Eruption of Mount Vesuvius in 79 AD

Eruption of Mount Vesuvius in AD 79
Destruction of Pompeii and Herculaneum by John Martin
Volcano	Mount Vesuvius
Date	August 24, AD 79 or October 24, AD 79
Type	Plinian
Location	Campanian volcanic arc, Italy 40°49′N 14°26′E
VEI	5
Impact	Buried the Roman settlements of Pompeii, Herculaneum, Oplontis, and Stabiae.

The eruption of Mount Vesuvius in 79 AD was one of the most catastrophic and infamous volcanic eruptions in European history. Historians have learned about the eruption from the eyewitness account of Pliny the Younger, a Roman administrator and poet.

Mount Vesuvius spewed a deadly cloud of volcanic gas, stones, and ash to a height of 33 kilometres (21 mi), ejecting molten rock and pulverized pumice at the rate of 1.5 million tons per second, ultimately releasing a hundred thousand times the thermal energy of the Hiroshima bombing. Several Roman settlements were obliterated and buried underneath massive pyroclastic surges and ashfall deposits, the most well known being Pompeii and Herculaneum.

The number of deaths is difficult to evaluate. The remains of about 1500 people have been found at Pompeii and Herculaneum, but it is not known whether they represent a small or a large part of the overall deaths.

Precursors and foreshocks

The Last Day of Pompeii. Painting by Karl Brullov, 1830–1833

The AD 79 eruption was preceded by a powerful earthquake seventeen years beforehand on February 5, AD 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. The deaths of 600 sheep from "tainted air" in the vicinity of Pompeii reported by Seneca the Younger leads Haraldur Sigurdsson to compare them to similar deaths of sheep in Iceland from pools of volcanic carbon dioxide and to speculate that the earthquake of 62 was related to new activity by Mount Vesuvius.

Another smaller earthquake took place in AD 64; it was recorded by Suetonius in his biography of Nero, and by Tacitus in Annales because it took place while 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, while Tacitus wrote that the theatre collapsed shortly after being evacuated.

The Romans grew accustomed to minor earth tremors in the region; the writer Pliny the Younger wrote that they "were not particularly alarming because they are frequent in Campania". Small earthquakes started taking place on 20 August 79, becoming more frequent over the next four days, but the warnings were not recognized.

Nature of the eruption

Vesuvius erupting. Painting by Norwegian painter J.C. Dahl, 1826.

Reconstructions of the eruption and its effects vary considerably in the details but have the same overall features. The eruption lasted for two days. The morning of the first day, August 24, was perceived as normal by the only eyewitness to leave a surviving document, Pliny the Younger, who at that point was staying at Misenum, on the other side of the Bay of Naples about 30 kilometres (19 mi) from the volcano, which may have prevented him from noticing the early signs of the eruption. He was not to have any opportunity, during the next two days, to talk to people who had witnessed the eruption from Pompeii or Herculaneum (indeed he never mentions Pompeii in his letter), so he would not have noticed early, smaller fissures and releases of ash and smoke on the mountain, if such had occurred earlier in the morning. Around 1:00 p.m., Mount Vesuvius violently exploded, throwing up a high-altitude column from which ash began to fall, blanketing the area. Rescues and escapes occurred during this time. At some time in the night or early the next day, August 25, pyroclastic flows in the close vicinity of the volcano began. Lights seen on the mountain were interpreted as fires. People as far away as Misenum fled for their lives. The flows were rapid-moving, dense, and very hot, knocking down wholly or partly all structures in their path, incinerating or suffocating all population remaining there and altering the landscape, including the coastline. These were accompanied by additional light tremors and a mild tsunami in the Bay of Naples. By evening of the second day the eruption was over, leaving only haze in the atmosphere through which the sun shone weakly.

Pliny the Younger wrote an account of the eruption:

Broad sheets of flame were lighting up many parts of Vesuvius; their light and brightness were the more vivid for the darkness of the night... it was daylight now elsewhere in the world, but there the darkness was darker and thicker than any night.

Stratigraphic studies

Herculaneum and other cities affected by the eruption of Mount Vesuvius. The black cloud represents the general distribution of ash and cinder. Modern coast lines are shown; Pliny the Younger was at Misenum.

