Brief Chronology of the 79 Eruption

A sattelite image of the region surrounding Mt. Vesuvius.

The 79 eruption of Vesuvius had two distinct phases: first a Plinian phase, where material was ejected in a tall column, spread in atmosphere and fell to earth like rain; followed by a Peléan phase where material flowed down the sides of the volcano as fast-moving avalanches of gas and dust, called pyroclastic flow (pyroclasts are rock fragments formed by a volcanic explosion or ejected from a volcanic vent). The term Plinian derives from the name of Pliny the Younger, whose written observations of the 79 eruption form an important part of the historic record of Pompeii. The term Peléan derives from the name of Mount Pelée on the island of Martinique, where the phenomenon of pyroclastic flow was first documented in a 1902 eruption. The pyroclastic flows of the Peléan phase at Pompeii were the primary cause of volcanic damage to walls, however the air-fall pumice and ash fall during the Plinian phase was also significant since the deposits collapsed roofs and buried low structures, shielding them from the effects of the pyroclastic flow that followed.

A Plinian eruption ejects a column of tephra high into the atmoshpere (tephra refers to any material that is ejected from a volcano into the atmosphere), creating a form similar to the mushroom cloud of a nuclear explosion. A Plinian eruption of Vesuvius began at midday on 24 August 79 AD created a Plinian column approximately 20 km (66,000 feet) high. This phase created a rain of ash and pumice over a broad area primarily to the south of Vesuvius, carried by prevailing winds. This phase lasted approximately eighteen hours, when approximately 2.5 meters (8.2 feet) of pumice stones fell on Pompeii, an initial layer of 1.3 to 1.4 meters (4.3 to 4.6 feet) of white pumice, followed by 1.1 to 1.3 meters (3.6 to 4.3 feet) of denser gray pumice. The average diameter of the pumice fallout was 1 cm (0.4 in), and posed little direct threat to human life.

Sigurdsson [1985, p. 351] estimates that roofs began to collapse with an accumulation of approximately 40 cm (16 inches) of pumice. Carey [1987, p. 309] measured the density of the white pumice layer as ranging from 0.58 to 0.67 g/cm³ (36 to 42 lb/ft³), and the density of the grey pumice layer at 1.1 g/cm³ (69 lb/ft³), so that a 40 cm (16 inch) layer of white pumice corresponds to a distributed load of approximately 250 kg/m² (51 lb/ft²). Although some stoutly-built timber roofs might have been able to sustain this load, it is very reasonable to assume that any timber roof structure would have collapsed under the full weight of the white pumice layer, corresponding to a load of approximately 844 kg/m² (172 lb/ft²). The full weight of the white and grey pumice layers corresponds to a load of approximately 2330 kg/m² (476 lb/ft²); this load is nearly double that prescribed by modern building codes for a heavy storage structure [UBC 1994, p. 2-29], such as a reinforced concrete warehouse, so it is reasonable to assume that any timber roof or floor structure would have collapsed at some point during the Plinian phase.

By the morning of 25 August, it is clear that all covered buildings in Pompeii were uninhabitable due to collapsed floors and roofs, and it is likely that there was a mass exodus from the city; of Pompeii's estimated 20,000 residents, only about 2,000 have been found in excavations, and the majority of those have been found on top of the pumice layer [Sigurdsson 1985, p. 352]. The Plinian phase created a nearly deserted city of buildings without roofs or floors, where the bottom story level was submerged in a layer of pumice; this set the stage for the pyroclastic flow of the Peléan phase that began on the morning of 25 August.

The Peléan phase brought a much more damaging eruption, in the form of high-temperature avalanches of gas and dust hugging the ground at high velocity. There are several terms in volcanology to describe various types and aspects of this type of eruption. Although some researchers apply the terms somewhat differently, most agree on two broad categories of ground flow eruption, defined by Sigurdsson [1982, pp. 40-41] as follows:

• Pyroclastic Flow: A hot, chaotic avalanche of pumice, ash, and gasses. Pyroclastic flows can move at high speeds along the ground and pass over substantial obstacles. Their distribution is, however, strongly controlled by topography.

• Pyroclastic Surge: A turbulent cloud of volcanic ash and hot gasses, which hugs the ground and travels at speeds often exceeding 100 km per hour. Surge deposits are more widely distributed than pyroclastic flow deposits, although not as widespread as air-fall pumice layers.

The key basis for distinction is the amount and nature of the pyroclastic material included in the mixture of volcanic solids and gas. Denser mixtures which include larger fragments at higher solid concentrations are typically categorized as pyroclastic flow, while less dense mixtures where the pyroclasts are primarily fine dust and ash are categorized as pyroclastic surge. The term "glowing avalanche" is sometimes used to describe pyroclastic flow, while the terms "glowing cloud" and "ground surge", are sometimes used to describe pyroclastic surge. The French term "nuée ardente" is often used to describe a common phenomenon where an avalanche of coarse material, a pyroclastic flow, is accompanied by an overriding ash cloud of fine material, a pyroclastic surge. The diagram below shows how the ash cloud (surge) layer of a nuée ardente separates from the ash-and-block (flow) layer. The surge layer may separate from the flow layer climbing hills and travelling greater distances [Fisher 1995, p. 262. fig. 18].

A diagram of a typical pyroclastic flow progression, showing the separation into underflow and surge [Fisher 1982, p. 366].

A series of images showing the progression of pyroclastic flow. Mt. Pelée, 25 January 1903 [LaCroix 1904, pl. XII].

Pyroclastic flow can result from a variety of eruptive mechanisms. In the image series shown above, the flow essentially spilled over the rim of the volcanic vent and poured down the side of the mountain. The 79 A.D. pyroclastic flows at Vesuvius were the result of a "column collapse" mechanism, where material is first ejected high into the atmosphere, and then falls to earth at high velocity. The image below shows the results of a numeric simulation of column collapse at Vesuvius [Dobran 1996].

Numeric simulation of pyroclastic flow resulting from eruptive column collapse at Vesuvius. The colors indicate temperature and pyroclast concentration, with red indicating high and blue indicating low. There is also an mpeg animation of the eruption sequence. (image and animation courtesy of Flavio Dobran). [Dobran 1996]

Pyroclastic surge and flow were both significant factors in the Peléan phase that began on the morning of 25 August. The deposits reveal that the city was hit first by a pyroclastic surge, leaving a deposit of 10 to 20 cm, closely followed by a pyroclastic flow that left a deposit varying in thickness from 200 cm at the north wall of the city to 50 cm in the Necropolis to the south [Sigurdsson 1982]. Shortly thereafter, there was a second pyroclastic surge leaving a deposit of 10 to 20 cm, rich in fragments of limestone and dense volcanic rocks. The eruptive activity concluded with a 70 cm of air-fall ash and accretionary lapilli, small pellets of cemented ash formed by the interaction of hot ash with water in the air [MacDonald 1972, p. 133]; like the initial Plinian phase, this final phase of air-fall material posed little threat to the masonry walls.

There is no doubt that the pyroclastic surge and flow events during the Peléan phase inflicted significant structural damage; the presence of bricks and roof tiles in the surge deposits attest to this damage [Sigurdsson 1982, p. 50]. However, they were not the only source of damage, there was significant seismic activity on the morning of 25 August, during the eruption, and there was the destructive earthquake of 62 A.D. In order to distinguish seismic-related damage from volcanic-related damage, it is necessary to closely examine the effects of each. Towards this objective, the following discussion examines the structural effects of pyroclastic flow.