British Museum: Life and Death in Pompeii

In AD 79 the beautiful Bay of Naples in southern Italy, famous in Roman times for its fertile soil, welcoming climate and luxurious living, was convulsed by a catastrophic eruption of the volcano Mount Vesuvius. In just one day, two cities, Pompeii and Herculaneum, were buried, along with smaller settlements such as Oplontis and Stabiae and countless farms, villas, estates and villages.
Vesuvius had been dormant for hundreds of years and so the eruption was particularly violent, producing not lava but something more deadly, a gigantic volcanic cloud of ash some 30 km high. Pompeii was directly in its path and throughout the eruption was gradually submerged under a rain of heavy ash from the
cloud, which made structures collapse under its weight. Herculaneum was shaken by earthquakes but because of the wind direction no ash fell in the early phase of the eruption.
But both cities were destroyed and buried as the volcanic cloud finally collapsed, sending several deadly ‘pyroclastic surges’ – avalanches of superheated ash and gas – down the slopes of Vesuvius. About 4–5 m of debris buried Pompeii while Herculaneum was submerged to a depth of as much as 23 m. 
No-one who was still in the cities at that point could have survived; the extreme temperatures of the surges killed them instantly. At Pompeii, ash formed a hard coating around their corpses, preserving the forms of their bodies, and sometimes even their clothes. The higher temperature of the surges that hit the people of Herculaneum vaporized them instantly down to the bone. But this high temperature at Herculaneum preserved articles of wood and even foodstuffs, which do not normally survive at Pompeii.
Källa: Paul Roberts: Life and Death in Pompeii and Herculaneum. Utställningen pågår till 26 september 2013