanding these links means we can start to look at real PDC deposits around active volcanoes and begin to pick apart what the flow that formed them may have been like.
While our experiments are beginning to explore the behaviours we see in PDC, and are even starting to capture some of the complexity in their deposits, we are still quite a long way from being able to accurately simulate the behaviour of any individual current. There are so many variables to grasp; temperature, grainsize, the rate of material being added to the flow, the slope angle, the valley shapes, the particle densities, what the ground surface is like… And how all those things vary in space and time as an eruption varies.
We may never get to a completely perfect simulation of how PDCs behave, but experiments are perhaps the key route to being able to explore different behaviours under different conditions, and hopefully – with time – can start to improve our understanding of the hazard these flows pose to the millions of people living on and near active volcanoes around the world.
Feature Image: Pyroclastic density current at Mt St Helens 7th August 1980
Credit: U.S. Geological Survey
Department of the Interior/USGS
U.S. Geological Survey/photo by P. Lipman