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Come morirono le vittime dell'eruzione del Vesuvio? Relazione scientifica.

How did the victims of the eruption of the Vesuvius die? E' il titolo di unarelazione scientifica di un gruppo di ricercatori di Napoli con cui viene spiegato in termini scientifici come avvenne realmente il decesso di migliaia di vittime della famosa eruzione del Vesuvio, definita "eruzione pliniana". Varie ipotesi si sono succedute nel tempo per fornire una spiegazione convincente ed esuriente del fenomeno noto in tutto il mondo per il numero delle vittime e per la rapida successione dei tempi, tanto da bloccare uomini e cose in una città fantasma ma viva nonostante tutto.

scavi

How did the victims of the Plinean Eruption of Vesuvius die?

Category: Archaeology

Posted on: June 27, 2010 12:15 PM, by Greg Laden

Even at the most extreme edges of the flow of stuff out of the volcano Pompeii, at the far edge of the mud and ash that came from the volcano’s explosion, the heat was sufficient to instantly kill everyone, even those inside their homes.

And that is how the people at Pompeii, who’s remains were found trapped and partly preserved within ghostly body-shaped tombs within that pyroclastic flow, died. They did not suffocate. They did not get blown apart by force. They did not die of gas poisoning. They simply cooked. Instantly.

That is the conclusion of a study just published in PLoS ONE by Giuseppe Mastrolorenzo, Pierpaolo Petrone, Lucia Pappalardo and Fabio Guarino, entitled Lethal Thermal Impact at Periphery of Pyroclastic Surges: Evidences at Pompeii.

This research was carried out specifically to address the question of survivability of certain kinds of volcanic eruptions. When this type of volcano explodes, a mixture of hot gas and fine ash flow at very high speed outwards and down slope, causing widespread devastation. It is generally assumed that people would not survive within the pyroclastic flow zone, but the question remained open as to whether people would survive at the outermost edge, where they may not be impacted directly by the flow itself.

… the evaluation of PDCs (pyroclastic flow) effects near the flow termination is widely debated being crucial for risk mitigation management, particularly for distal areas often densely inhabited. Nevertheless, these aspects are documented only in a few modern eruptions and difficult to retrieve from historical record, due to scarce availability of data on thin distal layers.

And thus, Pompeii becomes an important data point. At Pompeii, the remains of hundreds of individuals were found in the pyroclastic deposits from the famous 79 AD eruption, the one documented by Pliny the Younger and memorialized in countless books, a few movies, and a museum exhibit or two.

The study modeled the physics of the eruption, tying a model to the ground with the physical and archaeological data. For context:

The 79 AD Vesuvius eruption generated a sequence of six distinctive pyroclastic surges (S1 to S6) and flows with increasing power, which caused landscape modification as well widespread building collapse and fatalities [3]. The resulting ash deposits have thicknesses ranging from tens of metres near the vent to few millimetres at the flow periphery. In particular, the early three surges (S1 to S3) stopped ahead the northwestern walls of Pompeii, while the later ones (S4 to S6) over passed the town. The last two surges traveled up to a distance even exceeding 15 kilometres from the vent, whereas the S4 surge deposit least traces are confined within a few hundred of metres next to the south and southeastern walls.

The S4 surge, which deposited only a small blanketing of ash at the site, probably caused most of the fatalities in Pompeii. The 3 cm or so thick layer of ash was "emplaced suddenly in a single depositional event resulting from dusty gas mixture deflation in response to horizontal velocity and turbulence dumping at the flow termination."

In other words, it was like "phooofff" and everybody was dead and this dusting of ash was everywhere. And I don’t really mean to be flip about this … it is hard to imagine what is being described by this analysis in real terms.

Consider this: Imagine an area about 10 kilometers (just over 6 miles) around the volcano. Now imagine that area being covered with a blanket of very hot ashy air over the course of about 3 minutes. To help imagine this, pretend you are looking at the volcano when the explosion starts, and there is an airliner over the volcano heading for you at the same time. It is moving at top speed for such an aircraft, and when it reached you it has slowed to just over 100 mph. That represents the leading edge of the blanket of hot air and ash. Just at that moment, the blanket of air and ash deflates/dissipates, the air cooling and the ash settling. But first, all humans, probably all tetrapods (birds, mammals, etc) within that few miles of space have simply dropped dead. Since there is only a little ash, they are dirtied by it, but later, a larger deposit of ash is spewed out, and now all the dead are deeply buried. Or at least, that’s what I’m getting from this paper, more or less.

