The eruption of Mount Vesuvius in 79 AD was one of the most catastrophic volcanic eruptions in European history. Historians have learned about the eruption from the eyewitness account of Pliny the Younger, a Roman administrator and poet.It is the namesake for Plinian eruptions.
Mount Vesuvius spewed a deadly cloud of volcanic gas, stones, and ash to a height of 33 kilometres (21 mi), ejecting molten rock and pulverized pumice at the rate of 1.5 million tons per second, ultimately releasing a hundred thousand times the thermal energy of the Hiroshima bombing. Several Roman settlements were obliterated and buried underneath massive pyroclastic surges and ashfall deposits, the most well known being Pompeii and Herculaneum
Reconstructions of the eruption and its effects vary considerably in the details but have the same overall features. The eruption lasted for two days. The morning of the first day, August 24, was perceived as normal by the only eyewitness to leave a surviving document, Pliny the Younger, who at that point was staying at Misenum, on the other side of the Bay of Naples about 30 kilometres (19 mi) from the volcano, which may have prevented him from noticing the early signs of the eruption. He was not to have any opportunity, during the next two days, to talk to people who had witnessed the eruption from Pompeii or Herculaneum (indeed he never mentions Pompeii in his letter), so he would not have noticed early, smaller fissures and releases of ash and smoke on the mountain, if such had occurred earlier in the morning. Around 1:00 p.m., Mount Vesuvius violently exploded, throwing up a high-altitude column from which ash began to fall, blanketing the area
According to a stratigraphic study (a study of the layers of ash) by Sigurdsson, Cashdollar, and Sparks, published in 1982, and now a standard reference, the eruption of Vesuvius of 79 AD unfolded in two phases a Plinian eruption that lasted eighteen to twenty hours and produced a rain of pumice southward of the cone that built up to depths of 2.8 metres (9 ft 2 in) at Pompeii, followed by a pyroclastic flow or nuée ardente in the second, Peléan phase that reached as far as Misenum but was concentrated to the west and northwest. Two pyroclastic flows engulfed Pompeii, burning and asphyxiating the stragglers who had remained behind. Oplontis and Herculaneum received the brunt of the flows and were buried in fine ash and pumice, and pyroclastic deposits.
Lanza used the magnetic characteristics of over 200 samples of lithic, roof-tile, and plaster fragments collected from pyroclastic deposits in and around Pompeii to estimate the equilibrium temperatures of the deposits. The deposits were placed by pyroclastic density currents (PDCs) resulting from the collapses of the Plinian column. The authors argue that fragments over 2–5 cm (0.8–2 in) were not in the current long enough to acquire its temperature, which would have been much higher, and therefore they distinguish between the depositional temperatures, which they estimated, and the emplacement temperatures, which in some cases based on the cooling characteristics of some types and fragment sizes of rocks they believed they also could estimate. Final figures are considered to be those of the rocks in the current just before deposition.
By 2003 around 1,044 casts made from impressions of bodies in the ash deposits had been recovered in and around Pompeii, with the scattered bones of another 100. The remains of about 332 bodies have been found at Herculaneum (300 in arched vaults discovered in 1980). What percentage these numbers are of the total dead or the percentage of the dead to the total number at risk remain completely unknown. Thirty-eight percent of the 1044 were found in the ash fall deposits, the majority inside buildings. These are thought to have been killed mainly by roof collapses, with the smaller number of victims found outside buildings probably killed by falling roof slates or by larger rocks thrown out by the volcano. This differs from modern experience, since over the last four hundred years only around 4% of victims have been killed by ash falls during explosive eruptions. The remaining 62% of remains found at Pompeii were in the pyroclastic surge deposits, and thus were probably killed by them.