Monday, August 31, 2015

[Geology2] The Enduring Mysteries Of Mount Vesuvius And The Destruction Of Pompeii

Aug 25, 2015

The Enduring Mysteries Of Mount Vesuvius And The Destruction Of Pompeii

David Bressan


Opinions expressed by Forbes Contributors are their own.

Mt. Vesuvius today. (GABRIEL BOUYS/AFP/Getty Images)

Mt. Vesuvius today. (GABRIEL BOUYS/AFP/Getty Images)

Overlooking the bay of Naples, Mount Vesuvius is today part of one of the most recognized skylines in the world. But how how did Vesuvius Mons (the name use by the ancients Roman) look on the day of its famous eruption in the August of 79 AD?

The appearance of Mount Vesuvius and its surrounding area before that catastrophic eruption has been – and still is – a debated topic for geologists and archaeologists alike. But it's not a purely speculative topic – we not only have some geological clues about the area, but also written descriptions and some contemporary drawings as well. Roman authors who cite Vesuvius in their works include Strabo, Vitruvius and Diodorus Siculus.

In his Geographia, Strabo describes the "burned" rocks of the mountain and compares Vesuvius to the more active Mt. Etna. Additionally, Diodorus and Vitruvius seem to have grasped the volcanic origin of the mountain:

It is said, that once a fire burned below Vesuvius and spilled out a boiling flood, inundating the nearby countryside: so that the rock now called Pompeian Pumice, once was another sort of rock, reduced by fire to its actual quality.

Though some naturalists at the time recognized and described Vesuvius' volcanic nature, the quiet mountain was not considered a real danger anymore. Pliny the Elder, a naturalist who owned a villa near the gulf of Naples and who died during the eruption, never even mentioned the volcano in his works. Additionally, paintings found in the ruins of the city of Pompeii drawings of mountains are not very prominent. The only exception to this is one fresco, discovered between 1879 and 1881 in the so called "House of the Centenary." This fresco of Bacchus and Vesuvius shows the mountain with steep slopes and a single summit.

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Bacchus, god of pleasure and vine, shown with a mountain covered by vineyards. Soils of volcanic origin are very fertile and the sunny slopes of Vesuvius perfect for vineyards, however the interpretation of the painting as showing Mount Vesuvius is somehow dubious as the figure doesn't match necessarily written contemporary descriptions of Vesuvius (image in public domain).

That fresco, however, doesn't quite match contemporary accounts. Indeed, Strabo described the slopes of Vesuvius as being covered by vineyards and forests, but he also mentioned a flat top or a volcanic dome (?) without vegetation – a detail not shown in the fresco.

Vesuvius Mons overlooks these cities, covered entirely, with the exception of the summit, by cultivated fields. The top is mostly flat, without vegetation and of grayish color, it shows deep fissures, whose reddish rocks seems have been eroded by time.

Another, lesser known drawing from Pompeii seems to support Strabo's description. In the background of a fresco from the House of the Citharist, discovered between 1853 and 1868, a couple (identified as either Aeneas and Dido or Mars and Venus) is shown relaxing in the shade of a flat-topped mountain.

One of the most detailed descriptions of Vesuvius was provided by the historian Dio Cassius. However, his description could very easily describe the mountain post-eruption:

… Mount Vesuvius overlooks the sea … and contains copious sources of fire, the summit is of regular shape, so that the fire is found in the center..  the fire consumes the rocks in the middle, however the peaks around retain their ancient height, but the inner part, consumed by fire and time, has become hollow and was refilled by sediments, so that the entire mountain looks like an amphitheater. The higher ground of that mountain is covered by many trees and vines…

Available geological data isn't good enough to settle this debate, but it also suggests that Vesuvius was probably a flat-topped, unimpressive, mountain at the time of the 79 AD eruption. The distribution over a large area of the "Avellino pumice" suggests that Vesuvius lost much mass and height during a prehistoric eruption dated to 1,880 – 1,680 BC.

The crater that formed during the 79 AD eruption was later destroyed by an eruption in 472 AD. Since 1631, though, pictures of Vesuvius show today's familiar outline, with its two peaks formed by the caldera rim of Monte Somma, surrounding the inner Gran Cono.


"View of Vesuvius and surrounding area after the destruction by the eruption of the year 1631" by Giovanni Morghen from G.M. Mecattis "Racconto storico-filosofico del Vesuvio" (1752) shows the volcano already with his modern topography (image in public domain).

But it may surprise you to learn that the date of the 79 AD eruption itself is disputed.

Pretty much all textbooks provide a data of August 24 as the beginning of the eruption. That dates is based on two letters from the Roman author Pliny the Younger (nephew of Pliny the Elder) to Tacitus, a historian who had asked his friend for information about the death of his uncle.

However, the original letters by Pliny didn't survive into modern times, so their text is only known from transcriptions from the Medieval era. By that point, various versions of the letters existed, showing dates ranging from August to November (and some without any reference to a date at all). This discrepancy can be explained by various translation and transcription errors occurring over time, almost inevitable considering that the eruption happened almost 19 centuries ago.

Some circumstantial evidence suggests the eruption occurred later than August:

  • The famous gypsum-casts found in Pompeii show people wearing thick cloths, unusual for August but appropriate for the cool temperatures of early autumn. Portable stoves, many ready to use, were also discovered in many buildings.
  • Despite the fact that the volcanic sediments preserved organic remains very well, fruits typically found in summer are rare in the ruins of Pompeei. Autumn fruits, like olives and figs, were plentiful in uncovered shops. This could suggest that those stores were buried some time after the harvest, perhaps late October.
  • Large jars, used to ferment wine, were discovered already sealed in Pompeii. Considering that the grapes used matured in early autumn, the fermenting wine could also suggest a late October eruption date.
  • A certain coin – a Capricorn Silver Denarius issued by emperor Titus in July – June 79 AD, was discovered with the corpse of a woman buried in the ashes of Pompeii. This also suggests the eruption occurred late in summer/early autumn, as the coin would not have been circulating earlier. However, the exact identification of the coin (there exist various editions and the inscriptions of the discovered coin are difficult to read) is still disputed.
  • Archaeologists have uncovered the preserved remains of garum, a spicy fish sauce that's made using the fish species Boops boops (bogue), which abounds in the Mediterranean Sea from July to August. This could also (or also not) point to a time of the eruption sometime between late August – September, given the time needed for fishermen to provide fresh fish to manufacturers, who in turn needed about a month to produce the garum.

In addition to the above, there's also some geological evidence – such as the distribution of ash deposits – that casts doubt on the August date. The mapped ash layers suggest that during the eruption, the wind came from the east. This wind pattern is unusual for summers in Naples, but common in the rest of the year.

Though we know more about the 79 AD eruption of Mount Vesuvius than other historic eruptions, it's still a cold case in the history of volcanic investigations.

Interested in reading more? Try:

DE CAROLIS, E. & PATRICELLI, G. (2003): Vesuvio 79 d.C. la distruzione di Pompei ed Ercolano. L´ERMA di BRETSCHNEIDER: 129

STEFANI, G. (2006): Scoperte Campania – La vera data dell´eruzione. ARCHEO 260 – Ottobre: 10-13

PARASCANDOLA, A. (1938): L' attività e la forma del Vesuvio nell' antichità e l' origine del suo nome. Gli Abissi, Vol.1.

ROLANDI, G.; PAONE, A.; LASCIO, M.di & STEFANI, G. (2008): The 79 AD eruption of Somma: The relationship between the date of the eruption and the southeast tephra dispersion. Journal of Volcanological and Geothermal research. Vol. 169(1-2): 87-98


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