Investigating the Taphonomy of Volcanic eruptions: How volcanoes kill

The first scientists and journalist arrived on the island on May 21., on board of the American vessel “Dixie”, researchers from the United Kingdom and France soon followed.

13 days before the city of Saint-Pierre had been destroyed by a volcanic eruption. The geologist were baffled by the extant and pattern of the destruction - the eruption killed 30.000 people and destroyed the entire city - but despite the damage and eyewitnesses reports no signs of a lava flow were discovered.
The amount of destruction decreased gradually or suddenly, tand he geologist Edmund Hovey of the American Museum of Natural history claimed: "In many places the limit passes on single trees, letting one side dark and burned, the other green as if an eruption never happened."
Then, on the 9th of July, the English geologists Tempest Anderson and John S. Flett of the Royal Society of London became eyewitnesses of the true phenomenon that destroyed the city:

“The cloud had a spherical form and resembled rounded protuberances amplifying and doubling with terrifying energy. They extended to the sea, in our direction, boiling and changing shape in every moment. It didn’t spread laterally. It didn’t rise up in the atmosphere, but it descended on the sea in a turbulent mass interspersed by thunderbolts”

For the first time geologists observed a deadly “nueé ardente” as later the phenomenon was called by the French volcanologist Alfred Lacroix (1863-1948), or pyroclastic flow. According to later observations of molten glass (melt temperature 700°C) and unaltered copper tubes (melt temperature) found in the ruins of the city the temperature of the pyroclastic flow was estimated to 700-1.000°C.
The deathly factor of a volcanic eruption differs considerable with the kind of eruption and the distance to it. The most dangerous eruption are surely of explosive or phreatomagmatic nature. Pyroclastic density currents, turbulent hot mixtures of fine ash and gas flowing down volcano slopes at high speeds, are common in volcanic explosive eruptions. They can devastate large areas and cause numerous fatalities by exposure to mechanical impact, extreme heat and dusty gas.
To understand how exactly these phenomena act and how far their deathly effects reach is essential to plan and improve risk mitigation management.
A research published by MASTROLORENZO et al. 2010 address these questions using one of the most popular cases of a deadly eruption associated with pyroclastic event: the eruption of Mount Vesuvius in the year 79 A.D, that destroyed the villages and cities surrounding the volcano, the greater one being Pompeii.

Fig.1. Simplified stratigraphy of the vulcanic deposits in Pompeii correlated to the chronology of the destruction of the cities surrounding Mount Vesuvius. The 79 AD Vesuvius eruption generated a sequence of six distinctive pyroclastic surges (S1 to S6) and flows with increasing power, which caused widespread building collapse and fatalities.
The pyroclastic surge S4 caused most of the fatalities in Pompeii, even if the resulting deposit are only 3 centimetres thick, because it was the first surge to actually reach and cover the city, devastating an area of ca. 80 km2.

The first human remains were discovered in Pompeii only two months after the beginning of systematic excavations, on 19. April 1748 at the crossing of Via Stabia and Via Nola. During the more or less scientific motivated excavation campaigns in the following centuries further human and animal remains were discovered, in Herculaneum (destroyed by an Lahar) 328 bodies, in Pompeii the known bodies until 2002 are 1.150, not considering some hundred bodies discovered during the centuries, but later buried for reverence or lost.
Hester Lynch, visiting Pompeii in 1786 remembers: "some people would take away some parts, as I did to possess in my little museum a bone older then 17 centuries; .[].. as I observed a French gentleman, when I saw him put a human bone in his pocket.”

Fig.2. Discovery of human remains during the visit of the emperor Giuseppe II in Pompeii, Jean-Claude Ricard de Saint-Non (1781-1786) (image from DE CAROLIS & PATRICELLI 2003).

The new research compared artificially heated recent bones with bones recovered from the surge deposits of Pompeii. Also the documented position of bodies in relation to the stratigraphy of ash and surge deposits was considered.
In Pompeii, within the lapilli bed were found 394 skeletons of victims of the early fallout eruptive phase, 90% of whom died within buildings probably due to roof and floor collapse. Deposits of the later S4 surge preserved the remains of 650 victims heretofore supposed to have died by ash suffocation. From these 93 plaster casts, in addition to 37 corpses from Oplontis (a seaside suburbia site) and 78 skeletons of Herculaneum, were classified in a scheme considering the posture of the corpse, for example life-like when showing an apparent "freezing" in the act of movement - most bodies were found in such a posture (73%).

The damage on the ancient bones, showing micro-cracks on the surface and recrystallistaion of the interior structure, are signs of thermal modification. Comparing these bones with the observations made on modern bones, heated in experiments, the researchers were able to determinate a temperature range inside the surge that killed the people, at least 500-600°C at Oplonis and Herculaneum, and 300-250°C at Pompeii.

The temperature at Herculaneum and Oplontis was enough to vaporize the flesh of the victims, so that the ash could embed the skeletons, where in Pompeii the bodies remained intact inside the volcanic sediments. After decaying of the organic material a void remains, that today can be grouted with plaster to form a cast.

The published result questions some earlier assumptions, like the supposed main death cause of the people by ash suffocation. The new study indicate that heat was the main cause of death, the exposure to the at least 250°C hot surges at a distance of 10 kilometres from the vent was sufficient to cause instant death and spasm ("freezing" the people’s movement), even if people were sheltered within buildings.
Despite the fact that impact force and exposure time to dusty gas of the pyroclastic flow declined toward the periphery of the surge, theoretically improving survival possibilities, lethal temperatures were maintained up to the extreme depositional limits of the flow.

References:

DE CAROLIS, E. & PATRICELLI, G. (2003): Vesuvio 79 d.C. la distruzione di Pompei ed Ercolano. L´ERMA di BRETSCHNEIDER: 129
GIACOMELLI, L.; PERROTTA, A.; SCANDONE, R. & SCARPATI, C. (2003): The eruption of Vesuvius of 79 AD and its impact on human environment in Pompeii Episodes. Vol. 26, No. 3
LEWIS, T.A.(ed) (1985): Volcano (Planet Earth). Time-Life Books: 176
LUONGO, G.; PERROTTA, A. & SCARPATI, C. (2003): Impact of the AD 79 explosive eruption on Pompeii, I. Relations amongst the depositional mechanisms of the pyroclastic products, the framework of the buildings and the associated destructive events. Journal of Volcanology and Geothermal Research 126: 201-223 doi:10.1016/S0377-0273(03)00146-X
LUONGO, G.; PERROTTA, A.; SCARPATI, C.; DE CAROLIS, E.; PATRICELLI, G.; CIARALLO, A. (2003): Impact of the AD 79 explosive eruption on Pompeii, II. Causes of death of the inhabitants inferred by stratigraphic analysis and areal distribution of the human casualties. Journal of Volcanology and Geothermal Research 126: 169-200 doi:10.1016/S0377-0273(03)00147-1
MASTROLORENZO, G.; PETRONE, P.; PAPPALARDO, L. & GUARINO, F.M. (2010): Lethal Thermal Impact at Periphery of Pyroclastic Surges: Evidences at Pompeii. PLoS ONE 5(6): e11127. doi:10.1371/journal.pone.0011127