According to a stratigraphic study (a study of the layers of ash) by Sigurdsson, Cashdollar, and Sparks, published in 1982, and now a standard reference, the eruption of Vesuvius of AD 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 metres (9 ft 2 in) 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 pumice, and pyroclastic deposits.

In an article published in 2002 Sigurdsson and Casey elaborate on the stratigraphic evidence based on excavations and surveys up until then. In this interpretation, the quasi-initial explosion (not quite initial) produced a column of ash and pumice ranging between 15 kilometres (49,000 ft) and 30 kilometres (98,000 ft) high, which, due to northwest winds, rained on Pompeii to the southeast but not on Herculaneum upwind. The eruption is viewed as primarily phreatomagmatic; that is, the chief energy supporting the column came from the escape of steam superheated by the magma, created from seawater seeping over time into the deep faults of the region, that came into interaction with magma and heat.

Subsequently the cloud collapsed as the gases densified and lost their capability to support their solid contents, releasing it as a pyroclastic surge, which reached Herculaneum but not Pompeii. Additional explosions reinstituted the column. The eruption alternated between Plinian and Peléan six times. Surges 4 and 5 are believed by the authors to have destroyed Pompeii. Surges are identified in the deposits by dune and cross-bedding formations, which are not produced by fallout.

The authors suggest that the first ash falls are to be interpreted as early-morning, low-volume explosions not seen from Misenum, causing Rectina to send her messenger on a ride of several hours around the Bay of Naples, then passable, providing an answer to the paradox of how the messenger might miraculously appear at Pliny's villa so shortly after a distant eruption that would have prevented him.

Magnetic studies

Inside the crater of Vesuvius

A 2006 study by Zanella, Gurioli, Pareschi, and Lanza used the magnetic characteristics of over 200 samples of lithic, roof-tile, and plaster fragments collected from pyroclastic deposits in and around Pompeii to estimate the equilibrium temperatures of the deposits. The deposits were placed by pyroclastic density currents (PDCs) resulting from the collapses of the Plinian column. The authors argue that fragments over 2–5 cm (0.79–1.97 in) were not in the current long enough to acquire its temperature, which would have been much higher, and therefore they distinguish between the depositional temperatures, which they estimated, and the emplacement temperatures, which in some cases based on the cooling characteristics of some types and fragment sizes of rocks they believed they also could estimate. Final figures are considered to be those of the rocks in the current just before deposition.

All crustal rock contains some iron or iron compounds, rendering it ferromagnetic, as do Roman roof tiles and plaster. These materials may acquire a residual field from a number of sources. When individual molecules, which are magnetic dipoles, are held in alignment by being bound in a crystalline structure, the small fields reinforce each other to form the rock's residual field. Heating the material adds internal energy to it. At the Curie temperature, the vibration of the molecules is sufficient to disrupt the alignment; the material loses its residual magnetism and assumes whatever magnetic field might be applied to it only for the duration of the application. The authors term this phenomenon unblocking. Residual magnetism is considered to "block out" non-residual fields.

A rock is a mixture of minerals, each with its own Curie temperature; the authors therefore looked for a spectrum of temperatures rather than a single temperature. In the ideal sample, the PDC did not raise the temperature of the fragment beyond the highest blocking temperature. Some constituent material retained the magnetism imposed by the Earth's field when the item was formed. The temperature was raised above the lowest blocking temperature and therefore some minerals on recooling acquired the magnetism of the Earth as it was in AD 79. The overall field of the sample was the vector sum of the fields of the high-blocking material and the low-blocking material.

This type of sample made possible estimation of the low unblocking temperature. Using special equipment that measured field direction and strength at various temperatures, the experimenters raised the temperature of the sample in increments of 40 °C (72 °F) from 100 °C (212 °F) until it reached the low unblocking temperature. Deprived of one of its components, the overall field changed direction. A plot of direction at each increment identified the increment at which the sample's resultant magnetism had formed. That was considered to be the equilibrium temperature of the deposit. Considering the data for all the deposits of the surge arrived at a surge deposit estimate. The authors discovered that the city, Pompeii, was a relatively cool spot within a much hotter field, which they attributed to interaction of the surge with the "fabric" of the city.

