Worth a Thousand Words

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Volcanic eruptions have been in the news quite a lot lately, what with Europe’s air traffic getting disrupted by the one in Iceland, and other smaller eruptions happening in other places as well. If you are not a professional volcanologist, it is likely that you have heard at least about the most famous volcanic eruption in history – the 79 AD eruption of Vesuvius that obliterated the city of Pompeii. I remember how fascinated I was with it when I was a kid and first read about it. What was most fascinating about that whole episode is how quickly people (and animals, like horses) died, even if they were miles away from the volcano itself. The death was so sudden, that most of the victims died in mid-movement and they were preserved in these eerie postures, a moment in time in the life of a city, frozen. You may have seen pictures (or if you are lucky, you visited Pompeii) like this:

One thing I remember from the time I read about this as a kid is the cause of death – suffocation with ash. And while some of the people died in poses that indicate suffering, others are captured in seemingly ordinary movements, as if they died of something much more swift than minutes of suffocating agony. And now, those suspicions I had have been resolved.

In last week’s PLoS ONE article Lethal Thermal Impact at Periphery of Pyroclastic Surges: Evidences at Pompeii, authors Giuseppe Mastrolorenzo, Pierpaolo Petrone, Lucia Pappalardo and Fabio Guarino of Napoli, Italy, performed a detailed analysis of volcanic rock and ash, as well as bones of human and equine victims of the eruption. And their conclusion is that people and animals died of high temperature – Vesuvius had six flashes of heat that suddenly heated up the air as high as 250 degrees Celsius (482 degrees Fahrenheit) at the distance of 10 kilometers from the mountain – hot enough to instantly kill.

Here is the Abstract and you can read the details of the paper itself as well:

Background

The evaluation of mortality of pyroclastic surges and flows (PDCs) produced by explosive eruptions is a major goal in risk assessment and mitigation, particularly in distal reaches of flows that are often heavily urbanized. Pompeii and the nearby archaeological sites preserve the most complete set of evidence of the 79 AD catastrophic eruption recording its effects on structures and people.

Methodology/Principal Findings

Here we investigate the causes of mortality in PDCs at Pompeii and surroundings on the bases of a multidisciplinary volcanological and bio-anthropological study. Field and laboratory study of the eruption products and victims merged with numerical simulations and experiments indicate that heat was the main cause of death of people, heretofore supposed to have died by ash suffocation. Our results show that exposure to at least 250°C hot surges at a distance of 10 kilometres from the vent was sufficient to cause instant death, even if people were sheltered within buildings. Despite the fact that impact force and exposure time to dusty gas declined toward PDCs periphery up to the survival conditions, lethal temperatures were maintained up to the PDCs extreme depositional limits.

Conclusions/Significance

This evidence indicates that the risk in flow marginal zones could be underestimated by simply assuming that very thin distal deposits, resulting from PDCs with poor total particle load, correspond to negligible effects. Therefore our findings are essential for hazard plans development and for actions aimed to risk mitigation at Vesuvius and other explosive volcanoes.

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