Elsevier

Quaternary International

Volumes 173–174, October–November 2007, Pages 57-66
Quaternary International

The dark nature of Somma-Vesuvius volcano: Evidence from the ∼3.5 ka B.P. Avellino eruption

https://doi.org/10.1016/j.quaint.2007.03.001Get rights and content

Abstract

The interpretation of high-resolution seismic profiles and core data collected in the western Somma-Vesuvius volcano documents a stratigraphic unit consisting of a voluminous debris avalanche that was covered by pyroclastic gravity current deposits after entering the sea.

Based on the stratigraphic position together with the ages of the dated Somma-Vesuvius Plinian eruptions, this unit was linked to the ∼3.5 ka-old Avellino Plinian eruption and correlated and mapped in offshore and onshore areas. The deposition of the thick Avellino unit had a strong impact on the physical environment of the volcano's western sector, producing hundreds of metres of prograding coastline and the formation of a morphological barrier in the alluvial plain. The striking erosional and depositional features that characterise the top of the Avellino unit along the coast suggest that the entrance of the debris avalanche into the sea triggered a tsunami that struck the Naples Bay coast. The hazardous geologic events include both the sea-facing flank failure of the volcano (producing the debris avalanche and tsunami) and the Avellino Plinian eruption (producing pumice fall and surge clouds) that profoundly affected several human settlements located within a 20 km radius of the volcano, causing the decline of the Bronze Age communities.

Introduction

Volcanic eruptions may influence the environment and hence the human population on a local and, potentially, global scale (Rampino and Self, 1993; Baxter et al., 1999) through climate modification, acid rain/aerosol impact, proximal and distal tephra fall, blast, pyroclastic flow, gas emission, and tsunami. Following the eruptions of Mount St. Helens (1980), Nevado del Ruiz (1985), and Mount Pinatubo (1991), interest in the danger that volcano activity may pose has increased, leading to the improvement of risk assessment and the evaluation of the vulnerability of human populations.

The Vesuvius area has been inhabited for several millennia. During this time several large eruptions, such as the Plinian events of Avellino (∼3500 year B.P.) and Pompei (79 A.D.), and the subPlinian eruption of 1631 A.D. (Sigurdsson et al., 1985; Rolandi et al., 1993a; Albore Livadie et al., 1998) have had a profound and devastating impact on the surrounding territory and its inhabitants.

The first documented large-scale eruption that affected the human settlements located around the volcano was the Avellino eruption. During this event, approximately 4 km3 of volcanic products were deposited onto the surrounding countryside, with ash and pumice fall reaching the mountains of Irpinia. The subsequent pyroclastic density currents travelled more than 20 km from the volcano and deposited thick layers of debris over an area of more than 400 km2 (Lirer et al., 1973; Rolandi et al., 1998). This enormous eruption was even more devastating than that of 79 A.D. It caused the evacuation of thousands of people and the desertification of the inhabitable environment leading to the socio-demographic collapse of a large part of the Campanian Bronze Age communities who were substantially dependent on agriculture (Albore Livadie et al., 1998).

The 79 A.D eruption of Vesuvius, described by Pliny the Younger, ejected between 3 and 4 km3 of material, throughout a 500 km2 area. The material affecting the southeast of the volcano was mostly falling ash and pumice from the eruption cloud, whereas the area to the south–southwest was affected by the impact of pyroclastic surges and flows (Sigurdsson et al., 1985). Pompeii received a constant rain of ash, pumice, and lapilli that caused the roofs of most of the buildings to collapse. Herculaneum was subsequently devastated by surges and flows that killed hundreds of people. The first surge that hit Pompeii in the early morning of the following day killed many people who had returned to the city during the tephra fall phase, and before the beginning of the phreatomagmatic activity that produced very powerful surges and flows (Sigurdsson et al., 1985).

