Pyroclastic Flow: How the People of Pompeii Died

What Actually Killed the People of Pompeii

Most of the people who died in Pompeii were not buried alive or slowly suffocated. They were killed almost instantly in the early morning of the second day, when pyroclastic surges of superheated gas and ash — around 250 to 300 degrees Celsius — swept through the city, causing fatal thermal shock. The eruption had two very different phases, and understanding the split is the key to understanding how the people of Pompeii died.

This page explains the difference between the long pumice-fall phase and the lethal pyroclastic phase, why the timing decided who lived and who died, and why the victims' bodies were preserved so vividly in the ash. For the dramatic hour-by-hour story, see the last day of Pompeii.

The Two Phases of the Eruption

The destruction of Pompeii in 79 AD did not happen all at once. It unfolded over roughly 18 to 24 hours in two distinct stages, and almost everyone who died did so in the second.

1. The Plinian (pumice-fall) phase — first ~18 hours. Beginning around midday, Vesuvius blasted a towering column of gas, ash, and pumice some 30 kilometers into the sky. Light pumice stones and ash rained down on Pompeii, piling up at roughly 15 centimeters per hour. The streets filled, and the real danger of this phase was roof collapse: beams gave way under the accumulating weight, killing and trapping people who had sheltered indoors. Crucially, this slow burial also gave most residents time to flee.
2. The pyroclastic phase — early morning of the second day. When the eruption column could no longer support its own weight, it collapsed. Instead of rising, the superheated mass poured back down the mountain as pyroclastic surges and flows — ground-hugging avalanches of incandescent gas and ash moving faster than 100 kilometers per hour. A series of these surges reached Pompeii in the predawn hours and killed everyone still in the city.

Why the Pyroclastic Surges Were So Lethal

A pyroclastic surge is not lava. It is a turbulent cloud of volcanic gas suspending fine ash and rock, and its danger is its heat and speed combined. The surges that overtopped Pompeii's walls carried temperatures estimated at 250 to 300 degrees Celsius — far hotter than boiling water. At that temperature, exposed people died from thermal shock in moments, before suffocation or burial could play any part.

This is why the victims are so often found in lifelike, unguarded postures rather than curled up gasping for air: death was effectively instantaneous. At Herculaneum, on the volcano's western flank, the first surge was even hotter — hot enough to vaporize soft tissue and carbonize wood. Pompeii, slightly further away and downwind, was struck by surges that were cooler but still far beyond survivable.

Phase	Timing	Main hazard	Cause of death
Pumice fall (Plinian)	Midday to ~18 hrs	Accumulating pumice	Roof collapse, injury
Pyroclastic surges	Early hours, day 2	Superheated gas & ash (~250–300°C)	Thermal shock, near-instant

Why the Bodies Were Preserved in the Ash

The same event that killed the victims also preserved them. After a surge passed, fine ash settled and hardened around the bodies where they had fallen. Over the following centuries the soft tissue decayed away, leaving hollow cavities inside the solidified ash that held the precise shape of each person — the curve of a back, a hand raised to the face, the folds of clothing.

When 19th-century archaeologist Giuseppe Fiorelli realized what these voids were, he began pouring liquid plaster into them, then chipping away the surrounding deposit to reveal the cast within. The result is the haunting gallery of figures that visitors see today. To understand exactly how this process worked and where to see the casts, read about the bodies and plaster casts of Pompeii, and for who these people were, see the victims of Pompeii.

How Many Stayed, and How Many Escaped

Of Pompeii's estimated 11,000 to 15,000 inhabitants, the majority escaped during the long pumice-fall phase, streaming out of the city gates with pillows tied over their heads against the falling stones. The roughly 2,000 who stayed behind — sheltering in their homes, guarding property, or simply waiting for the storm to end — were the ones overtaken by the surges in the morning. The lesson written into the geology of the site is stark: in this eruption, leaving early meant survival, and waiting meant death. The same volcano remains active today, as our page on Vesuvius eruptions explains.

Related Pages

• The Last Day of Pompeii — The full hour-by-hour account of the eruption
• The Victims of Pompeii — Who the people killed in 79 AD were
• The Bodies and Plaster Casts — How the victims were preserved and where to see them
• Vesuvius Eruptions — The volcano's eruptive history, then and now