The investigators reconstruct the sequence of volcanic events as follows. On the first day of the eruption a fall of white pumice containing clastic fragments of up to 3 centimetres (1.2 in) fell for several hours. It heated the roof tiles to 120–140 °C (248–284 °F). This period would have been the last opportunity to escape. Subsequently a second column deposited a grey pumice with clastics up to 10 cm (3.9 in), temperature unsampled, but presumed to be higher, for 18 hours. These two falls were the Plinian phase. The collapse of the edges of these clouds generated the first dilute PDCs, which must have been devastating to Herculaneum, but did not enter Pompeii.

Early in the morning of the second day the grey cloud began to collapse to a greater degree. Two major surges struck and destroyed Pompeii. Herculaneum and all its population no longer existed. The emplacement temperature range of the first surge was 180–220 °C (356–428 °F), minimum temperatures; of the second, 220–260 °C (428–500 °F). The depositional temperature of the first was 140–300 °C (284–572 °F). Upstream and downstream of the flow it was 300–360 °C (572–680 °F).

The variable temperature of the first surge was due to interaction with the buildings. Any population remaining in structural refuges could not have escaped, as the city was surrounded by gases of incinerating temperatures. The lowest temperatures were in rooms under collapsed roofs. These were as low as 100 °C (212 °F), the boiling point of water. The authors suggest that elements of the bottom of the flow were decoupled from the main flow by topographic irregularities and were made cooler by the introduction of ambient turbulent air. In the second surge the irregularities were gone and the city was as hot as the surrounding environment.

During the last surge, which was very dilute, one meter more of deposits fell over the region.

The two Plinys

Pompeii, with Vesuvius towering above

The only surviving eyewitness account of the event consists of two letters by Pliny the Younger, who was 17 at the time of the eruption, to the historian, Tacitus. Observing the first volcanic activity from Misenum across the Bay of Naples from the volcano, approximately 35 kilometres (22 mi), the elder Pliny launched a rescue fleet and went himself to the rescue of a personal friend. His nephew declined to join the party. One of the nephew's letters relates what he could discover from witnesses of his uncle's experiences. In a second letter the younger Pliny details his own observations after the departure of his uncle.

Pliny the Younger

Further information: Pliny the Younger

The two men saw an extraordinarily dense cloud rising rapidly 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.

These events and a request by messenger for an evacuation by sea prompted the elder Pliny to order rescue operations in which he sailed away to participate. His nephew attempted to resume a normal life, continuing to study, and bathing, but that night a tremor awoke him and his mother, prompting them to abandon the house for the courtyard. At another tremor near dawn the population abandoned the village. After still a third "the sea seemed to roll back upon itself, and to be driven from its banks", which is evidence for a tsunami. There is, however, no evidence of extensive damage from wave action.

The early light was obscured by a black cloud through which shone flashes, which Pliny likens to sheet lightning, but more extensive. The cloud obscured Point Misenum near at hand and the island of Capraia (Capri) across the bay. Fearing for their lives the population began to call to each other and move back from the coast along the road. Pliny's mother requested him to abandon her and save his own life, as she was too corpulent and aged to go further, but seizing her hand he led her away as best he could. A rain of ash fell. Pliny found it necessary to shake off the ash periodically to avoid being buried. Later that same day the ash stopped falling and the sun shone weakly through the cloud, encouraging Pliny and his mother to return to their home and wait for news of Pliny the Elder. The letter compares the ash to a blanket of snow. Evidently the earthquake and tsunami damage at that location were not severe enough to prevent continued use of the home.

Pliny the Elder

Further information: Pliny the Elder

Pliny's uncle Pliny the Elder was in command of the Roman fleet at Misenum, and had meanwhile decided to investigate the phenomenon at close hand in a light vessel. As the ship was preparing to leave the area, a messenger came from his friend Rectina (wife of Tascius) living on the coast near the foot of the volcano, explaining that her party could only get away by sea and asking for rescue. How the messenger escaped remains unexplained. Suddenly grasping the full significance of events, Pliny ordered the immediate launching of the fleet galleys to the evacuation of the coast. He continued in his light ship to the rescue of Rectina's party.

He set off across the bay but in the shallows on the other side encountered thick showers of hot cinders, lumps of pumice, and pieces of rock. Advised by the helmsman to turn back he stated "Fortune favors the brave" and ordered him to continue on to Stabiae (about 4.5 km or 2.8 mi from Pompeii), where Pomponianus was. It is not clear whether he was abandoning the effort to reach Rectina's villa or believed Pomponianus was a member of Rectina's party. Pliny does not mention her again. Pomponianus had already loaded a ship with possessions and was preparing to leave, but the same onshore wind that brought Pliny's ship to the location had prevented anyone from leaving.

