| Mount Vesuvius | |
|---|---|
| Monte Vesuvio | |
Mt. Vesuvius as seen from the ruins of Pompeii, which was destroyed in the eruption of AD 79. The active cone is the high peak on the left side; the smaller one on the right is part of the Somma caldera wall. |
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| Elevation | 1,281 m (4,203 ft) |
| Prominence | Gran Cono |
| Location | |
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Mount Vesuvius
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| Geology | |
| Type | Somma volcano |
| Age of rock |
25,000 years before present to 1944 age of volcano = c. 17,000 years to present |
| Volcanic arc/belt | Campanian volcanic arc |
| Last eruption | 1944 |
| Climbing | |
| Easiest route | Walk |
Mount Vesuvius (Italian: Monte Vesuvio, Latin: Mons Vesuvius) is a stratovolcano in the Gulf of Naples, Italy, about 9 kilometres (5.6 mi) east of Naples and a short distance from the shore. It is the only volcano on the European mainland to have erupted within the last hundred years, although it is not currently erupting. The two other major active volcanoes in Italy, Etna and Stromboli, are located the islands of Sicily and Stromboli respectively.
Mount Vesuvius is best known for its eruption in AD 79 that led to the burying and destruction of the Roman cities of Pompeii and Herculaneum. They were never rebuilt, although surviving townspeople and probably looters did undertake extensive salvage work after the destructions. The towns' locations were eventually forgotten until their accidental rediscovery in the 18th century.
The eruption also 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. Vesuvius itself underwent major changes – its slopes were denuded of vegetation and its summit changed considerably due to the force of the eruption. Vesuvius has erupted many times since and is today regarded as one of the most dangerous volcanoes in the world because of the population of 3,000,000 people living nearby and its tendency towards explosive (Plinian) eruptions. It is the most densely populated volcanic region in the world.[1]
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Vesuvius has a long historic and literary tradition. It was considered a divinity of the Genius type at the time of the eruption of 79 AD: it appears under the inscribed name Vesuvius as a serpent in the decorative frescos of many lararia, or household shrines, surviving from Pompeii. An inscription from Capua[2] to IOVI VESVVIO indicates that he was worshipped as a power of Jupiter; that is, Jupiter Vesuvius.[3]
The historian Diodorus Siculus relates a tradition that Hercules, in the performance of his labors, passed through the country of nearby Cumae on his way to Sicily and found there a place called "the Phlegraean Plain" (phlegraion pedion, "plain of fire"), "from a hill which anciently vomited out fire ... now called Vesuvius."[4] It was inhabited by bandits, "the sons of the Earth," who were giants. With the assistance of the gods he pacified the region and went on. The facts behind the tradition, if any, remain unknown, as does whether Herculaneum was named after it. An epigram by the poet Martial in 88 AD suggests that both Venus, patroness of Pompeii, and Hercules were worshipped in the region devastated by the eruption of 79.[5] Whether Hercules was ever considered some sort of patron of the volcano itself is debatable.
Vesuvius was a name of the volcano in frequent use by the authors of the late Roman Republic and the early Roman Empire. Its collateral forms were Vesaevus, Vesevus, Vesbius and Vesvius.[6] Writers in ancient Greek used Οὐεσούιον or Οὐεσούιος. Many scholars since then have offered an etymology. As peoples of varying ethnicity and language occupied Campania in the Roman Iron Age, the etymology depends to a large degree on the presumption of what language was spoken there at the time. Naples was settled by Greeks, as the name Nea-polis, "New City", testifies. The Oscans, a native Italic people, lived in the countryside. The Latins also competed for the occupation of Campania. Etruscan settlements were in the vicinity. Other peoples of unknown provenience are said to have been there at some time by various ancient authors.
On the presumption that the language is Greek, Vesuvius might be a Latinization of the negative οὔ (ve) prefixed to a root from or related to the Greek word σβέννυμι = "I quench", in the sense of "unquenchable".[6][7] In another derivation it might be from ἕω "hurl" and βίη "violence", "hurling violence", *vesbia, taking advantage of the collateral form.[8]
Some other theories about its origin are:
Vesuvius is a distinctive "humpbacked" mountain, consisting of a large cone (Gran Cono) partially encircled by the steep rim of a summit caldera caused by the collapse of an earlier and originally much higher structure called Monte Somma.[10] The Gran Cono was produced during the eruption of AD 79. For this reason, the volcano is also called Somma-Vesuvius or Somma-Vesuvio.
