Happy Anniversary - Mt. Vesuvius

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Vesuvius has erupted many times. The famous eruption in 79 was preceded by numerous others in prehistory, including at least 3 significantly larger ones, the best known being the Avellino eruption around 1800 BC which engulfed several Bronze Age settlements. Since 79, the volcano has also erupted repeatedly, in 172, 203, 222, possibly 303, 379, 472, 512, 536, 685, 787, around 860, around 900, 968, 991, 999, 1006, 1037, 1049, around 1073, 1139, 1150, and there may have been eruptions in 1270, 1347, and 1500. The volcano erupted again in 1631, six times in the 18th century, eight times in the 19th century (notably in 1872), and in 1906, 1929, and 1944. There has been no eruption since 1944, and none of the post-79 eruptions were as large or destructive.

The eruptions vary greatly in severity but are characterized by explosive outbursts of the kind dubbed Plinian after Pliny the Younger, the Roman naturalist who observed the 79 eruption, to which his uncle Pliny the Elder possibly fell victim. On occasion, the eruptions have been so large that the whole of southern Europe has been blanketed by ash; in 472 and 1631, Vesuvian ash fell on Constantinople (Istanbul), over 1,200 km away. A few times since 1944, landslides in the crater raised clouds of ash dust, which caused false alarms of an eruption.

Before 79
The mountain started forming 25,000 years ago. Although the area has been subject to volcanic activity for at least 400,000 years, the lowest layer of eruption material from the Somma mountain lies on top of the 34,000 year-old Campanian Ignimbrite produced by the Campi Flegrei complex, and was the product of the Cordola plinian eruption 25,000 years ago. It was then built up by a series of lava flows, with some smaller explosive eruptions interspersed between them. However, the style of eruption changed around 19,000 years ago to a sequence of large explosive plinian eruptions, of which the 79 one was the last. The eruptions are named after the tephra deposits produced by them:

The Basal Pumice (Pomici di Base) eruption, 18,300 years ago, VEI 6, was probably the most violent of these eruptions and saw the original formation of the Somma caldera. The eruption was followed by a period of much less violent, lava producing eruptions.

The Green Pumice (Pomici Verdoline) eruption, 16,000 years ago, VEI 5.

The Mercato eruption (also known as Pomici Gemelle or Ottaviano), 8,000 years ago, VEI 6, followed a smaller explosive eruption around 11,000 years ago (called the Lagno Amendolare eruption).

The Avellino eruption (Pomici di Avellino), 1660 BC ± 43 years, VEI 6, followed two smaller explosive eruptions around 5,000 years ago. The Avellino eruption vent was apparently 2 km west of the current crater, and the eruption destroyed several Bronze Age settlements. The remarkably well-preserved remains of one were discovered in May 2001 near Nola by Italian archaeologists, with huts, pots, livestock and even the footprints of animals and people, as well as skeletons. The residents had hastily abandoned the village, leaving it to be buried under pumice and ash in much the same way that Pompeii was later preserved. The eruption was larger than the ones of 79 (VEI 5) and 1631 (VEI 4) with pyroclastic surge deposits distributed to the northwest of the vent, the surges travelling as far as 15 km from it, and lie up to 3 m deep in the area now occupied by Naples.

The volcano then entered a stage of more frequent, but less violent, eruptions until the most recent plinian eruption which destroyed Pompeii.

The last of these may have been in 217 BC. There were earthquakes in Italy during that year and the sun was reported as being dimmed by a haze or dry fog. Plutarch wrote of the sky being on fire near Naples and Silius Italicus mentioned in his epic poem Punica that Vesuvius had thundered and produced flames worthy of Mount Etna in that year, although both authors were writing around 250 years later. Greenland ice core samples of around that period show relatively high acidity, which is assumed to have been caused by atmospheric hydrogen sulfide.

The mountain was then quiet for hundreds of years and was described by Roman writers as having been covered with gardens and vineyards, except at the top which was craggy. Within a large circle of nearly perpendicular cliffs was a flat space large enough for the encampment of the army of the rebel gladiator Spartacus in 73 BC. This area was doubtless a crater. The mountain may have had only one summit at that time, judging by a wall painting, "Bacchus and Vesuvius", found in a Pompeiian house, the House of the Centenary (Casa del Centenario).

Several surviving works written over the 200 years preceding the 79 eruption describe the mountain as having had a volcanic nature, although Pliny the Elder did not depict the mountain in this way in his Naturalis Historia:

The Greek historian Strabo (ca 63 BC-24 AD) described the mountain in Book V, Chapter 4 of his Geographica as having a predominantly flat, barren summit covered with sooty, ash-coloured rocks and suggested that it might once have had "craters of fire". He also perceptively suggested that the fertility of the surrounding slopes may be due to volcanic activity, as at Mount Etna.

In Book II of De Architectura, the architect Vitruvius (ca 80-70 BC -?) reported that fires had once existed abundantly below the mountain and that it had spouted fire onto the surrounding fields. He went on to describe Pompeiian Pumice as having been burnt from another species of stone.