Pliny and his party saw flames coming from several parts of the mountain, which Pliny and his friends attributed to burning villages. After staying overnight, the party was driven from the building by an accumulation of material, presumably tephra, which threatened to block all egress. They woke Pliny, who had been napping and emitting loud snoring. They elected to take to the fields with pillows tied to their heads to protect them from rockfall. They approached the beach again but the wind had not changed. Pliny sat down on a sail that had been spread for him and could not rise even with assistance when his friends departed, escaping ultimately by land. Very likely, he had collapsed and died, which is the most popular explanation of why his friends abandoned him, although Suetonius offers an alternative story of his ordering a slave to kill him to avoid the pain of incineration. How the slave would have escaped to tell the tale remains a mystery. There is no mention of such an event in his nephew's letters.

In the first letter to Tacitus his nephew suggested that his death was due to the reaction of his weak lungs to a cloud of poisonous, sulphurous gas that wafted over the group. However, Stabiae was 16 km (9.9 mi) 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. An asthmatic attack is also not out of the question. His body was found with no apparent injuries on the next day, after dispersal of the plume.

Casualties from the eruption

File:Ring Lady.JPG

The skeleton called the "Ring Lady" unearthed in Herculaneum

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.

By 2003 around 1,044 casts made from impressions of bodies in the ash deposits had been recovered in and around Pompeii, with the scattered bones of another 100. The remains of about 332 bodies have been found at Herculaneum (300 in arched vaults discovered in 1980). What percentage these numbers are of the total dead or the percentage of the dead to the total number at risk remain completely unknown.

Thirty-eight percent of the 1044 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 buildings probably 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. It was initially believed that due to the state of the bodies found at Pompeii and the outline of clothes on the bodies it was unlikely that high temperatures were a significant cause. But in 2010, studies indicated that during the fourth pyroclastic surge – the first surge to reach Pompeii – temperatures reached 300 °C (572 °F). Volcanologist Giuseppe Mastrolorenzo, who led the study noted that "(It was) enough to kill hundreds of people in a fraction of a second". In reference as to why the bodies were frozen in suspended action, "The contorted postures are not the effects of a long agony, but of the cadaveric spasm, a consequence of heat shock on corpses."

Herculaneum, which was much closer to the crater, was saved from tephra falls by the wind direction, but was buried under 23 metres (75 ft) of material deposited by pyroclastic surges. It is likely that most, or all, of the known 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.

These people were all caught on the former seashore by the first surge and died of thermal shock but not of carbonization, although some were partly carbonized by later and hotter surges. The arched vaults were most likely boathouses, as the crossbeams in the overhead were probably for the suspension of boats. No boats have been found, indicating they may have been used for the earlier escape of some of the population. The rest were concentrated in the chambers at a density of as high as 3 persons per square meter. As only 85 metres (279 ft) of the coast have been excavated, the casualties waiting to be excavated may well be as high as the thousands.

Date of the eruption

The year of the eruption is pinned to AD 79 (that is, the corresponding year of the Roman ab urbe condita calendar era) by references in contemporary Roman writers, a number of them apart from Pliny the Younger, and has never been seriously questioned. It is determined by the well-known events of the reign of Titus. Vespasian died that year. When Titus visited Pompeii to give orders for the relief of the displaced population, he was the sole ruler. In the year after the eruption, AD 80, he faced another disaster, a great fire at Rome.

The time of year is stated once in one historical document, the first letter of Pliny the Younger to Tacitus, as "nonum kal. Septembres", which is not a regular syntactic unit and has no syntax (the grammarians say, indeclinable), but would seem to be an abbreviation of a standard date. By 79 the Julian Calendar was in use. The inscribing of dates was abbreviational and formulaic. Whether anyone knew exactly what the abbreviation stood for is questionable (compare English Mr. and Mrs.); certainly, literary representations such as Pliny's left out or misinterpreted key elements that would be required for the understanding of a produced meaning. Pliny's date (supposing that the date we now find in the text is the same one given by Pliny) would have been a.d. IX kal. sept., to be interpreted as "the ninth day before the Kalends of September", which would have been eight days before September 1, or August 24 (the Romans counted September 1 as one of the nine).

