Mount Vesuvius, visible from the city, had not erupted in recorded history. The mountain was farmed on its slopes and was not understood to be the kind of geological structure that would destroy a city. When the eruption began, probably in the late morning of a day in the late summer or early fall — the exact date has been disputed between August 24 and a date in late October, with a graffito discovered in 2018 suggesting an October eruption — the people of Pompeii had no precedent for what was coming.
The eruption occurred in two main phases. The first, the Plinian phase, lasted approximately eighteen to twenty hours. A column of ash, gas, and rock rose to heights estimated at more than twenty kilometers and then began to collapse. Pumice fell on Pompeii — first white pumice, then grey — building up a layer estimated at roughly 2.8 meters in depth to the south of the volcano, in the direction of the city. Buildings collapsed under the weight. Some people were killed by falling roofs and walls. Many fled; the streets leading out of Pompeii show evidence of the evacuation.
Those who could not or did not flee in time were killed in the second phase. Pyroclastic surges — dense, fast-moving currents of superheated gas and volcanic rock — swept down from the volcano during the night and into the following morning. The temperatures in these surges, as measured by researchers studying the deposits, ranged from approximately 180 degrees Celsius in the first surge affecting Pompeii to 220 to 260 degrees Celsius in the second. The effect on the human body was near-instantaneous. The contorted positions in which victims have been found — knees drawn up, arms raised, bodies twisted — were long interpreted as evidence of agony. Forensic analysis suggests instead that these postures resulted from heat shock acting on the muscular system after death, a cadaveric phenomenon that produces characteristic flexion, not the recorded final movement of a living person.