What happened in 79 CE
In 79 CE Mount Vesuvius, a volcano on the Bay of Naples, erupted violently and buried Pompeii, Herculaneum and several smaller settlements. Pompeii was not swallowed by a single blast or a river of lava. The town was destroyed in stages over roughly a day, and each stage affected it differently. The order of those stages is reasonably well understood from two sources: the deposits that geologists have studied in the ground, and a letter by Pliny the Younger, who watched the eruption from across the bay.
The popular image of Pompeii as a place “frozen” in an instant hides this sequence. For many hours the people there had time to act, and many appear to have left. The phrase is a shorthand for how well the ash preserved what remained.
Before the eruption
Vesuvius had not behaved like a dangerous volcano in living memory, and the Romans of the region do not seem to have recognised it as one. A strong earthquake struck the area in 62 CE, or perhaps 63, and damaged Pompeii badly. Repairs were still under way in many buildings in 79. Whether the earthquake was an early sign of the volcano’s reawakening is a matter that scholars discuss, and there is no reliable way to say how many residents saw any connection.
Pompeii’s population in this period is usually put at somewhere around ten thousand, but the figure is an estimate and scholars give a range. Herculaneum, closer to the volcano on the coast, was smaller.
Phase one: the eruption column
Pliny the Younger, who was a teenager staying at Misenum across the bay, described a cloud rising above the mountain that looked like an umbrella pine, with a tall trunk and spreading branches. This matches what volcanologists call a Plinian eruption column, named after him. Gas and fragmented rock were thrown high into the atmosphere, and the wind carried material towards the south and south-east, which was the direction of Pompeii.
According to Pliny, this began in the early afternoon. His uncle, Pliny the Elder, commanded the Roman fleet at Misenum, and set out across the bay to observe the event and to attempt rescues. He died on the shore near Stabiae. The cause of his death is debated, and Pliny the Younger’s account suggests a collapse from the fumes, though that is an inference from one report.
Phase two: pumice and ash fall
Pumice is light, frothy volcanic rock. For many hours it fell on Pompeii as small lapilli, first white and later grey, together with ash. Deposits steadily grew in the streets. By the end of this phase they reached a depth of a few metres, enough to bury ground-floor doorways and to put heavy loads on roofs.
This phase was dangerous but not instantly lethal. Falling stones could injure people, and the darkness, the noise and the rising layer of pumice made movement hard. Roofs gradually collapsed under the weight. Some residents died in collapsing buildings, while others sheltered indoors and were trapped when the pumice piled up against doors. Many others walked out across the soft deposit and left the town. We know this because the number of bodies recovered in Pompeii is considerably smaller than the likely population, though the exact share who escaped cannot be known.
Households that fled often took valuables with them, and archaeologists have noticed that some houses are strikingly empty of portable goods. Other people returned or lingered, perhaps hoping to retrieve possessions, and were caught by what followed.
Phase three: pyroclastic surges
The decisive change came when the eruption column became unstable and started to collapse. Instead of rising, the mixture of hot gas, ash and rock flowed down the mountain as pyroclastic surges and flows. These currents are fast and extremely hot, and they cannot be outrun once they arrive.
Herculaneum, much closer to the crater, was reached by the first of the surges, apparently during the night. Pompeii was hit later, in the early hours of the following morning, by several surges that came one after another. Those still in the town were killed quickly, probably by the heat and by the inhalation of hot ash rather than by suffocation under rubble alone. The surges also knocked down upper storeys and walls that the pumice had left standing.
Ash from the surges settled around the bodies already lying in the pumice. This is the origin of the voids that nineteenth-century excavators later filled, a technique described in the account of the plaster body casts.
Pompeii compared with Herculaneum
Because the towns were hit differently, they preserve different things. At Pompeii, pumice fall buried the town under a thick, relatively loose layer, and later surges added more. Herculaneum was buried under much deeper, harder material, and the heat carbonised wood. Roofs, beams, furniture and even food were charred rather than destroyed. Hundreds of skeletons have been found in the boat chambers along the former shoreline at Herculaneum, where people had apparently gathered hoping for rescue by sea.
Why this matters for reading the site
Knowing the phases helps explain much of what visitors see. Rooms with a thick layer of pumice often contain little, because their owners had already left. Collapsed upper floors reflect the later surges. Gaps and cavities in the deposits record the shape of bodies, and the age of those remains can now be studied with methods such as ancient DNA, which has in some cases challenged earlier assumptions about who was sheltering together.
Many victims were not Roman citizens of high rank. Enslaved people, who made up a significant share of the population as the study of Roman slavery shows, were among those who could not simply choose to leave.
The Bronze Age eruption that buried Akrotiri on Thera was a different event on a different scale. Nonetheless, it offers a useful comparison for how volcanic burial can preserve a town while most of its people had already gone.
Dating and what is still uncertain
The traditional day for the eruption, 24 August, comes from manuscripts of Pliny’s letter, but the evidence on the ground and a charcoal inscription discovered in 2018 have led many scholars to favour a later, autumn date. The details are covered in the discussion of the eruption date debate.
Other points remain uncertain, including the exact death toll, which is usually described only as thousands across the region, and the share of Pompeii’s residents who escaped. What is secure is the basic sequence: a rising column, hours of pumice fall, then surges that ended life in the town.