The Avellino 3780-yr-B.P. catastrophe as a worst-case scenario for a future eruption at Vesuvius - PubMed (original) (raw)

The Avellino 3780-yr-B.P. catastrophe as a worst-case scenario for a future eruption at Vesuvius

Giuseppe Mastrolorenzo et al. Proc Natl Acad Sci U S A. 2006.

Abstract

A volcanic catastrophe even more devastating than the famous anno Domini 79 Pompeii eruption occurred during the Old Bronze Age at Vesuvius. The 3780-yr-B.P. Avellino plinian eruption produced an early violent pumice fallout and a late pyroclastic surge sequence that covered the volcano surroundings as far as 25 km away, burying land and villages. Here we present the reconstruction of this prehistoric catastrophe and its impact on the Bronze Age culture in Campania, drawn from an interdisciplinary volcanological and archaeoanthropological study. Evidence shows that a sudden, en masse evacuation of thousands of people occurred at the beginning of the eruption, before the last destructive plinian column collapse. Most of the fugitives likely survived, but the desertification of the total habitat due to the huge eruption size caused a social-demographic collapse and the abandonment of the entire area for centuries. Because an event of this scale is capable of devastating a broad territory that includes the present metropolitan district of Naples, it should be considered as a reference for the worst eruptive scenario at Vesuvius.

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Conflict of interest statement

Conflict of interest statement: No conflicts declared.

Figures

Fig. 1.

Fig. 1.

The area covered by Avellino pyroclastic surge and fall deposits in the southeastern Campanian plain and surrounding uplands. Black dots indicate the identified Old Bronze Age archaeological sites buried by the products of the Avellino eruption. Because of the stratospheric winds, fallout lapilli and ash were deposited east-by-northeast of the volcano, whereas the surge clouds flowed down the volcano slopes in a prevalent NNW direction. Most sites within the fallout area (the yellow zone) were buried by a pumice lapilli blanket thicker than 30 cm, which is above the limit for roof collapse. Structures in the surge area closer than 12 km to the volcano (the dark red zone) could be swept away by the cloud impact force, whereas those at a greater distance would suffer less impact damage but still be affected by decimeters (the light red zone) to centimeters of fine ash bed or floods (the blue zone). In the present metropolitan area of Naples, the Avellino surge bed is as thick as 3 m (bar scale values are in meters).

Fig. 2.

Fig. 2.

Diagrammatic illustration of the transport and deposition mechanisms of the final Avellino pyroclastic surge sequence (cloud and deposit thickness are not to scale) along the prevalent northwest propagation direction. With increasing distance from the volcano, surge cloud velocity (v), temperature (T), average particle concentration, and deposit thickness decrease. In the proximal zone within 10 km of the volcano, surge clouds were highly energetic and inflated, and the nearly homogeneous dispersal of particles within a cloud with a volume fraction of 1–10% allowed the deposition of sandwave (from dunes and antidunes to low-amplitude dunes) beds. At intermediate distances between 10 and 15 km, the prevalence of gravitational settling over turbulence caused the lower part of the cloud to be denser (particle volume fraction, ≈10%) and the upper part to be dilute (particle volume fraction, <0.1%), causing low-amplitude dunes and planar beds to be formed. Within this distance, velocity ranged between 100 and 10 m/s, and temperature was ≈100°C. In distal zones from 15 to 25 km, the cloud cooled below 100°C, and deceleration (v = 10 to 1 m/s) and turbulent damping caused steam condensation, particle aggregation, and sudden settling, thus forming planar and massive beds. Within a radius of 10 km from Vesuvius, an area that includes the city of Naples, ≈50% of the entire ash and lapilli volume was deposited with a thickness decreasing from ≈20 m to ≈5 m. The photograph shows a scanning electron microscope image of a typical fine-ash aggregate of particles (≈100 μm diameter) from the distal deposits.

Fig. 3.

Fig. 3.

Archeological evidence for the catastrophe. (A) The Old Bronze Age village of Nola. The mould of one of a group of huts found 15 km northeast of Vesuvius. The huts were partially buried under ≈1 m of fallout lapilli, 20 cm of planar surge, and 20–40 cm of flood deposit. The hut roof partially collapsed, but its interior, including pottery, was filled by surge ash and perfectly preserved. (B) One of the two human victims of the Avellino eruption found at San Paolo Belsito, near Nola. The victim, a young woman buried by 1 m of pumice lapilli, was found in a self-protecting position typical of death by suffocation. (C) Footprints of two fugitives in the surge ash deposit of the Avellino eruption, found ≈15 km NNW of Vesuvius. Thousands of footprints directed NNW away from the volcano testify to an en masse exodus from the devastated zone.

Fig. 4.

Fig. 4.

Computer simulation of the areal distribution and dynamic overpressure of a pyroclastic surge cloud that is an analogue to the Avellino final surge at Vesuvius. Total devastation, corresponding to a dynamical overpressure exceeding 25 KPa, occurs within 12 km from the summit around the volcano flanks (initial velocity = 100 m/s, thickness = 50 m, density = 50 kg/m3, viscosity = 100 Pa/s, yield strength = 0 Pa). Within the urban area of Naples, dynamic overpressure has high to moderate values, ranging from 40 to 2 KPa. Dynamic overpressure values between 10 and 25 KPa produce severe to extremely high damage to buildings and other objects. The numerical simulation is based on a simple model of a gravity-driven pyroclastic current that stops by en masse freezing.

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