Destruction of Pompeii

Pompeii was the largest of several towns buried by the paroxysmal eruption of Mt. Vesuvius that began at about noon on August 24 of 79 AD. The main phases of the eruption lasted into the next day, and there may have been smaller scale activity in following days -- no historical record survives of the immediate aftermath of the explosive first 24 hours. The pumice fall deposits at Pompeii, 8 kilometers south of Vesuvius, were 2.5 to 3 meters deep. Much thicker pyroclastic flow deposits (up to 20 meters deep) overwhelmed Herculanum and other towns on the eastern slope. Tephra (bits of lava and other materials blown out in pyroclastic flows) was found 74 miles away, and finer ash flew for hundred of miles before settling out. The pillar of smoke and ash was visible from Rome, and some people there claimed they had heard the volcano's rumbling. Modern scientists estimate the volume of eruptive material at almost four cubic kilometers of magma. Contrary to earlier assessments, volcanologists today know that there were no lava flows in the 79 AD eruption. What came out was violently ejected from the top and sides of the volcano, mostly in the form of "inflated" pumice, that is, glassy fragments that had been expanded by volcanic gasses and steam. The measured volume of the inflated pumice is almost 9 cubic kilometers. Towns were flattened by devastating pyroclastic flows and by the weight of volcanic debris, the countryside was devastated, and thousands were killed, some of them still undiscovered today.
Among the dead was the local military commander, Pliny the Elder, the author of the great Natural History, who had been given a sinecure command in the resort town of Pompeii as a reward for his scholarship. His heroic death while trying to mount a rescue of the townspeople is only known from the account of his nephew and pupil, Pliny the Younger, who witnessed the eruption from 30 kilometers away and even at that distance had to flee for his own life. Communications to the stricken area were completely cut off, although there is evidence, in the form of prints of distinctive Roman Army shoes left in the top layers of ash in Pompeii, that military assistance arrived soon after the eruption ended, probably from nearby garrisons that had somehow survived. The devastated towns and villages were not rebuilt and eventually their very locations were forgotten.
A fortified Oscan town, already called Pompeii, was on the site of Roman Pompeii, on a lava promontory from a prehistoric eruption of Vesuvius, even before Rome was founded. Through the centuries the town had passed through Greek-colonial, Etruscan, Samnite, and, eventually Roman hands. By 200 BC Pompeii was a Roman ally, and, after participating in the "Social Wars", the Latin League's unsuccessful revolt against Rome in the 80s BC, it was captured outright and thoroughly Romanised.
The Pompeii of the eruption housed more than twenty thousand souls and was a resort town that provided fleshly and other pleasures, primarily to the upper middle class and to lower levels of the aristocracy. The upper level aristocrats took their pleasures more privately in their villas in the hills around the Bay of Naples, but some of them may also have been in Pompeii, perhaps in the baths, when the eruption started. They would undoubtedly have headed back for the hills. Pompeii had a larger than usual service class, free and slave, which were there to handle the needs of the semi-permanent residents and transients, the tourists and pleasure-seekers. Pompeii was also a thriving port, so chandlers, shipping offices, and other maritime services were available. The crews of merchant and military ships and the marine soldiers that the latter carried would also have found use for the pleasures that the town provided. There was a forum where the town's business was conducted, not, as usual, in the center of town but in the more level southwestern corner. An amphitheater at the eastern edge of town provided the kind of bloody entertainment that the locals and visiting Romans so much enjoyed.
The ferocity of the 79 AD eruption was obviously a surprise, but it was not completely without precursors. In 63 AD there had been a devastating earthquake, which today's vulcanologists believe was a consequence of the same tectonic activity that re-ignited the long dormant volcano. Damage from the earthquake was still being repaired, and many homes and public buildings in Pompeii had recently been redecorated when the eruption of 79 AD occurred: that accounts for the very "new" look of many of the structures, frescoes, and statues that were subsequently buried in the layers of pumice and ash. Pre-eruption Roman writings clearly indicate that people knew that the mountain was volcanic and that they associated the earthquake with the possibility that the volcano would again become devastatingly active. In the months before the climactic explosion, there had been occasional rumblings from the peak, a series of minor earthquakes, and at least one small ejection of ash.
Many separate eruptive events occurred on August 24-25 AD 79, and all have been counted and classified by modern scientists. But, from the viewpoint of the Pompeiians and their neighbors, there were just three important stages. First was the initial ejection what dropped several meters of inflated pumice on the city. Most of the population seems to have survived the pumice fall, which accumulated at a rate of more than to 15 centimeters per hour, by the simple expedient of staying inside sturdy buildings. Some roofs collapsed and people who ventured outside might have been injured and a few even killed by the stones up to 5 centimeters in diameter falling at more than 50 meters per second. This phase lasted about 12 hours, during which a column of ejecta, which was supported by convection from the heat of the volcano and by the pressure of continuous constricted eruption, rose to about 33 kilometers.
Then the second and really deadly phase started. Possibly because by that time the vent had widened or perhaps because the volatility of the ejected material had decreased, the 33-kilometer-high ejecta plume collapsed on itself and poured down the slopes carrying with it newly released gasses and pyroclastics. Superheated surges, 500 degrees centigrade or more, rushed over and through the towns surrounding Vesuvius, killing everything -- humans, animals, vegetation -- in their path. The people of Pompeii, made visible in plaster casts centuries later, were not overcome by poisonous gasses as had long been conjectured, but rather they were killed instantly by thermal shock. In some areas, the heat surges were augmented by pyroclastic flows. Herculanum, long thought to have been inundated by post-eruption mud flows (lehars), was really buried by pyroclastic flows, the leading edge of which covered the 6 kilometer distance from the summit to Herculanum in just four minutes following the collapse of the ejecta plume. Almost all of the 4-5000 known deaths occurred during this phase, and almost all of the dead were found on top of the first phase pumice fall layer.
The third phase, which may actually have continued for some days, was the burial of the whole area in a thick layer of much finer and lighter volcanic ash (tuff) which eventually consolidated into the tufa stone which until today is quarried from numerous pits in the area.
Vesuvius, because of its history, its continuing venting and associated tectonic activity, and its proximity to heavily populated areas, is still one of the most dangerous volcanoes in the world, and it is monitored constantly by earthbound and satellite carried instruments. It is one of sixteen "decade volcanoes" worldwide that were put under special watch beginning in the 1990s as part of the International Decade for Natural Disaster Reduction.
There are numerous hard copy descriptions of Pompeii, Vesuvius, and the surrounding areas.  The easiest to carry with you is the Blue Guide to Southern Italy, which gives more than enough detail for a visitor (in very small print!)
Note: Vesuvius is not considered the most dangerous volcano in the world. That distinction belongs to Mount Rainier, the highest peak in the Cascade Range in Washington state, which overshadows a population of 2.5 million and a National Park that receives over two million annual visitors. Rainier is the real reason why folks in Seattle, Tacoma, and other nearby towns should be sleepless.


Previous Posts:

An Interview of Pliny the Younger, the nephew of Pliny the Elder (the writer of the Natural Histories of Vesuvias)

Pompeii - According to Wikipedia

The Destruction of Pompeii, 79 AD