Copyrighted photograph of a street in Pompeii by Robert Decker, 1971. Vesuvius is in the background.

Probably one of the most detailed studies of a large explosive eruption anywhere was that done on the AD 79 Vesuvius eruption by Sigurdsson et al (1985) in the National Geographic Research vol. 1, no. 3, pp. 332-387. It should be required reading for any students of Vesuvius and Pompeii. It is a long article so I won't re-type more than just a short part that deals with the people being killed.

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The citizens of Pompeii became directly aware of Vesuvius' eruption sometime in the early afternoon of 24 August as coarse pumice-fall began to plunge the city into darkness. No trace remains of the initial fine ashfall that affected areas to the north and east farther up the slopes of the volcano. The initial explosive phase of the eruption (A-1) may have been witnessed from Pompeii -- an excellent view of the summit -- but without direct consequences; it probably only generated curiosity.

Pumice and lithics rained continuously from early afternoon on 24 August to early the next morning, according to accounts of Pliny the Younger. During this time 130-140 cm of white pumice (A-2) accumulated, on top of which another 110 to 130 cm of gray pumice was laid down (A-3 to A-5). Although the pumice layer appears relatively homogeneous, the diameter of lithics and pumice varies throughout (Figure 23), reflecting variations in height of the eruption column during the Plinian phase. For example, relatively large, dense pieces of pumice are concentrated about 10 cm above the base of the gray pumice layer.

Pompeii happened to be located on the secondary thickness maximum of the fallout deposit, and thus received the thickest accumulation of pumice- and lithic-fall. Roofs probably collapsed about halfway through the deposition of white pumice (some 40 cm). With most structures unsafe for habitation, an exodus from the city is likely to have begun. Escape of most of Pompeii's residents can thus be attributed to the extended yet comparatively innocuous Plinian fallout phase.

The first surge to reach Pompeii swept against the north wall of the city in the early morning of 24 August, depositing dark gray ash near the Herculaneum Gate (S-3). Neither Vesuvius Gate, 200 m to the east, nor other sites in the Pompeii area evidence this surge. Therefore it is possible that the surge cloud did not extend inside the city walls but flowed just west of Pompeii over the villa dei Misteri and Villa Diomede. The surge must have caused alarm and almost unbearable conditions in the city. Evidence from Mount St. Helens in 1980 and El Chichon in 1982 indicates a peripheral zone of high heat associated with the distal ends of surge clouds (Moore & Sisson 1981; Sigurdsson, Carey et al. 1984). The first surges of the eruption (S-1 and S-2), which form a characteristic doublet in the middle of the pumice-fall at Boscoreale and Oplontis, did not reach as far southeast as Pompeii.

Within the city, 3 cm of pumice- and lithic-fall (A-6) accumulated before the next surge (S-4) overwhelmed the city. This surge also extended to Bottaro, 1 km south of Pompeii (Figure 8), and to Tricino, 3 km east of Pompeii (Figure 10).

The majority of human remains discovered in the excavations have been found on top of the pumice-fall layer, lying within surge deposits S-4 and S-5, but principally buried by the thick S-6 surge (Figure 24). Because of their fine-grained, silty nature, the surges have preserved accurate molds of the victims including details of facial expressions and sometimes clothing. With time, the soft tissues have decayed, leaving only bones in the hollow cavities. In 1860, Giuseppe Fiorelli developed the ingenious technique of making plaster casts of these impressions before the surrounding surge is disturbed (figure 25). Many hundreds of casts of the dead in Pompeii have since been made in this manner. In 1966, for example, casts of 13 victims were made in the Garden of the Fugitives, where they fell in various groups of adults and children on top of the pumice-fall deposit. Thus evidence is compelling that the S-4 surge was the lethal event in Pompeii. Since detailed stratigraphic studies of the deposits have not yet been feasible inside Pompeii, the extent of building damage resulting from the S-4 surge cannot be judged. By this time the ground-floor levels of buildings had already been buried and only the upper stories protruded from the pumice blanket.

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As I said, this is a very comprehensive study, and you should get a copy or two to read. It seems as if the people survived the fall of over a meter of pumice but that this pumice did start to collapse walls. When pyroclastic surges (fast-moving, horizontally-directed, ash and rock-laden blasts) occurred, the remaining people were killed, either by the heat, by lack of oxygen in the thick dusty cloud, or by being struck by larger blocks. They were buried by a later surge. Most of what you see as the "bodies" are apparently plaster casts rather than preserved bodies.

Scott Rowland

The mold of a dog is shown in the above photo. The poor animal was chained to a post and struggled for hours before finally succumbing to the ash.

Have a look at Vesuvius and Po! An excellent description of the eruption and conditions in Pompeii and Herculaneum can be found in Peter Francis' Volcanoes text.

Sources of Information:
Francis, P., 1994, Volcanoes a planetary perspective: Oxford University Press, New York, 443 p.

Steve Mattox

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