The Enigma of the 79 AD Eruption of Vesuvius
By Henrik Posted 12/08/2016 on Volcano Cafe
(One of my favourite topics.)
Artist’s impression of the 79 AD eruption as seen from Pompeii. Image is probably a screen-grab from the movie “Last Days of Pompeii”.
The August 24th eruption of Vesuvius in 79 AD is the most famous and well-known volcanic eruption of all time. By now, volcanologists have pieced together the sequence of events to form a coherent and comprehensive picture and the only official dilemma is the actual date with meteorological evidence arguing a date towards the middle or end of September. Be that as it may, this is the history of the 79 AD eruption as it stands today.
Seventeen years before the eruption, the Neapolis (literally New City) area was struck by a powerful earthquake which seems to have had its epicentre close to Pompeii as the destruction was heaviest there. There are reports of hundreds of sheep dying from tainted air, probably carbon dioxide poisoning, which argues that the 62 AD earthquake was a precursor of the 79 AD eruption. In 64 AD, the emperor Nero visited Neapolis and performed at the theatre. During his performance, another earthquake struck but legend has it that the emperor concluded his performance after which the theatre was evacuated and then collapsed.
Map showing the major locations such as Misenum from where Pliny the Younger observed the AD 79 eruption. Please note that at the time, Pompeii was situated on the coast and that one probable reason it was not rebuilt was that after the eruption, the location was inland! (Wikimedia Commons)
On August 20th, 79 AD, the area was struck by a series of small earthquakes which increased in number over the next four days but according to Pliny the Younger, people were not alarmed as they were used to it. From the distance of Misenium where he was staying, the morning of August 24th seemed to be normal but around 1 p.m., the volcano erupted. A messenger from a friend of Pliny the Elder, one Rectina of Stabiae south of Pompeii, reached Pliny just as the first eruption occurred with reports of earlier and smaller eruptions not seen from Misenium. Modern research, primarily by Haraldur Sigurdsson (1982 and 2002) paints the following picture of the event:
Possible profile of Vesuvius prior to the 79 AD eruption
The initial eruption was phreatomagmatic in nature, i.e. “throat-clearing”, and resulted in a column of pulverised, old material at least 15 km high. The column eventually collapsed and later, during the night, pyroclastic flows began. The first one is believed to have been about 550C hot and engulfed Herculaneum, killing some 5,000 inhabitants including those who had sought shelter in the boathouses. Mercifully death was quick as their brains quickly boiled and their skulls exploded.
According to Sigurdsson & associates, the eruption altered between plinian and peléan no less than six times. The main eruption when the eruption column is believed to have reached an altitude of 33 km and the peléan surges 4 and 5 that followed are believed to have been responsible for the destruction of Pompeii. Because of the greater distance from Vesuvius, about 11 km, the pyroclastic surges were not as hot at about 300C but hot enough to instantly cook the skin of its victims, effectively leaving them with third-degree burns covering 100% of their bodies, causing almost instantaneous death from heat shock. In the evening of August 25th, the eruption was over after some 30 hours.
Google Earth(?) image of Vesuvius clearly showing the scarp of the pre-eruption volcano named Monte Somma.
The 79 AD eruption has been assigned an “official” VEI of 5 but there are several reasons to doubt this. First of all, let us compare with the 1991 eruption of Mount Pinatubo! On July 16th 1990, the central Luzon was struck by a magnitude 7.7 earthquake which may have been a precursor to the eruption. On March 15th, earthquakes were felt by inhabitants close to the volcano and on April 2nd, phreatic explosions began and continued for several weeks. On June 7th, the first magmatic explosion occurred with an ash column some 7 km high. On June 12th, the large explosions began with a 30 min eruption which produced a 19 km high column and pyroclastic flows up to 4 km from the volcano. The next day, a 15 min explosion sent an ash column some 24 km high, then another eruption occurred lasting five minutes that produced a 24 km high column. The next day, June 14th, a three minute eruption resulted in a 21 km high column. On June 15th, Pinatubo was shattered by a cataclysmic eruption that lasted three hours and sent a 34 km high ash column skywards.
From this description, it is clear that Vesuvius eruption was comparable to that of Pinatubo, especially if one considers the brevity of the several Pinatubo eruptions in comparison to those of Vesuvius. The main Pinatubo eruption on June 15thlasted for three hours and produced an ash column 34 km high against the at least six hours long Vesuvius main eruption that resulted in a column 33 km high. The figure given for the Plinian phase of Vesuvius 79 AD eruption is eighteen hours. The depth of the pumice and pyroclastic flow deposits, 20 metres at Herculaneum and 2.8 m pumice plus several metres of pyroclastics at Pompeii, 11 km from the volcano, are comparable.