The caldera started forming during an eruption around 17,000[11][12] (or 18,300)[13] years ago and was enlarged by later paroxysmal eruptions[14] ending in the one of AD 79. This structure has given its name to the term "somma volcano", which describes any volcano with a summit caldera surrounding a newer cone.[15]
The height of the main cone has been constantly changed by eruptions but is 1,281 m (4,202 ft) at present. Monte Somma is 1,149 m (3,770 ft) high, separated from the main cone by the valley of Atrio di Cavallo, which is some 3 miles (5 km) long. The slopes of the mountain are scarred by lava flows but are heavily vegetated, with scrub and forest at higher altitudes and vineyards lower down. Vesuvius is still regarded as an active volcano, although its current activity produces little more than steam from vents at the bottom of the crater. Vesuvius is a stratovolcano at the convergent boundary where the African Plate is being subducted beneath the Eurasian Plate. Layers of lava, scoria, volcanic ash, and pumice make up the mountain. Their mineralogy is variable, but generally silica-undersaturated and rich in potassium, with phonolite produced in the more explosive eruptions.[16]
Vesuvius was formed as a result of the collision of two tectonic plates, the African and the Eurasian. The former was pushed beneath the latter, deeper into the earth. As the water-saturated sediments of the oceanic African plate were pushed to hotter depths in the earth, the water boiled off and caused the melting point of the upper mantle to drop enough to create partial melting of the rocks. Because magma is less dense than the solid rock around it, it was pushed upward. Finding a weak place at the Earth's surface it broke through, producing the volcano.
The volcano is one of several which form the Campanian volcanic arc. Others include Campi Flegrei, a large caldera a few kilometres to the north west, Mount Epomeo, 20 kilometres (12 mi) to the west on the island of Ischia, and several undersea volcanoes to the south. The arc forms the southern end of a larger chain of volcanoes produced by the subduction process described above, which extends northwest along the length of Italy as far as Monte Amiata in Southern Tuscany. Vesuvius is the only one to have erupted within recent history, although some of the others have erupted within the last few hundred years. Many are either extinct or have not erupted for tens of thousands of years.
Mount Vesuvius has erupted many times. The famous eruption in 79 AD was preceded by numerous others in prehistory, including at least three significantly larger ones, the best known being the Avellino eruption around 1800 BC which engulfed several Bronze Age settlements. Since 79 AD, 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.[14] 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 as the Pompeian one.
The eruptions vary greatly in severity but are characterized by explosive outbursts of the kind dubbed Plinian after Pliny the Younger, a Roman writer who published a detailed description of the AD 79 eruption, including his uncle's death.[17] On occasion, eruptions from Vesuvius 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 kilometres (750 mi) away. A few times since 1944, landslides in the crater have raised clouds of ash dust, raising false alarms of an eruption.
Scientific knowledge of the geologic history of Vesuvius comes from core samples taken from a 2,000 m (6,600 ft) plus bore hole on the flanks of the volcano, extending into Mesozoic rock. Cores were dated by potassium-argon and argon-argon dating.[18] 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 Codola plinian eruption 25,000 years ago.[10]
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 AD 79 one was the last. The eruptions are named after the tephra deposits produced by them, which in turn are named after the location where the deposits were first identified:[19]
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.[14] 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[23] 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.[24]
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 AD 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:[25]
The AD 79 eruption was preceded by a powerful earthquake seventeen years beforehand on 5 February, AD 62, which caused widespread destruction around the Bay of Naples, and particularly to Pompeii.[29] Some of the damage had still not been repaired when the volcano erupted.[30] 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 Vesuvius.[31]
Another smaller earthquake took place in 64 AD; it was recorded by Suetonius in his biography of Nero,[32] and by Tacitus in Annales because it took place while Nero was in Naples performing for the first time in a public theatre.[33] 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 even wrote that they "were not particularly alarming because they are frequent in Campania". Small earthquakes started taking place on 20 August, 79[30] becoming more frequent over the next four days, but the warnings were not recognised.[34]
Reconstructions of the eruption and its effects vary considerably in the details but have the same overall features. The eruption lasted two days. The morning of the first day was perceived as normal by the only eyewitness to leave a surviving document, Pliny the Younger. In the middle of the day an explosion threw 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 pyroclastic flows in the close vicinity of the volcano began. Lights were seen on the mountain 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. Those are the historical facts as far as they can be discovered. Many studies both scientific and speculative have attempted to fill in additional detail, most claiming to be based "on new evidence." Much of this evidence is scientifically cogent, but the authors of the studies present different views. How to reconcile them remains an on-going concern.
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 79 AD unfolded in two phases:[35] 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 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 phreatic; that is, the chief energy supporting the column came from the escape of steam superheated by the magma, created from ground water seeping over time into the deep faults of the region.
