May 3, 2012 | 2
The first U.S. scientists and journalists arrived May 21, 1902 and soon researchers from the United Kingdom and France followed.
Just 13 days earlier the city of Saint-Pierre, on the Caribbean island of Martinique, had been annihilated by an unknown volcanic phenomenon. The geologists were baffled by the extant and pattern of the destruction inside the city, almost six kilometers distant to the summit of the active volcano Mount Pelée. The eruption killed estimated 30.000 people and destroyed almost all buildings in the city. However despite the damage and single eyewitnesses’ reports no lava flows – supposedly the main culprit of the devastation – were discovered.
The geologist Edmund Hovey of the American Museum of Natural History describes the scenery: “In many places the limit [of the devastation] passes on single trees, letting one side dark and burned, the other green as if an eruption never happened.”
Then, on July 9, the geologists Tempest Anderson and John S. Flett of the Royal Society of London observed and survived 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 as a turbulent mass….“
For the very first time geologists observed a deadly “nueé ardente” – an “incandescent cloud” or “glowing avalanche” as the phenomenon was later named by the French volcanologist Alfred Lacroix (1863-1948). A “nueé ardente“, today referred as pyroclastic density current, is a mixture of volcanic particles and hot gases that flows according to its density over long distances. Pyroclastic flows can originate from the collapse of parts of the cooling eruption column, from laterally blasts or from hot avalanches derived from the collapse of a lava dome or parts of a volcano.
After recognizing this peculiar volcanic phenomenon geologists became concerned: Why are pyroclastic density current so dangerous and how far do the deathly effects reach?
In the ruins of Saint-Pierre various hinds were observed that helped solve this mystery. Some bodies were horrible mangled, exploded from inside and their guts exposed on the outside. Other corpses were horrible burnt, but surprisingly the cloths covering the bodies were untouched. The heat inside the pyroclastic flow that destroyed Saint-Pierre was estimated based on observations of molten glass (melt temperature ca. 700°C) and unaltered copper tubes (melt temperature ca 1.100°C) found in the ruins between 700-1.000°C.
This intense heat burns the skin, the outer layers of the body shrink due the thermal impact and are torn apart and the inner organs were squeezed out. Other people in areas not directly hit by the 1.000°C hot flow inhale cooler gases (300°C), they suffocate on the ashes inside the lungs and were cooked from inside to the outside, letting the clothes intact.
Further evidence how pyroclastic currents kill was provided by the examination of an eruption that occurred almost 2.000 years ago.
In the year 79 A.D. the eruption of Mount Vesuvius destroyed the villages and cities surrounding the volcano, also the famous city of Pompeii.
The first human remains were discovered in Pompeii only two months after the beginning of systematic excavations, April 19, 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 the nearby village of Herculaneum 328 bodies, in Pompeii the known bodies until 2002 are 1.150, not considering some hundred bodies discovered earlier but later buried again or lost forever.
The early tourist 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 as 17 centuries; ... as I observed a French gentleman, when I saw him put a human bone in his pocket.“
Research published in 2010 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.
Fig.2. Simplified stratigraphy of the volcanic deposits in Pompeii correlated to the chronology of the destruction of the cities surrounding Mount Vesuvius. The eruption started with ash and tephra-fall (A1-A8), this continuous deposition was interrupted by a sequence of six distinctive pyroclastic currents or surges (S1 to S6) of 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 is only 3 centimeters thick, because it was the first surge to actually reach and cover the city, devastating an area of ca. 80 square-km. Volcanologist “Fitzgabbro” sheared some images of the various deposits at Pompeii and surrounding areas.
In Pompeii, within the ash beds of the early eruptive phase, 394 skeletons were found. 90% of these victims died within buildings, probably due to roof and floor collapse. Deposits of the later S4 surge preserved the remains of 650 persons, supposedly killed by the effects of the surge: heat and ash suffocation.
From these 93 plaster casts of the cavity left by the decomposed corpse, 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 bone 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 Oplontis 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, in Pompeii, due the cooler temperatures, the bodies remained intact and were preserved inside the volcanic sediments. After decomposition of the organic material a void remains, that today can be filled with plaster to form a cast of the unfortunate victims.
The published result questions some earlier assumptions, like the supposed main death cause of the people by ash suffocation. The new research indicates that heat was the main cause of death, the exposure to the at least 250°C hot surges at a distance of 10 kilometers from the volcano was sufficient to cause instant death and spasms (“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, the temperatures remained lethal up to the outer depositional limits of the pyroclastic flows.
These are important results and must be considered when hazard maps of the area surrounding a volcano are compiled. Only at Mount Vesuvius estimate 300.000 people live today within an area considered at high risk of pyroclastic flows during a larger, explosive eruption (the state agency I.N.G.V. released some videos of the computer simulations, showing the possible spread of pyroclastic clouds around Vesuvius).
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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
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