This article originally appeared on Changing the Climate, a blog written by postgraduate students in Earth Sciences at Oxford.

Among the many hazards posed by subduction zone volcanoes, some of the deadliest and most viscerally terrifying are pyroclastic density currents (PDCs), ground-hugging clouds of gas and fragmented magma (ash) that can reach temperatures of 1000°C. The very largest can reach speeds of up to 700 km/h and typically travel up to 15km away from the volcanic crater. There are two main types of PDC, an ash rich current, known as a pyroclastic flow, or a gas-rich current, known as a pyroclastic surge.

Both types form in various ways. An explosive eruption may ‘boil over’ the crater rim, or an eruption column may become so dense that it collapses, producing continuous flows. More short-lived, discrete flows can be generated during ‘lateral blasts’ or when lava domes in the crater disintegrate, either by gravitational collapse or by an explosion. This last process is thought to have caused the most infamous and deadly volcanic event in the 20^th Century, the 8^th May 1902 eruption of Mont Pelée, on the island of Martinique in the Caribbean.

All told, around 28,000 people were in the city of Saint Pierre, only 6km away from Mont Pelée, when it erupted. Within three minutes of the initial explosion a large black pyroclastic flow rolled down the Riviere Blanche valley on the south flank of the volcano. It was directed towards Saint Pierre by a natural V-shaped notch in the crater, and hit the city at over 160 km/h. The impact was forceful enough to dislodge and transport a three-tonne statue, reduce a cathedral to only its foundations, and to capsize a steamship when it reached the harbour. This one event makes up just under half of the 60,000 recorded fatalities caused by PDCs since 1500 AD.

Those that explored the ruined city in the aftermath of the eruption were in for some gruesome sights. Many of the corpses found in the open were lying prone in the direction the flow had travelled in, often with flexed limbs as their muscles had contorted in temperatures of over 200°C. Many corpses were noted to have been shrunken and desiccated from the heat of the flow and the remaining ash. Amazingly, two people from within Saint Pierre managed to survive and give first-hand accounts of the experience. Leon Compere-Leandre, a shoemaker who lived in a house at what would be the margin of the pyroclastic flow, described the disaster:

“I felt a terrible wind blowing, the earth began to tremble, and the sky suddenly became dark. I turned to go into the 
house, with great difficulty climbed the three or four steps that separated me from my room, and felt my arms and legs 
burning, also my body. I dropped upon a table. At this moment four others sought refuge in my room, crying and writhing 
with pain, although their garmets (sic) showed no sign of having been touched by flame.”

Another man, Louis-August Cyparis, had been jailed in an underground cell the night before the eruption, and although he experienced terrible burns to his hands, arms, legs, and back, he managed to survive long enough to be rescued four days later. He went on to recount his tale many times as part of a circus group as “The Man Who Survived Doomsday”. Outside of Saint Pierre, as many as 123 people survived after being rescued from ships in the harbour and the edges of where the pyroclastic flow had struck. These survivors were the ones who had only suffered from external burns to the skin. A further 40, who had also suffered internal burns to the upper respiratory tract, survived the event but did not make it beyond the hospital.

As extraordinary as these accounts are, the 1902 Mont Pelée eruption was only the first of at least seven eruptions in the 20^th Century to record people surviving encounters with this most deadly of volcanic phenomena. In the present day, where 29 million people live within 10km of an active volcano, using these records to understand where and how people can survive PDCs is vital information for focussing disaster response efforts. This is particularly important for populations on small volcanic islands, or for densely populated urban areas around a volcano.

The first autopsies of fatalities from pyroclastic flows were conducted after the 18^th May 1980 eruption of Mt St Helens, USA. In combination with the historical accounts and medical experiments since, the results of these have been used to determine the critical factors that control whether a pyroclastic flow is survivable.

One of the first medical experiments was conducted in the 1950s, in order to find out what temperatures a human can handle for a certain length of time. In a somewhat dubious sounding set up, the researchers exposed their clothed forearms to a 600°C radiator until “unbearable pain occurred”, and concluded that humans could survive temperatures of 200°C in dry, unmoving air for 2-5 minutes. Fire safety researchers in the 1990s also found that humans can even tolerate breathing dry, particle-free air at these temperatures for a few minutes.