Subsequently the cloud collapsed as the gases expanded 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 3 and 4 are believed by the authors to have destroyed Pompeii.[36] 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.
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.[37] 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 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.[38]
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.[39] 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 79 AD. 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 (104 °F) from 100 °C (212 °F) until it reached the low unblocking temperature.[40] 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.[41] 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.[42]
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.[43] It heated the roof tiles to 120 °C (248 °F) to 140 °C (284 °F).[44] 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 °C (356 °F) to 220 °C (428 °F), minimum temperatures; of the second, 220 °C (428 °F) to 260 °C (500 °F). The depositional temperature of the first was 140 °C (284 °F) to 300 °C (572 °F). Upstream and downstream of the flow it was 300 °C (572 °F) to 360 °C (680 °F).[45]
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.[46] 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.[47]
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,[48] to the historian, Tacitus.[49] 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.[50] In a second letter the younger Pliny details his own observations after the departure of his uncle.[51]
The two men saw an extraordinarily dense cloud rising rapidly above the mountain:[50]
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 lightening, 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’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 Bassus) 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 from Pompeii), where Pomponianus was. It is not clear whether he was abandoning the effort to reach Rectina's villa or believed Pompianus 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.[52] 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 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.[53] 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.
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.[54]
It is not known how many people the eruption killed. 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.[55] The remains of about 332 bodies have been found at Herculaneum (300 in arched vaults discovered in 1980).[56] 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 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,[55] 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 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. Death was not immediate; the remains show some evidence of final agony. 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.[57]
The year of the eruption of AD 79 was referred to by contemporary writers (apart from being described by Pliny) and has never been seriously questioned, being 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, 80 AD, 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,[50] as "nonum kal. Septembres", which is not a produced meaning 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 (if it was Pliny's) would have been a.d. IX kal. sept., to be interpreted as "the ninth day before the Kalends of September", which would have been 8 days before Sept. 1, or August 24 (the Romans counted Sept. 1 as one of the nine).
August 24 is not necessarily the date given by Pliny. It represents an editorial collusion to use the text of Codex Laurentianus Mediceus (a manuscript), which also appears in the 1508 printed edition of Aldus Manutius, in all recensions since then, even though the numerous Pliny manuscripts as well as the works of other authors offer many alternatives.[58] 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. Archaeological evidence from Pompeii suggest that the town was 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.[59]
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.[60] 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 established that the winds of 79 produced long depositional patterns and were therefore not weak, and that the transition occurs in September, not August. 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, whatever it was. They focus on manuscript variants looking for possible sources of copyist alteration of Pliny's date. In many manuscripts the month has been omitted. If some original had no month then the copyists may have felt obliged to provide one, but chose wrongly. The authors 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 nature. This view is currently gaining wide acceptance. In its index of volcanic activity on Earth, the Global Volcanism Program of the Smithsonian Institution takes the view that the eruption did start around October 24 (?), 79 AD and ended on October 28 ±1 day.[14]
Since the eruption of 79 AD, Vesuvius has erupted around three dozen times. It erupted again in 203, during the lifetime of the historian Cassius Dio. In 472, it ejected such a volume of ash that ashfalls were reported as far away as Constantinople. The eruptions of 512 were so severe that those inhabiting the slopes of Vesuvius were granted exemption from taxes by Theodoric the Great, the Gothic king of Italy. Further eruptions were recorded in 787, 968, 991, 999, 1007 and 1036 with the first recorded lava flows. The volcano became quiescent at the end of the 13th century and in the following years it again became covered with gardens and vineyards as of old. Even the inside of the crater was filled with shrubbery.
Vesuvius entered a new phase in December 1631, when a major eruption buried many villages under lava flows, killing around 3,000 people. Torrents of boiling water were also ejected, adding to the devastation. Activity thereafter became almost continuous, with relatively severe eruptions occurring in 1660, 1682, 1694, 1698, 1707, 1737, 1760, 1767, 1779, 1794, 1822, 1834, 1839, 1850, 1855, 1861, 1868, 1872, 1906, 1926, 1929, and 1944.
The eruption of 1906 killed over 100 people and ejected the most lava ever recorded from a Vesuvian eruption.
