Eruption of Mount Vesuvius

Mount Vesuvius is best known for its eruption in AD 79 that led to the destruction of the Roman cities of Pompeii and Herculaneum.

They were never rebuilt, although surviving townspeople and probably looters did undertake extensive salvage work after the destructions. The towns' locations were eventually forgotten until their accidental rediscovery in the 18th century.

The eruption also changed the course of the Sarno River and raised the sea beach, so that Pompeii was now neither on the river nor adjacent to the coast. Vesuvius itself underwent major changes – its slopes were denuded of vegetation and its summit changed considerably due to the force of the eruption.

He [Pliny the Elder] was at Miseno leading the fleet. On August 24th, barely an hour had passed after midday, when my mother pointed out to him a cloud which had appeared, the like of which had never been seen before in size and shape. [...] We could not be sure which mountain the cloud rose from, as we were looking from a great distance; only later did we realise that it was from Mt. Vesuvius. This cloud resembled a pine (Mediterranean pine) more than any other tree.
As from an enormous trunk, the cloud rose high into the sky and spread out as if it were growing branches. I believe, because it was first blown high by a strong gust of wind, all in one piece, then as it grew smaller it was dropped, or, maybe because of its overpowering weight, the cloud expanded umbrella-like: it shone brilliantly white at times, then at others appeared dirty, darkened by different colour stains depending on the prevalence of the ash or sand that it had lifted up with it.”

— Pliny the Younger

Pliny's letters reveal that eruption phenomena were not the only threat to buildings in 79, there was also significant seismic ground motion associated with the eruption. In the city of Stabiae, approximately 7 kilometers south of Pompeii, ground tremors on the evening of the 24th motivated residents to move outside [Sigurdsson 1982, p. 45]. In Misenum, approximately 35 kilometers from Pompeii, tremors on the morning of the 25th were strong enough to move vehicles even with stones blocking their wheels [Sigurdsson 1982, p. 46].

Finally, on the morning of August 24th, 79 A. D. the volcano burst open with an earsplitting crack. Smoke, mud, flames and burning stones spewed from the summit of the mountain, sending a rain of ash and rock through the surrounding countryside. The mud seeped down the sides of Vesuvius, swallowing nearby farms, orchards and villas. Adding to the destruction were the mephitic vapors that accompanied the falling debris; the fumes first caused deliriousness in their victims, then suffocated them.

Some people of Pompeii grabbed their beasts of burden and attempted to flee the area; others perhaps chose to wait until the streets were clear of the panicked masses; still others sealed themselves up in rooms, supposing that the ashes and poisonous gasses would not harm them there.