But how representative of a PDC are these conditions? While the 2-5 minute timescale agrees with the general duration of PDCs reported by survivors, and the temperature is reasonable for the margins or the tail end of a current, we know that they could not possibly be described as ‘unmoving’. The speed at which the current travels is important because, in the same way that a windy day will feel colder than the actual air temperature, faster flows will impart more heat to the skin per unit of time and cause more thermal injury. This can be exacerbated by high concentrations of water vapour in the flow, which can reduce the survivable temperatures to just 50°C.

Additional heat flux can also result from skin contact with, or inhalation of, ash. Because pyroclastic flows are essentially fragmented magma, they tend to be laden with crystals and very small, sharp, and hot pieces of volcanic glass, in concentrations far exceeding that thought necessary to burn and block the trachea and prevent breathing.  Even if the particle concentration isn’t high enough to obstruct the airways directly, the lasting damage to alveoli (the fine structures in the lungs where exchange of oxygen and carbon dioxide takes place) from ‘respirable’ particles (<10 µm in size) is a frequent cause of death in those who survived the immediate impact of the flow. If that respiratory danger wasn’t enough, PDCs are typically free of oxygen, and can instead contain asphyxiating gases such as hydrogen sulphide, sulphur dioxide, and carbon dioxide. Thermal injury and asphyxiation from a number of these factors are thought to have caused 90% of the 1,565 deaths caused by the eruption of La Soufriere, St Vincent that occurred within hours of the eruption at Mont Pelée. The other 10% of deaths were mostly attributed to roofs collapsing from the weight of ash, or from fires.

So what’s the best way to try and survive a pyroclastic density current? The common thread in accounts of previous eruptions is that the few survivors in each event were typically healthy adults who found robust shelter. If a PDC has travelled far enough to lose a significant amount of its (building-destroying) energy, then seeking shelter indoors offers the best chance of survival. As long as a room, cellar, or the building itself can be effectively isolated from any influx of ash or asphyxiating gases, it’s likely that anyone inside would mostly be affected by the rapid heating of the air in the room. Someone in a poorer-quality building or caught outside at the same place and time is going to have a much lower chance of survival, even though the marginal reaches of a flow are where rescue efforts would be focussed.

There is, however, one very sure-fire, volcanologist-approved way of surviving a pyroclastic density current. And that is to heed evacuation warnings when given. It can’t get you if you’re not there to be got.

References and Further Reading:

drvitelli.typepad.com/providentia/2011/09/when-mount-pelee-exploded-part-2.html Contains the account of Leon Compere Leandre, one of the survivors of the Mont Pelée eruption.
www.geology.sdsu.edu/how_volcanoes_work/Pelee.html Descriptions of events before, during, and after the Mont Pelée 1902 eruption.

Baxter, P. J. (1990) Medical effects of volcanic eruptions: I. Main causes of death and injury. Bulletin of Volcanology, 52, 532-544. link.springer.com/content/pdf/10.1007%2FBF00301534.pdf

Baxter, P. J., Neri, A. & Todesco, M. (1998) Physical Modelling and Human Survival in Pyroclastic Flows. Natural Hazards, 17, 163-176. link.springer.com/content/pdf/10.1023%2FA%3A1008031004183.pdf

Brown, S. K., Jenkins, S. F., Sparks, S. J., Odbert, H. & Auker, M. R. (2017) Volcanic fatalities database: analysis of volcanic threat with distance and victim classification. Journal of Applied Volcanology, 6: 15. link.springer.com/article/10.1186/s13617-017-0067-4

Buettner, K. (1950) Effects of Extreme Heat on Man. Journal of the American Medical Association, 144, 732-738. jamanetwork.com/journals/jama/fullarticle/297251

Hansell, A. L., Horwell, C. J. & Oppenheimer, C. (2006). The health hazards of volcanoes and geothermal areas. Occupational & Environmental Medicine, 63, 149-156. www.ncbi.nlm.nih.gov/pmc/articles/PMC2078062/

Mastrolorenzo G, Petrone P, Pappalardo L, Guarino FM (2010) Lethal Thermal Impact at Periphery of Pyroclastic Surges: Evidences at Pompeii. PLoS ONE 5(6): e11127. journals.plos.org/plosone/article?id=10.1371/journal.pone.0011127