A multidisciplinary volcanological and bio-anthropological study of the eruption products and victims, merged with numerical simulations and experiments, indicates that at Vesuvius and surrounding towns heat was the main cause of death of people, previously believed to have died by ash suffocation. The results of the study, published in 2010, show that exposure to at least 250 °C (482 °F) hot surges (known as pyroclastic flows) at a distance of 10 kilometres (6 miles) from the vent was sufficient to cause instant death, even if people were sheltered within buildings.
Pompeii and other cities affected by the eruption of Mount Vesuvius. The black cloud represents the general distribution of ash and cinder. Modern coast lines are shown.
The people and buildings of Pompeii were covered in up to twelve different layers of tephra, in total 25 meters deep, which rained down for about six hours. Pliny the Younger provided a first-hand account of the eruption of Mount Vesuvius from his position across the Bay of Naples at Misenum, in a version he wrote 25 years after the event. His uncle, Pliny the Elder, with whom he had a close relationship, died while attempting to rescue stranded victims. As admiral of the fleet, Pliny the Elder had ordered the ships of the Imperial Navy stationed at Misenum to cross the bay to assist evacuation attempts. Volcanologists have recognised the importance of Pliny the Younger's account of the eruption by calling similar events "Plinian".
The eruption was documented by contemporary historians and is generally accepted as having started on 24 August 79, relying on one version of the text of Pliny's letter. However the archeological excavations of Pompeii suggest that the city was buried about three months later. This is supported by another version of the letter, which gives the date of the eruption as November 23.
People buried in the ash appear to have been wearing heavier clothing than the light summer clothes typical of August. The fresh fruit and vegetables in the shops are typical of October—and conversely the summer fruit typical of August was already being sold in dried, or conserved form. Wine fermenting jars had been sealed, which would have happened around the end of October. Coins found in the purse of a woman buried in the ash include one with a fifteenth imperatorial acclamation among the emperor's titles. These cannot have been minted before the second week of September. So far there is no definitive theory as to why there should be such an apparent discrepancy.
(source)