Lethal Thermal Impact at Periphery of Pyroclastic Surges: Evidences at Pompeii

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 250uC 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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