What happened at Pompeii? The eruption of Vesuvius in AD 79

What happened at Pompeii is that in AD 79 the nearby volcano Mount Vesuvius erupted and buried the Roman town in about two days. For most of a day, pumice rained down and piled up; then, early the next morning, avalanches of scorching ash and gas swept through and killed everyone still there. The town lay buried and largely forgotten until excavation began in the 18th century, which is why it is now the fullest picture of everyday Roman life.

In short

• The eruption came in two stages: a towering column that dropped pumice for 18–20 hours, then pyroclastic surges, fast-moving clouds of hot ash and gas.
• The pumice drove many people out and collapsed roofs on some who stayed; the surges killed those still in the town, within minutes.
• The traditional date is 24 August, but an inscription found in 2018 points to October, and the question is still argued.

How do we know what happened?

Two kinds of evidence tell the story, and they agree remarkably well. The first is an eyewitness. Pliny the Younger, then about 17, watched the eruption from Misenum, across the Bay of Naples, where his uncle Pliny the Elder commanded the Roman fleet. Years later the historian Tacitus asked him how his uncle died, and Pliny answered in two letters, the only surviving eyewitness account.

The second is the ground itself. Volcanologists have dug through the layers the eruption laid down and read them like a diary: pale pumice at the bottom, then gray pumice, then thin, hard layers left by the surges. In the 1980s Haraldur Sigurðsson and colleagues used those layers to work out that the eruption alternated between a tall column and collapsing surges six times, as summarized in the account of the eruption on Wikipedia.

What happened, hour by hour?

The letters and the rock together give a timeline. Times are approximate; Romans counted hours from sunrise.

When	What happened	Where the evidence comes from
Day 1, about 1 pm	A cloud rises over the mountain, shaped "more like a pine-tree than anything else"	Pliny's first letter, "about the seventh hour"
Day 1, afternoon	The column climbs to about 33 km; winds blow it southeast, and pumice begins falling on Pompeii	Deposits; Pliny describes pumice and blackened stones landing on his uncle's ships
Day 1, evening and night	About 2.8 m of pumice piles up in Pompeii; roofs give way under the weight	The pumice layer
Night	The column partly collapses; surges reach Herculaneum, on the coast to the west, and bury it	Deposits up to 23 m deep at Herculaneum
Day 2, early morning	Surges 4 and 5 sweep through Pompeii, and the town disappears under about 1.8 m more of ash	The surge layers; most bodies lie on top of the pumice, inside the ash
Day 2, morning	At Misenum, about 30 km away, a black cloud covers the bay; Pliny and his mother flee in darkness	Pliny's second letter

Pliny the Elder had sailed across the bay to help, landed at Stabiae, and died on the beach there. His nephew wrote that his body was found two days later "untouched, uninjured," which suggests he collapsed, perhaps from the fumes or a heart problem, rather than from burns or blows.

Why did so many people die?

Pumice is frightening but far more survivable: it is light, frothy rock, and people could walk through it with pillows tied over their heads, as Pliny describes. Many of Pompeii's residents seem to have left during the first afternoon and night. Those who stayed sheltered indoors, or tried to leave too late, met the surges.

A pyroclastic surge is a ground-hugging cloud of hot ash and gas that moves faster than anyone can run. It kills by heat and by the ash it forces into the lungs, and anyone caught in the open had no escape. That shows in where the bodies were found. Of the 1,044 victims recorded at Pompeii by 2003, about 62% lay in the surge deposits, on top of the pumice, in streets, doorways and rooms; the other 38% were found within the pumice fall itself, mostly inside buildings, where roofs and walls gave way under its weight. Counting scattered bones, the remains of about 1,150 people had been recovered at Pompeii by 2003, and more have been found since in new excavations. Nobody knows the full death toll, because the town has not been fully dug and many who fled may have died on the roads.

How the eruption grew from the magma beneath Vesuvius is the subject of the note on how volcanoes form: the gas dissolved in sticky magma expands violently as it rises, shattering the rock into pumice and ash.

When exactly did Vesuvius erupt?

For centuries the answer was 24 August, from printed editions of Pliny's letter. But the date passed through many hand-copied manuscripts, and the surviving copies don't all agree. Archaeologists had also long noticed autumn clues: remains of fruits such as pomegranates, braziers for heating, and wine already sealed in jars.

In 2018, excavators in a house being remodeled found a charcoal scrawl on a wall. It gives a date equal to 17 October, without a year. If it was written in AD 79, the eruption came after that day, and many scholars now favor about 24 October.

Still argued

Supporters of an October date say a charcoal scribble on a wall under repair would not have lasted long, so it was probably written that year, and it fits the autumn finds. Others answer that the inscription has no year and could be older, that seasonal finds can be read either way, and that the manuscript evidence is muddled rather than wrong. A 2022 study proposed a date between 24 October and 1 November, and that too has been challenged. Neither side has a decisive document.

How were the plaster casts made?

The ash that buried the victims hardened around their bodies. As the bodies decayed over the centuries, they left hollows in the shape of the person. In 1863 the archaeologist Giuseppe Fiorelli, director of the excavations, had the idea of pouring liquid plaster into such a hollow, letting it set, and chipping away the ash. What came out was the person's last posture, often with bones and teeth inside. According to the Archaeological Park of Pompeii, about a hundred casts have been made since the 19th century.

The casts are the remains of real people, and the stories told about them have not always been right. In 2024, geneticists who recovered DNA from the bones inside five casts reported in Current Biology that an adult long described as a mother holding her child was a man unrelated to the child, and that a pair thought to be sisters included at least one male. DNA now lets researchers test what earlier generations assumed.

Why does Pompeii matter today?

Because it preserves ordinary life, not just monuments. Most Roman sites were lived in, rebuilt, stripped and reused for centuries, so what survives is the stone. Pompeii was sealed in a matter of hours, with its bakeries, street-corner food counters, painted walls, election notices and graffiti in place. Historians of the Roman Empire use it to see how a provincial town of the 1st century actually worked, and it is a case study in how historians weigh evidence, since even a "frozen moment" holds only what the ash happened to catch.

It is also a warning that hasn't expired. Vesuvius has erupted many times since, most recently in 1944, and hundreds of thousands of people live on and around its slopes. The same layers that tell Pompeii's story are what volcanologists use to plan evacuations around Naples, as part of the wider study of what the Earth is made of and how it moves.

Check yourself

• Without looking back: why did the pumice fall drive people out, while the surges killed those who remained?
• Why do most of the bodies at Pompeii lie on top of the pumice layer rather than at the bottom of it?
• What evidence supports an October date for the eruption, and what is the strongest objection to it?

Sources

• Letters of Pliny the Younger, book 6, letters 16 and 20 — Pliny the Younger, translated by J. B. Firth, 1900
• Eruption of Mount Vesuvius in 79 AD — Wikipedia
• The plaster casts of Pompeii: an itinerary through the eruption of AD 79 — Archaeological Park of Pompeii, 2026
• Ancient DNA challenges prevailing interpretations of the Pompeii plaster casts — Pilli and colleagues, Current Biology, 2024
• Giuseppe Fiorelli — Wikipedia