The last major eruption was in March 1944. It destroyed the villages of San Sebastiano al Vesuvio, Massa di Somma, Ottaviano, and part of San Giorgio a Cremano.[61] From 18 March to 23 March 1944, lava flows appeared within the rim. There were outflows. Small explosions then occurred until the major explosion took place on 18 March 1944.[62]
At the time of the eruption, the United States Army Air Force (USAAF) 340th Bombardment Group was based at Pompeii Airfield near Terzigno, Italy, just a few kilometers from the eastern base of the mountain. The tephra and hot ash damaged the fabric control surfaces, the engines, the Plexiglass windshields and the gun turrets of the 340th's B-25 Mitchell medium bombers. Estimates ranged from 78 to 88 aircraft destroyed.[63]
The eruption could be seen from Naples. Different perspectives and the damage caused to the local villages were recorded by USAAF photographers and other personnel based nearer to the volcano.[64]
Large plinian eruptions which emit lava in quantities of about 1 cubic kilometre (0.24 cu mi), the most recent of which overwhelmed Pompeii, have happened after periods of inactivity of a few thousand years. Subplinian eruptions producing about 0.1 cubic kilometres (0.024 cu mi), such as those of 472 and 1631, have been more frequent with a few hundred years between them. Following the 1631 eruption until 1944 every few years saw a comparatively small eruption which emitted 0.001-0.01 km³ of magma. It seems that for Vesuvius the amount of magma expelled in an eruption increases very roughly linearly with the interval since the previous one, and at a rate of around 0.001 cubic kilometres (0.00024 cu mi) for each year. This gives an extremely approximate figure of 0.06 cubic kilometres (0.014 cu mi) for an eruption after 60 years of inactivity.[65]
Magma sitting in an underground chamber for many years will start to see higher melting point constituents such as olivine crystallising out. The effect is to increase the concentration of dissolved gases (mostly steam and carbon dioxide) in the remaining liquid magma, making the subsequent eruption more violent. As gas-rich magma approaches the surface during an eruption, the huge drop in pressure caused by the reduction in weight of the overlying rock (which drops to zero at the surface) causes the gases to come out of solution, the volume of gas increasing explosively from nothing to perhaps many times that of the accompanying magma. Additionally, the removal of the lower melting point material will raise the concentration of felsic components such as silicates potentially making the magma more viscous, adding to the explosive nature of the eruption.
The government emergency plan for an eruption therefore assumes that the worst case will be an eruption of similar size and type to the 1631 VEI 4[66] one. In this scenario the slopes of the mountain, extending out to about 7 kilometres (4.3 mi) from the vent, may be exposed to pyroclastic flows sweeping down them, whilst much of the surrounding area could suffer from tephra falls. Because of prevailing winds, towns to the south and east of the volcano are most at risk from this, and it is assumed that tephra accumulation exceeding 100 kg/m² – at which point people are at risk from collapsing roofs – may extend out as far as Avellino to the east or Salerno to the south east. Towards Naples, to the north west, this tephra fall hazard is assumed to extend barely past the slopes of the volcano.[65] The specific areas actually affected by the ash cloud will depend upon the particular circumstances surrounding the eruption.
The plan assumes between two weeks and 20 days' notice of an eruption and foresees the emergency evacuation of 600,000 people, almost entirely comprising all those living in the zona rossa ("red zone"), i.e. at greatest risk from pyroclastic flows.[1][67] The evacuation, by trains, ferries, cars, and buses is planned to take about seven days, and the evacuees will mostly be sent to other parts of the country rather than to safe areas in the local Campania region, and may have to stay away for several months. However the dilemma that would face those implementing the plan is when to start this massive evacuation, since if it is left too late then thousands could be killed, while if it is started too early then the precursors of the eruption may turn out to have been a false alarm. In 1984, 40,000 people were evacuated from the Campi Flegrei area, another volcanic complex near Naples, but no eruption occurred.[67]
Ongoing efforts are being made by the government at various levels (especially of Regione Campania) to reduce the population living in the red zone, by demolishing illegally constructed buildings, establishing a national park around the upper flanks of the volcano to prevent the erection of further buildings[67] and by offering financial incentives to people for moving away.[68] One of the underlying goals is to reduce the time needed to evacuate the area, over the next 20 or 30 years, to two or three days.[69]
The volcano is closely monitored by the Osservatorio Vesuvio in Naples with extensive networks of seismic and gravimetric stations, a combination of a GPS-based geodetic array and satellite-based synthetic aperture radar to measure ground movement, and by local surveys and chemical analyses of gases emitted from fumaroles. All of this is intended to track magma rising underneath the volcano. Currently no magma has been detected within 10 km of the surface, and so the volcano is classified by the Observatory as at a Basic or Green Level.[70]
The area around Vesuvius was officially declared a national park on 5 June 1995.[71] The summit of Vesuvius is open to visitors and there is a small network of paths around the mountain that are maintained by the park authorities on weekends.
There is access by road to within 200 metres (660 ft) of the summit (measured vertically), but thereafter access is on foot only. There is a spiral walkway around the mountain from the road to the crater.
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