In 79 AD, Mount Vesuvius, a stratovolcano located in the modern-day region of Campania, Italy, erupted, causing one of the deadliest eruptions in history. Vesuvius violently ejected a cloud of super-heated tephra and gases to a height of 33 km (21 mi), ejecting molten rock, pulverized pumice and hot ash at 1.5 million tons per second, ultimately releasing 100,000 times the thermal energy of the atomic bombings of Hiroshima and Nagasaki. The event gives its name to the Vesuvian type of volcanic eruption, characterised by columns of hot gases and ash reaching the stratosphere, although the event also included pyroclastic flows associated with Peléan eruptions.
The event destroyed several Roman towns and settlements in the area. Pompeii and Herculaneum, obliterated and buried underneath massive pyroclastic surges and ashfall deposits, are the most famous examples. Archaeological excavations have revealed much of the towns and the lives of the inhabitants, leading to the area becoming Vesuvius National Park and a UNESCO World Heritage Site.
The total population of both cities was over 20,000. The remains of over 1,500 people have been found at Pompeii and Herculaneum. The total death toll from the eruption remains unknown.
A detailed firsthand account of the eruption is preserved in two letters of Pliny the Younger, who personally witnessed the event from the town of Misenum.
Precursor earthquakes
A major earthquake caused widespread destruction around the Bay of Naples, particularly to Pompeii, on February 5, 62 AD. Some of the damage had still not been repaired when the volcano erupted in 79 AD.
Another smaller earthquake took place in 64 AD; 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 theater. Suetonius recorded that the emperor continued singing through the earthquake until he had finished his song, while Tacitus wrote that the theater collapsed shortly after being evacuated.
Minor earthquakes were reported in the four days before the 79 AD eruption, but the warnings were not recognized. The inhabitants of the area surrounding Mount Vesuvius had been accustomed to minor tremors in the region; Pliny the Younger wrote that they "were not particularly alarming because they are frequent in Campania".
Nature of the eruption
Reconstructions of the eruption and its effects vary considerably in the details but have the same overall features. The eruption lasted for two days. Pliny the Younger, author of the only surviving written testimony, described the morning before the eruption as normal; however, he was staying at Misenum, 29 kilometres (18 mi) from the volcano across the Bay of Naples. The first day of the eruption had little effect on Misenum. Pompeii is never mentioned in Pliny the Younger's letter.
In the early afternoon, Mount Vesuvius erupted violently, spewing up a high-altitude column from which ash and pumice began to fall, blanketing the area and causing roofs to collapse. Rescues and escapes occurred over the next few hours. One or more major earthquakes also occurred at this time making walls collapse and killing some refugees. These were accompanied by a mild tsunami in the Bay of Naples. At some time in the night or early the next day, pyroclastic flows in the close vicinity of the volcano began; lights seen on the mountain were interpreted as fires, and people as far away as Misenum fled for their lives.
The first pyroclastic flow left a few centimetres of ash immediately above the grey pumice but did not cause major damage. A second brief phase left a thin layer mainly of lava and pumice.
In the early morning, a second pyroclastic flow was rapid-moving, dense and very hot, knocking down many walls in its path, and incinerating or suffocating the remaining population. This thick layer of ash sealed many bodies in cavities which preserved their shapes and later allowed plaster casts devised by Giuseppe Fiorelli.
By the evening of the second day, the eruption stopped affecting Misenum, with only haze in the atmosphere, screening sunlight.
Pliny the Younger wrote:
broad flames shone out in several places from Mount Vesuvius, which the darkness of the night contributed to render still brighter and clearer... It was now day everywhere else, but there a deeper darkness prevailed than in the thickest night.
Stratigraphic studies
Sigurðsson, Cashdollar, and Sparks undertook a detailed stratigraphic study of ash layers based on excavations and surveys, published in 1982. They concluded that the eruption unfolded in two phases, Vesuvian and Pelean, which alternated six times.
A first Plinian phase projected a column of volcanic debris and hot gases between 15 km (9 mi) and 30 km (19 mi) into the stratosphere. This phase lasted 18 to 20 hours and spread pumice and ashes, forming a 2.8 m (9 ft) layer to the south, towards Pompeii. An earthquake caused buildings in Pompeii to collapse at this time.
The following Pelean phase produced pyroclastic surges of molten rock and hot gases that reached as far as Misenum, to the west. Concentrated to the south and southeast, two pyroclastic surges engulfed Pompeii with a 1.8-metre-deep (6 ft) layer, burning and asphyxiating any living beings who had remained behind. Herculaneum, Pompeii, and Oplontis received the brunt of the surges and were buried in fine pyroclastic deposits, pulverized pumice and lava fragments up to 20 m (70 ft) deep. Surges 4 and 5 are believed to have destroyed and buried Pompeii. Surges are identified in the deposits by dune and cross-bedding formations, which are not produced by fallout.
The eruption is considered primarily phreatomagmatic, i.e. a blast driven by energy from escaping steam produced by seawater seeping into the deep-seated faults and interacting with hot magma.
Timing of explosions
In an article published in 2002, Sigurðsson and Casey concluded that an early explosion produced a column of ash and pumice which rained on Pompeii to the southeast but not on Herculaneum, which was upwind. Subsequently, the cloud collapsed as the gases densified and lost their capability to support their solid contents.
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
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 (0.8–2 in) were not in the current long enough to acquire its temperature, which would have been much higher. 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. The final figures are considered to be those of the rocks in the current just before deposition.
All crystal 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 several 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 materials retained the magnetism the Earth's field imposed when the item was formed. The temperature was raised above the lowest blocking temperature; therefore, some minerals on recooling acquired the magnetism of the Earth as it was in 79 AD. The broad 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 it possible to estimate 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 (70 °F) from 100 °C (210 °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 the equilibrium temperature of the deposit. Considering the data for all the surge deposits arrived at a surge deposit estimate. The authors discovered that the city of Pompeii was a relatively cool spot within a much hotter field, which they attributed to the 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 in) fell for several hours. It heated the roof tiles to 120–140 °C (250–280 °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 (4 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 second morning, 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 (360–430 °F), minimum temperatures; of the second, 220–260 °C (430–500 °F). The depositional temperature of the first was 140–300 °C (280–570 °F). Upstream and downstream of the flow it was 300–360 °C (570–680 °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 gases of incinerating temperatures surrounded the city. 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 made cooler by introducing turbulent ambient 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, an additional 1 metre (3.3 ft) of deposits fell over the region.
The two Plinys
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, written to the historian Tacitus some 25 years after the event. Observing the first volcanic activity from Misenum across the Bay of Naples from the volcano, approximately 29 kilometres (18 mi) away, Pliny the Elder (Pliny the Younger's uncle) 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
Pliny the Younger saw an extraordinarily dense cloud rising rapidly above the mountain:
but in likeness and form it more closely resembled a pine-tree than anything else, for what corresponded to the trunk was of great length and height, and then spread out into a number of branches, [...] At times it looked white, and at other times dirty and spotted, according to the quantity of earth and cinders that were shot up.
— Pliny the Younger, Pliny the Younger: Letters
Book 6, Letter 16 to Tacitus