… most of the victims are typically frozen in suspended actions (73% life-like stance, 27% sleep-like stance), showing as well as limb contraction (76%) and a large number of corpses presenting the pugilistic attitude (64%).
That "pugilistic attitude" bit is the appearance that the victim was fighting for his or life, but which actually occurs as a side effect of being cooked very quickly (or drying out more slowly in some cases). Everyone was killed and their bodies frozen in the position they were in at that time of instant death, plus a small bit of involuntary perimortem spasm. They were not suffocated, they were not killed by being blasted with an explosive force that knocked them down. They were cooked in situ.

Finally, contrary to previous hypotheses, our findings based on the interdisciplinary volcanological and bio-anthropological study of the deposits and victims of the 79 AD Plinian eruption reveal that even at the extreme periphery of the S4 surge neither asphyxia nor impact force but heat caused the deaths. Actually, while impact force and exposure time to dusty gas dropped below lethal conditions, the pyroclastic cloud retained its high temperature thus being the main cause of instantaneous mortality for the Vesuvius area inhabitants, including people who were sheltered within buildings as far as in Pompeii.

The temperature was 250 to 300 degrees C, and the time of exposure was about 30 seconds or so. Death would have been during the first few seconds of that time. That temperature is enough to light gasoline and other fuels under certain conditions, but not enough to ignite clothing.
So, now you know how they died.

Mastrolorenzo, G., Petrone, P., Pappalardo, L., & Guarino, F. (2010). Lethal Thermal Impact at Periphery of Pyroclastic Surges: Evidences at Pompeii PLoS ONE, 5 (6) DOI: 10.1371/journal.pone.0011127

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COMMENTO

Se fra i lettori c’è chi è in grado di tradurre e comprendere il testo di questa relazione, frutto di un lavoro di ricerca a cura di un gruppo di esperti vulcanologi, geofisici, archeologi, troverà sicuramente tutti gli elementi utili per una comprensione finalmente chiara e accettabile sotto il profilo scientifico di ciò che avvenne in questa sfortunata città 2000 anni fa.

Fra i vari numerosi tentativi di offrire al mondo una spiegazione di ciò che realmente accadde nel 79 d.C. con una ricostruzione molto fedele dei fatti, devo ammettere che qualche anno fa ha fatto testo un documentario televisivo quanto mai efficace e rispettoso dei fatti presentato da A. Angela della serie “Il piacere della scoperta”. Ma questa relazione ha il merito di aver affrontato i fatti da un punto di vista rigidamente scientifico, tanto da presentarsi più che come una semplice ipotesi, una diagnosi rigidamente medico-scientifica relativa al decesso immediato delle vittime.

Non manca una serie di immagini filmati presentateci grazie a YouTube in cui la ricostruzione storica dell’evento si concilia abbastanza bene con gli effetti devastanti dell’eruzione e con il clima di terrore che vissero gli abitanti in fuga.

Questi gli autori della ricerca, con i loro titoli accademici e l’indicazione degli istituti di appartenenza:
Giuseppe Mastrolorenzo*, Pierpaolo Petrone, Lucia Pappalardo, Fabio M. Guarino .
-1 Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli, Osservatorio Vesuviano, Napoli, Italy,
-2 Museo di Antropologia, Centro Musei delle Scienze Naturali, Università degli Studi di Napoli Federico II, Naples, Italy,
-3 Dipartimento di Biologia Strutturale e Funzionale, Università degli Studi di Napoli Federico II, Naples, Italy.

NOTA BENE

1. Questo è il link, presente nell’articolo di cui sopra, che vi porta direttamente al testo originale della ricerca. Ovviamente il testo è in lingua inglese perché indirizzato alla comunità scientifica internazionale.

2. Questo link vi porta a uno dei tanti film che hanno come soggetto la fine di Pompei. Di sicuro uno dei più riusciti per il realismo delle scene.

FILM

Intervista con gli studiosi che hanno contribuito a ricostruire nei minimi dettagli ciò che avvenne il 24 agosto del 79 d.C. Qui di seguito le varie fasi di questa interessantissima intervista, nella quale vengono messi in rilievo i rischi connessi con le costruzioni abusive sorte lungo i fianchi del Vesuvio, fra le quali, sembra un paradosso, un ospedale in piena zona rossa!

il Vesuvio è un pericolo sottovalutato