On this day August 24, 79

The volcano Mount Vesuvius, erupted, burying the towns of Pompeii, Herculaneum, and Stabiae in Italy, the eruption was preceded by smaller earthquakes started taking place on 20 August becoming more frequent over the next four days, but the warnings were not recognised and on the afternoon, a catastrophic eruption of the volcano started.

Herculaneum skeleton

Herculaneum skeleton

The eruption of Vesuvius 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 pyroclastic deposits.

The only surviving reliable eyewitness account of the event was recorded by Pliny the Younger, who was 17 at the time of the eruption, in two letters to the historian Tacitus.

Observing it from Misenum (across the bay, approximately 35 kilometres (22 mi) from the volcano) whilst his uncle sailed closer, he saw an extraordinarily dense and rapidly rising cloud appearing above the mountain:

I cannot give you a more exact description of its appearance than by comparing to a pine tree; for it shot up to a great height in the form of a tall trunk, which spread out at the top as though into branches. … Occasionally it was brighter, occasionally darker and spotted, as it was either more or less filled with earth and cinders. (Sixth Book of Letters, Letter 16.)

After some time he described the cloud rushing down the flanks of the mountain and covering everything around it, including the surrounding sea. This is known today as a pyroclastic flow, which is a cloud of superheated gas, ash, and rock that erupts from a volcano.

Geologists have used the magnetic characteristics of over 200 volcanic rocks and pieces of debris (e.g. roof tiles) found in Pompeii to estimate the temperature of this pyroclastic flow. (When molten rock solidifies, magnetic minerals in the rock record the direction of Earth’s magnetic field. If the material is heated above a certain temperature, known as the Curie temperature, the rock’s magnetic field may be modified or completely reset.)

Most of the materials analyzed experienced temperatures between 240 °C (464 °F) and 340 °C (644 °F) (with a few areas showing lower temperatures of only 180 °C (356 °F)). This suggests that the ash cloud had a temperature of 850 °C (1,560 °F) when emerging from the mouth of Vesuvius and had cooled to below 350 °C (662 °F) by the time it reached the city. It is theorized that turbulence may have mixed cool air into the ash cloud. (Cioni, et al., 2004). This is now called the Plinian stage of the eruption, named after both the younger and elder Plinys.

Pliny stated that several earth tremors were felt at the time of the eruption and were followed by a very violent shaking of the ground. He also noted that ash was falling in very thick sheets and the village he was in had to be evacuated, and then that the sun was blocked out by the eruption and the daylight hours were left in darkness. Also, the sea was sucked away and forced back by an “earthquake”, a phenomenon now called a tsunami.

Along with Pliny the Elder, the only other noble casualties of the eruption to be known by name were Agrippa (a son of the Jewish princess Drusilla and the procurator Antonius Felix) and his wife.

Estimates of the population of Pompeii range from 10,000 to 25,000, and Herculaneum is thought to have had a population of about 5,000. It is not known how many people the eruption killed, although around 1,150 remains of bodies – or casts made of their impressions in the ash deposits – have been recovered in and around Pompeii. The remains of about 350 bodies have been found at Herculaneum (300 in arched vaults discovered in 1980). However these figures must represent a great underestimation of the total number of deaths over the region affected by the eruption.

Thirty-eight percent of the victims at Pompeii 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 of buildings probably being 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 – probably from a combination of suffocation through ash inhalation and blast and debris thrown around. In contrast to the victims found at Herculaneum, examination of cloth, frescoes and skeletons show that it is unlikely that high temperatures were a significant cause.

Herculaneum, which was much closer to the crater, was saved from tephra falls by the wind direction, but was buried under 23 metres (75 ft) of material deposited by pyroclastic surges. It is likely that most, or all, of the victims in this town were killed by the surges, particularly given evidence of high temperatures found on the skeletons of the victims found in the arched vaults, and the existence of carbonised wood in many of the buildings.

Pompeii and Herculaneum were never rebuilt, although surviving townspeople and probably looters did undertake extensive salvage work after the destructions. The eruption changed the course of the Sarno River and raised the sea beach, so that Pompeii was now neither on the river nor adjacent to the coast.

The towns’ locations were eventually forgotten until their accidental rediscovery in the 18th century. Vesuvius itself underwent major changes – its slopes were denuded of vegetation and its summit had changed considerably due to the force of the eruption.

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