Pompeii

Twenty-five hours

The Plinian eruption of Mount Vesuvius lasted about twenty-five hours.

The first phase, beginning early in the afternoon, sent a 33-kilometre-high column of ash and pumice straight up into the stratosphere. For the next eighteen hours the column rained pumice onto Pompeii and the surrounding towns, the falling stones accumulating at the rate of fifteen centimetres an hour. Most Pompeians who left immediately survived.

Those who waited — perhaps two thousand of the city's twelve thousand inhabitants — were caught the next morning when the eruption column collapsed and produced a series of pyroclastic surges. Scorching mixtures of gas and ash that swept down the mountain at 100 km/h and instantly killed anyone in their path.

The date isn't what we thought

For almost two millennia the eruption was confidently dated to 24 August AD 79.

Then in 2018 a workman in the House of the Garden uncovered a charcoal graffito reading indulsit pro masumis esuritioni — in or near 17 October. A Pompeian writing in early October could only have done so if the eruption had not yet happened.

Other clues fit an autumn date. Charcoal braziers were set up in the doorways, unnecessary in late August. Wine fermentation vessels were sealed — grape harvest finished. The bodies wore heavier woollen clothing than late summer would warrant.

Most modern archaeologists now believe the eruption occurred on or about 24 October AD 79. Two months later than the traditional date.

Pliny investigates, and dies

The most famous casualty was Gaius Plinius Secundus — Pliny the Elder — author of the 37-volume Natural History and commander of the Roman naval fleet at Misenum.

When the eruption began Pliny ordered his ships to sea, partly to evacuate refugees and partly to satisfy his scientific curiosity. He landed at Stabiae, took dinner, slept through the worst of the ash fall, and died the next morning, almost certainly of acute respiratory failure from inhaled sulphur dioxide and superheated ash.

His seventeen-year-old nephew Pliny the Younger watched the eruption from across the Bay of Naples and wrote the only contemporary eyewitness account, in two letters to the historian Tacitus — the documents that gave volcanology its term "Plinian eruption".

The casts that aren't bodies

The famous "bodies of Pompeii" are not bodies.

When the pyroclastic surge buried the inhabitants under a layer of fine ash that hardened quickly, the bodies decomposed inside the hardened shells, leaving hollow cavities in the shape of the corpse. In 1863, the Italian archaeologist Giuseppe Fiorelli devised a technique of pumping liquid plaster into these cavities, allowing it to set, then chipping away the surrounding ash.

The resulting "casts" preserve the exact final position of the victim down to the texture of their clothing — a slave girl with her hands over her face, a watchdog still chained to its post, a couple embracing. But the casts are plaster shapes of an absent body, not the body itself.

The actual bones of most victims remain inside the casts.