But the best evidence that volcanologists have (most likely) gotten it wrong comes from linguistics. Just to the north of the modern cone of Vesuvius lies a scarp or ridge of a much larger volcano named Monte Somma, which is sometimes claimed to be the result of the VEI 6 Avellino eruption dated to approximately 1800-1650 BC even if other sources suggest an eruption 17,000 BP as the originator. A study of the Avellino eruption reconstructs a minimum eruption time of 3 hours in which an initial explosion raised a column of 23 km which deposited about 0.32 km3 of white pumice while a second, more intense explosion, raised a column of 31 km depositing 1.25 km3 of grey pumice with pyroclastic deposits up to 50 cm thick at Avellino. As can be inferred from this reconstruction, the VEI 6 Avellino eruption was less explosive and the deposits were far less extensive and thick as those of the VEI 5 eruption of 79 AD.
Vesuvius as it appeared prior to the 79 AD eruption as depicted in a mural discovered in Pompeii.
But let us return to the linguistic evidence! Somma is Latin for summit and can only be the result of a situation where the scarp at 1,132 m a.s.l. is higher than the cone of Vesuvius, i.e. after the 79 AD eruption! At the time of the Avellino eruption, the tribe that was to become the Romans had not yet arrived in Italy, therefore it is impossible that the name is derived from a feature left by that or an earlier eruption. Furthermore, “somma” is not Etruscan in origin. Also, paintings of Vesuvius prior to 79 AD found at Pompeii show no signs of this “Monte Somma”. They appear to show a single cone about 1,900 metres high, although some claim it may have been as high as 2,500 m. It was also where Spartacus had his lair during the Third Servile War (73–71 BC)
From this evidence it is clear that the 79 AD eruption was, not a VEI 5, but a VEI 6 eruption that resulted in a collapse of Vesuvius where the volcano lost over a km in height and created the caldera whose highest point was afterwards named Monte Somma. The linguistic evidence provided by the name Monte Somma is so strong that any reconstruction of Vesuvius 79 AD eruption must account for this. The gauntlet has been thrown
Henrik
What I have to say about this, now?
Last year I didn't know much about Avellino. It was a new to me knowing about strong eruptions of Vesuvius before Pompeii (the most famous one), and I was just starting to explore the VEI classification. I ended in this page exactly by searching about the supposely 'VEI-5' of Pompeii.
What Henrik wrote made sense, indeed. Volcanic deposit on Pompeii are too thick for a VEI-5. Before Pompeii excavation, deposits were 7 meters thick - so I read on my science book at school, years ago. VEI-5 deposits are normally one or two meters thick, three meters if we consider a major VEI-5 eruption like the last eruption of Mt.Fuji, however 7 meters is already too thick. Somewhere (including Wikipedia) I've read that 6 meters is the minimum deposits thickness for a VEI-6.
The assigned 'VEI 5' to that eruption is precisely a 'VEI-5?' with a question mark, and some expert already increased the classification to a full VEI-6. The deposits that are found south of Naples (They say 4 to 8 km3, it depends from the site) are just a percentage of what came out from Vesuvius. In fact, in reality, most of ejecta ended in the sea and on mountains, and for geographical reason, deposits in the sea and on mountains went quickly eroded by rain and water. This would means that Vesuvius, most likely erupted about 10 km3 of material (I found that the total ejecta was about 12,5 km3). Deposit from an explosive eruption, unlike lava, don't last long in time. The concave flank of stratovolcanoes (such as Vesuvius) look that way exactly for that reason. Tephra needs time to cement and meanwhile mere wind and especially water scratch it away from volcanoes flanks. Pumice float and went lost.
The graphic above is from IGNV Osservatorio Vesuviano. Pompeii is classified as VEI-6.
From Volcanism and associated hazards: the Andean perspective, those are the most well know eruptions of the last 2000 years.
It is not true that Avellino Eruption was less intense than Pompeii eruption. They are right to say it was actually stronger. Avellino is more than 30 km away from Vesuvius, and from Avellino, you can't see Vesuvius. It is hidden behind two mountains, as tall as Vesuvius. And at more than 30 km away from the volcano, behind two mountains, there were full 50 cm of deposits. Avellino eruption is also known to have altered the climate, however Vesuvius survived better back then, with a less damaged edifice.
Pompeii was a minor VEI-6, while Avellino was a full VEI-6.
Above, the distance between Vesuvius and Avellino.
As full power, Vesuvius is an average volcano (Most of well known volcanoes you can name had at least one VEI-6 eruption in the past), overall, Vesuvius is among the most violent volcanoes in the world, in good company with la Peleé and Merapi, worth deserving it's title as 'one of the most dangerous volcanoes on Earth'.
I forgot: the real date of Pompeii eruption is in late October, and not in August.
