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Lethal thermal impact at periphery of pyroclastic surges: evidences at Pompeii - PubMed

  • ️Fri Jan 01 2010

Lethal thermal impact at periphery of pyroclastic surges: evidences at Pompeii

Giuseppe Mastrolorenzo et al. PLoS One. 2010.

Abstract

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 degrees 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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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Numerical simulation of S4 PDC.

Areal distribution and dynamic overpressure at Vesuvius of a pyroclastic surge cloud that is an analogue to the 79 AD upper main surges. Initial velocity = 50÷100 m/s, thickness = 30÷130 m, density = 2÷50 kg/m3, viscosity = 2×10 −5 Pa/s, yield strength = 0 Pa (scale bar values in KPa). The numerical simulation is based on a simple model of a gravity-driven pyroclastic current that stops by en masse freezing.

Figure 2
Figure 2. Typical body postures assumed by human victims in PDCs at Pompeii.

“Life-like” stance: a1. infant and a2. adult female (House of the gold bracelet, Regio VI, Insula 17 [Insula Occidentalis], 42), b. adult male (The garden of the fugitives, Regio I, Insula 21), c. adult male (The Great Palaestra, Regio II, Insula 7); “sleep-like” stance: d. adult male (The garden of the fugitives, Regio I, Insula 21); “limb contraction” stance: e1. child (The garden of the fugitives, Regio I, Insula 21) and e2. left foot adult male (outdoor victim, XIX sec. findings); “pugilistic attitude” stance: a3. child (House of the gold bracelet) and f. adult male (outdoor victim, XIX sec. findings). Examination of the corpses' posture suggests the complete absence of any mechanical effect and an instantaneous death followed by sudden muscles contraction (cadaveric spam) due to the heat-shock induced by the PDC, as also testified by hyperflexion of hands and feet toes (flexor reflex) (e1, e2).

Figure 3
Figure 3. Human victims of the 79 AD eruption at Pompeii and Oplontis.

a. Cast of adult woman, part of 13 human victims died outdoor (Pompeii, Garden of Fugitives) (14), possibly showing some evidence of minor mechanical impact; b. cast of adult male, part of a group of 21 victims found outdoor (Pompeii, Porta Nola), with evidence of exposure to high temperature typical of fire victims or lethality in PDCs; c. the group of human victims found in the Villa B at Oplontis, partially just skeletons and to some extent casts.

Figure 4
Figure 4. Thermal modifications in human victims bones and in recent human bones heated in laboratory.

Adult bone victims analyzed with a light microscope (scale bar 100 µm) and a scanning electron microscope (scale bar 10 µm, 1700×): Femur from Pompeii showing linear cracking (a1) and an intact ultrastructure (a2); radius from Herculaneum showing both linear and polygonal cracking (b1) and incipient recrystallization (b2); fibula from Oplontis characterized by extreme polygonal cracking (c1) and advanced recrystallization (c2). SEM images of recent adult human hand phalanx heated to 200°C (a3), 500°C (b3) and 800°C (c3) (scale bar 5 µm, 2500×).

Figure 5
Figure 5. Cryostat cross-sections of decalcified ancient and heated recent human and horse bones stained with thionin.

a1. Pompeii femur, adult; a2. Herculaneum humerus, adult; a3. Oplontis femur, adult (scale bar, 30 µm). Recent adult human phalanx exposed at 150°C (b1), 200°C (b2), 250°C (b3) and 300°C (b4) (scale bar, 50 µm); recent adult horse (Equus caballus) femur exposed at 150°C (c1), 200°C (c2), 250°C (c3) and 300°C (c4) (scale bar, 28 µm). In the Pompeii sample (a1), some osteocyte nuclei and cement lines are clearly visualized. In Herculaneum (a2) and Oplontis (a3) osteocyte nuclei are not recognizable, while microstructural damage consisting in marked linear and polygonal cracking is apparent. In both stained series of heated recent human (b1–b4) and horse (c1–c4) bone samples, DNA is clearly evident in osteocyte lacunae up to 250°C and likely persistent up to 300°C.

Figure 6
Figure 6. Colour features of recent human bones (adult phalanx) heated in laboratory from 100°C to 800°C.

a1. 100°C, natural bone colour, light grey 25YR 8/1; a2. 200°C, pale yellow 25YR 8/3; b1. 300°C, bright brown 2.5YR 5/8; b2. 400°C, reddish black 2.5YR 1.7/1; c1. 500°C, black N 2/0; c2. 600°C, dark grey N 3/0 and pale reddish 2.5YR 7/3; d1. 700°C, brownish grey 7.5Y 4/1 and 5/1; d2. 800°C, grey N 6/0 and greyish white N 8/0 (scale bar, 2.0 mm). Bone colours are based on Munsell (1954) soil colour chart (for details on rates, limits and time of exposure see tab. S2).

Figure 7
Figure 7. Cross ground sections of undecalcified recent human and horse bone samples heated in laboratory.

Adult phalanx samples after exposure at 200°C (a1) and 300°C (a2). At 200°C the bone microstructure is well preserved, with some linear microcracking, while at 300°C it shows an evident pattern of linear as well as moderately polygonal cracking. Recent adult horse femur exposed at 200°C (b1), 300°C (b2), 400°C (b3) and 500°C (b4). At 200°C and 300°C, the well preserved bone histology shows linear microcracking. At 400°C, the bone matrix reveals an high degree of polygonal cracking and partial vanishing of the lamellar structure. At 500°C, only osteons and osteocyte lacunae are still visible, while lamellae are completely vanished. After 500°C, the increased friability of bone samples resulting in the rapid disintegration of specimen on the grinding plate did not allow to further obtain ground sections (scale bar, 100 µm).

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