Diodorus Siculus (ca 90 BC — ca 30 BC), another Greek writer, wrote in Book IV of his Bibliotheca Historica that the Campanian plain was called fiery (Phlegrean) because of the mountain, Vesuvius, which had spouted flame like Etna and showed signs of the fire that had burnt in ancient history.

By 79 the area was, as now, densely populated with villages, towns and small cities like Pompeii, and its slopes were covered in vineyards and farms.

Eruption of 79

By the 1st century, Pompeii was only one of a number of towns located around the base of Mount Vesuvius. The area had a substantial population which grew prosperous from the region's renowned agricultural fertility. Many of Pompeii's neighboring communities, most famously Herculaneum, also suffered damage or destruction during the 79 eruption, which is thought to have lasted about 19 hours, in which time the volcano released about 1 cubic mile (4 cubic kilometres) of ash and rock over a wide area to the south and south-east of the crater, with about 3 m (10 ft) of tephra falling on Pompeii. The white ash produced by this eruption is mainly of leucite and phonolite.

Foreshocks

The 79 eruption was preceded by a powerful earthquake seventeen years beforehand on 5 February 62, which caused widespread destruction around the Bay of Naples, and particularly to Pompeii. Some of the damage had still not been repaired when the volcano erupted. However, this may have been a tectonic event rather than one associated with the re-awakening of the volcano.

Another smaller earthquake took place in 64; it was recorded by Suetonius in his biography of Nero, in De Vita Caesarum, and by Tacitus in Book XV of Annales because it took place whilst Nero was in Naples performing for the first time in a public theatre. Suetonius recorded that the emperor continued singing through the earthquake until he had finished his song, whilst Tacitus wrote that the theatre collapsed shortly after being evacuated.

The Romans grew used to minor earth tremors in the region; the writer Pliny the Younger writing that they "were not particularly alarming because they are frequent in Campania". In early August of 79, springs and wells dried up. Small earthquakes started taking place on 20 August, 79 becoming more frequent over the next four days, but the warnings were not recognised (it is worth noting the Romans had no word for volcano, and only a hazy concept of other similar mountains like Mount Etna, home of Vulcan), and on the afternoon of 24 August, a catastrophic eruption of the volcano started. The eruption devastated the region, burying Pompeii and other settlements. By coincidence it was the day after Vulcanalia, the festival of the Roman god of fire.

Nature of the eruption
The eruption of Vesuvius on 24 and 25 August, 79, unfolded in two phases, a Plinian eruption that lasted eighteen to twenty hours and produced a rain of pumice southward of the cone that built up to depths of 2.8 meters at Pompeii, followed by a pyroclastic flow or nuée ardente in the second, Peléan phase that reached as far as Misenum but was concentrated to the west and northwest. Two pyroclastic flows engulfed Pompeii, burning and asphyxiating the stragglers who had remained behind. Oplontis and Herculaneum received the brunt of the flows and were buried in fine ash and pyroclastic deposits.

The Two Plinys
Pliny the Younger
The only surviving reliable eyewitness account of the event was recorded by Pliny the Younger in two letters to the historian Tacitus. Observing it from Misenum (across the bay, approximately 35 km from the volcano) whilst his uncle sailed closer, he saw an extraordinarily dense and rapidly-rising cloud appearing above the mountain:

“I cannot give you a more exact description of its appearance than by comparing to a pine tree; for it shot up to a great height in the form of a tall trunk, which spread out at the top as though into branches. ... Occasionally it was brighter, occasionally darker and spotted, as it was either more or less filled with earth and cinders. (Sixth Book of Letters, Letter 16.)”

This was the eruption column, now estimated to have been more than 32 km (20 miles) tall.

After some time he described the cloud rushing down the flanks of the mountain and covering everything around it, including the surrounding sea. This is known today as a pyroclastic flow, which is a cloud of superheated gas, ash, and rock that erupts from a volcano. Geologists have used the magnetic characteristics of over 200 volcanic rocks and pieces of debris (e.g. roof tiles) found in Pompeii to estimate the temperature of this pyroclastic flow. (When molten rock solidifies, magnetic minerals in the rock record the direction of Earth's magnetic field. If the material is heated above a certain temperature, known as the Curie temperature, the rock's magnetic field may be modified or completely reset.) Most of the materials analyzed experienced temperatures between 240 °C and 340 °C (with a few areas showing lower temperatures of only 180 °C). This suggests that the ash cloud had a temperature of 850 °C when emerging from the mouth of Vesuvius and had cooled to below 350 °C by the time it reached the city. It is theorized that turbulence may have mixed cool air into the ash cloud. (Cioni, et al., 2004). This is now called the Plinian stage of the eruption, named after both the younger and elder Plinys.

