Historians are able to piece together what happened at Pompeii and Herculaneum thanks to the first hand accounts of Pliny the younger. Pliny the Younger was a Roman aristocrat and poet, whose uncle, Pliny the elder, was in command of the Roman fleet stationed in Misenum. Pliny the younger writes of his own experiences watching Mount Vesuvius erupt, as well as recounting his uncles final days as he lead the rescue mission to Pompeii and Herculaneum. From this account, combined with what has been uncovered at the two dig sties, historians have been able to piece together a fairly accurate re-telling of what would have occured at Mount Vesuvius, Pompeii, and Herculaneum.
On the 24th of August CE 79, and lasting over 24 hours, Mount Vesuvius erupted spewing a noxious cloud of ash and rock over 30 kilometres into the air. The pumice cloud was blown in a mostly South-Easterly direction, burying Pompeii and Herculaneum in a pyroclastic flowup to 20 metres deep in some areas.
There had been warning signs in the lead up to Mount Vesuvius's eruption, many of which we use today to predict the eruptions of similar disasters. However, the Roman population below Mount Vesuvius was largely unaware of its volcanic nature and the danger that it posed, as its last eruption was more than 700 years earlier. The first, and biggest precursor to the August CE 79 eruption was a massive earthquake 16 years earlier in CE 63. This earthquake caused a huge amount of damage in Pompeii and Herculaneum, secondary warning signs also began to present themselves from the begining of August, with an ever increasing amount of small tremors and earthquakes hinting at the pressure bulding up below.
So, instead of worrying about the earthquakes and what they meant, the citizens of Pompeii and Herculaneum continued on with their daily lives. Many townspeople were even working on repairing the damage caused by the earthquake when Mount Vesuvius erupted.
As such, the Roman populace that surrounded Mount Vesuvius could react in only one way: Panic! Citizens who fled immediately without gathering posessions stood some chance of survival, with an estimated 80% of the population escaping. Those who stayed in their homes, or attempted to gather valuable, and often heavy personal items stood no chance.
For those that did not escape, two fates awaited them:
-A slow asphyxiation brought on by the tonnes of ash falling on the city during the first few hours of Mount Vesuvius's eruption. -A quick and sudden death from the extreme heat brought on by the pyroclastic flows that, upon reaching Pompeii, are specualted to have been as hot as 300 degrees.
Although the tonnes of ash and intense heat of the pyroclastic flow were the reasons for the death of so many people, these are also the two reasons why Pompeii and Herculaneum were preserved so well. Firstly, the ash provided a coating to art work and pottery that stopped the erosion of time wearing it away. Secondly, the pyroclastic flows, while reaching intense speeds that would obliterate buildings at their peak, had almost completely stopped by the time they reached the Roman towns. While the pyroclastic flows had lost their speed, the had retained their heat, and this heat was so intense that it quickly charred, not burnt, many of the wooden structures, thus preserving them.
A pyroclastic flow is a mass of rocks, gasses, and ash, that can reach speeds of several hundred kilometres an hour, and heats of over 100 degrees. This is different to lava which is a flow of molten rocks that moves at a much slower pace.