August 24 is not necessarily the date given by Pliny. It represents an editorial collusion to use the manuscript text of Pliny's letters in the Medicean Laurentian Library, which also appears in the edition printed by Aldus Manutius (1508) and in all recensions since then, even though the numerous other Pliny manuscripts, as well as the works of other authors, offer many alternative dates. Unfortunately, the portion of Tacitus' Histories in which he most likely made specific use of the letter – requested for that purpose from his friend Pliny – and where he would have mentioned the date, does not survive; although Tacitus would have made use of Pliny's letter, the textual traditions of the two works, with their likely references to the date, would have been completely separate and not contaminated by each other. Since the textual tradition of any of Tacitus works through the Middle Ages, up to the first printed editions, is much slimmer than that of Pliny's letters, and rests directly on relatively early textual witnesses (although the direct parent codices of these have been lost) the risk of multiple scribal errors and variants of the date creeping in would have been much smaller for this lost Tacitus text.

Archaeological dissent from this view began with the work of Carlo Maria Rosini in 1797, to be followed by a succession of archaeologists putting forward evidence to the contrary, though mainstream scholarly opinion has long been in favour of August 24. Discussion on the subject has increased somewhat in recent years. Some of the archaeological evidence from Pompeii does suggest that the town may likely have been buried about two or three months later. For example, people interred in the ashes appear to be wearing warmer clothing than the light summer clothes that would be expected in August. The fresh fruit, olives, 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.

A 2007 study by Rolandi, De Lascio, and Stefani of 20 years of data concerning wind direction at meteorological stations in Rome and Brindisi established wind patterns in the Vesuvius area above 14 kilometres (46,000 ft) with more precision than was previously known. From June through August the winds blow strongly from the west, for the rest of the time, from the east. This fact was known, but the easterly winds of the eruption were considered anomalous in August, caused (conjecturally) by the weak and shifting winds of the transition. The authors argued that the winds of 79 produced long depositional patterns and therefore would not have been this weak, and that the transition occurs in September, not August (their reference data, though, is from modern weather observations and might not match the patterns of those same months in antiquity with precision). The authors therefore reject the August date as being inconsistent with the patterns of nature.

The rejection is not of Pliny's eyewitness account or of Pliny's date, as transmitted in the text read in modern times. The rejection focuses on manuscript variants looking for possible sources of copyist alteration of Pliny's date. In some ancient and medieval manuscripts of other authors, the month has been omitted. If some original had no month, then the copyists may have felt obliged to provide one, but chose wrongly. Rolandi et al. suggest an original date of a.d. IX kal dec (November 23) or a.d. ix kal nov (October 24) more in line with the evidence of weather observations and wind patterns. The question remains an open one, and different reliable scholarly sources (modern secondary sources and discussions) continue to propose different dates.

References

• Rolandi, G.; Paone, A.; De Lascio, M.; Stefani, G. (2008). "The 79 AD eruption of Somma: the relationship between the date of the eruption and the southeast tephra dispersion". Journal of Volcanology and Geothermal Research. 169: 87–98. doi:10.1016/j.jvolgeores.2007.08.020.
• Sigurdsson, Haraldur (2002). "Mount Vesuvius before the Disaster". In Jashemski, Wilhelmina Mary Feemster; Meyer, Frederick Gustav (eds.). The natural history of Pompeii. Cambridge UK: The Press Syndicate of the University of Cambridge. pp. 29–36.{{cite book}}: CS1 maint: ref duplicates default (link)
• Sigurdsson, Haraldur; Carey, Steven (2002). "The Eruption of Vesuvius in AD 79". In Jashemski, Wilhelmina Mary Feemster; Meyer, Frederick Gustav (eds.). The natural history of Pompeii. Cambridge UK: The Press Syndicate of the University of Cambridge. pp. 37–64.{{cite book}}: CS1 maint: ref duplicates default (link)
• Zanella, E.; Gurioli, L.; Pareschi, M.T.; Lanza, R. (2007). "Influences of urban fabric on pyroclastic density currents at Pompeii (Italy): Part II: temperature of the deposits and hazard implications" (PDF). Journal of Geophysical Research. 112 (112). doi:10.1029/2006JB004775.

External links

• AD79 Destruction and Rediscovery Much information on the eruption, the locations destroyed, and subsequent rediscovery.