Pliny stated that several earth tremors were felt at the time of the eruption and were followed by a very violent shaking of the ground. He also noted that ash was falling in very thick sheets and the village he was in had to be evacuated, and then that the sun was blocked out by the eruption and the daylight hours were left in darkness. Also, the sea was sucked away and forced back by an "earthquake", a phenomenon now called a tsunami.

Pliny the Elder
Pliny’s uncle Pliny the Elder was in command of the Roman fleet at Misenum, and had meanwhile decided to take several ships to investigate the phenomenon at close hand. The fleet also attempted a rescue mission for those at the foot of the volcano when, as the ship was preparing to leave the area, a messenger arrived from a friend of Pliny’s living on the coast near the foot of the volcano imploring him to rescue her. He set off across the bay but encountered thick showers of hot cinders, lumps of pumice and pieces of rock which, altering the shoreline and water depths, blocked his approach to the shore and prevented him from landing there. The prevailing southerly wind also stopped him landing there, but he continued south under it to Stabiae (about 4.5 km from Pompeii) where he landed and took shelter with Pomponianus, a friend. Pomponianus had already loaded a ship with possessions and was preparing to leave, but the wind was against him.

Pliny and his party saw flames coming from several parts of the mountain (probably pyroclastic flows and surges, which would later destroy Pompeii and Herculaneum). After staying overnight, the party decided to evacuate in spite of the rain of tephra because of the continuing violent conditions threatening to collapse the building. Pliny, Pomponianus and their companions made their way back towards the beach with pillows tied to their heads to protect them from rockfall. By this time, there was so much ash in the air that the party could barely see through the murk and needed torches and lanterns to find their way. They made it to the beach but found the waters too violently disturbed from the continuous earthquakes for them to escape safely by sea.

Pliny the Elder collapsed and died, and in the first letter to Tacitus his nephew suggested that this was due to the inhalation of poisonous, sulphuric gases. However, Stabiae was 16 km from the vent (roughly where the modern town of Castellammare di Stabia is situated) and his companions were apparently unaffected by the fumes, and so it is more likely that the corpulent Pliny died from some other cause, such as a stroke or heart attack. His body was found with no apparent injuries on 26 August, after the plume had dispersed sufficiently for daylight to return.

Casualties from the eruption

Along with Pliny the Elder, the only other noble casualties of the eruption to be known by name were Agrippa (a son of the Jewish princess Drusilla and the procurator Antonius Felix) and his wife.

Estimates of the population of Pompeii range from 10,000 to 25,000, whilst Herculaneum is thought to have had a population of about 5,000. It is not known how many people the eruption killed, although around 1,150 remains of bodies — or casts made of their impressions in the ash deposits — have been recovered in and around Pompeii. The remains of about 350 bodies have been found at Herculaneum (300 in arched vaults discovered in 1980). However these figures must represent a great underestimation of the total number of deaths over the region affected by the eruption.

Thirty-eight percent of the victims at Pompeii were found in the ash fall deposits, the majority inside buildings. These are thought to have been killed mainly by roof collapses, with the smaller number of victims found outside of buildings probably being killed by falling roof slates or by larger rocks thrown out by the volcano. This differs from modern experience, since over the last four hundred years only around 4% of victims have been killed by ash falls during explosive eruptions. The remaining 62% of remains found at Pompeii were in the pyroclastic surge deposits, and thus were probably killed by them — probably from a combination of suffocation through ash inhalation and blast and debris thrown around. In contrast to the victims found at Herculaneum, examination of cloth, frescoes and skeletons show that it is unlikely that high temperatures were a significant cause.

Herculaneum, which was much closer to the crater, was saved from tephra falls by the wind direction, but was buried under 23 m (75 ft) of material deposited by pyroclastic surges. It is likely that most, or all, of the victims in this town were killed by the surges, particularly given evidence of high temperatures found on the skeletons of the victims found in the arched vaults, and the existence of carbonised wood in many of the buildings.

Pompeii and Herculaneum were never rebuilt, although surviving townspeople and probably looters did undertake extensive salvage work after the destructions. The eruption changed the course of the Sarno River and raised the sea beach, so that Pompeii was now neither on the river nor adjacent to the coast.

The towns' locations were eventually forgotten until their accidental rediscovery in the 18th century. Vesuvius itself underwent major changes — its slopes were denuded of vegetation and its summit had changed considerably due to the force of the eruption.

Date of the eruption
The eruption of 79 AD was documented by contemporary historians and is universally accepted as having started on August 24th. However the archeological excavations of Pompeii suggest that the town was buried a couple of months later. For example, people buried in the ash appear to be wearing warmer clothing than the light summer clothes that would be expected in August. The fresh fruit and vegetables in the shops are typical of October, and conversely the summer fruit that would have been typical of August was already being sold in dried, or conserved form. Wine fermenting jars had been sealed over, and this would have happened around the end of October. The coins found in the purse of a woman buried in the Ash include a commemorative coin that should have been minted at the end of September. So far there is no definitive theory as to why there should be such an apparent discrepancy.