According to Dobran (2006), the territory devastated by the eruption was only able to support life in the second and third centuries after the formation of new soil and underground water supplies. The 472 A.D. Pollena eruption was also a disruptive event, but at the time the area around the volcano was not densely populated due to the huge disruption caused by the 79 A.D. event (Rolandi et al., 2004b). This may explain the absence of any accurate and detailed record of the eruption, the major environmental impacts of which appear to be associated with secondary flows and the mobilization of unconsolidated pyroclastic fragments due to the heavy rainfall which buried buildings and destroyed essential agricultural infrastructure (e.g., Grattan, 2006). The subPlinian 1631 eruption destroyed many surrounding towns, and killed between 4000 and 10,000 people. It produced Strombolian and lava fountaining activity at the summit. Violent earthquakes caused additional destruction. A tsunami produced damage in the Bay of Naples as its 2–5 m high return wave struck the shore. More than 4000 people perished from pyroclastic gravity currents, while hundreds more died or were severely injured after the eruption due to buildings collapsing as a result of ash and mud flows (Rolandi and Russo, 1993; Rolandi et al., 1993a; Rosi et al., 1993; Dobran, 2006).

Vesuvius is currently experiencing a phase of quiescence following years of activity that ended with the eruption of 1944. This may explain why so little work has been conducted on the public perception of the hazard, risk and vulnerability of the Naples coastal area. One must also consider the possible negative impact such a perception would have on the tourist industry (e.g., Dobran, 2006).

For many years, however, numerous volcanological studies dealing with the reconstruction of the Somma-Vesuvius eruption and the on-land distribution of related products have been carried out (e.g., Delibrias et al., 1979; Santacroce, 1987; Barberi et al., 1990; Scandone et al., 1993; Andronico et al., 1995). The Italian Government has used these results to devise an Emergency Management Plan for the population living in the area of the volcano. The plan was based on an eruption scenario similar to the 1631 subPlinian event (Protezione Civile, 1995; Santacroce, 1996). Although no one can predict the scale and violence of the next eruption, the possibility that Vesuvius will experience a period of activity and explosive events comparable to those of the 3.5 ka B.P. and 79 A.D. Plinian eruptions or to the 472 A.D. event does not appear to be an unreasonable scenario (Lirer et al., 1997; Rossano et al., 1998; Mastrolorenzo et al., 2002, Mastrolorenzo et al., 2006; Rolandi et al., 2004b).

Apart from volcanic eruptions sensu stricto, including, among others, the passage of pyroclastic gravity flows through several metropolitan districts (Lirer et al., 1997; Rossano et al., 1998; Mastrolorenzo et al., 2006), one significant hazard associated with Vesuvius is the formation of tsunami. Triggered by the entrance of volcanic-related mass flows (e.g., debris avalanches and pyroclastic gravity currents) into the sea they would expose the community and the coastal area to great risk. This consideration arises from the recent discovery of mass wasting-related volcanic deposits that were discharged into Naples Bay during certain Vesuvius eruptions (Milia et al., 2003; Sacchi et al., 2005). The huge volumes involved (up to more than 1 km3) had the potential to generate anomalous waves that could strike the Neapolitan coastal area.

In the model of Heinrich et al. (1999), developed after studying the Montserrat (West Indies) eruption, entry of 0.04 and 0.08 km3 debris avalanches into the sea produced wave heights of around 15 m within a distance of 2 km from the entry location. Wave heights increase to more than 5 m at a distance of 5 km. Of course, the results of this model cannot be directly applied to our scenario due to differences in coastline geometry, bathymetry and collapsing mass dimensions.

Study of the 3.5 ka B.P. Avellino eruption deposits used seismic reflection profiles offshore of Somma-Vesuvius and onshore core data from an area to the west of the volcano. The results of these studies allowed: (a) recognition of the deposits of the Avellino eruption in the Volla plain; (b) correlation of the onshore–offshore distribution of the Avellino eruption products; (c) determination of the chronologic succession of events using the eruptive history (volcaniclastic mass wasting; pyroclastic fall and currents) and the areal distribution of their products; and (d) evaluation of the impact of this eruption on the surrounding physical environment and Bronze Age communities.

Section snippets

Geological setting

Somma-Vesuvius volcano is located on the northeastern coast of Naples Bay (southern Italy; Fig. 1). The latter is a peri-Tyrrhenian basin and corresponds to a Middle Pleistocene half graben. It represents the offshore counterpart of the Campania Plain and is filled by fourth-order (with a 100 ka frequency) depositional sequences that are arranged in sequence sets that display long-term aggradational–progradational stacking patterns (Milia, 1999a; Milia and Torrente, 1999). The Mesozoic–Cenozoic

Methods

The region west of Vesuvius was investigated using a lithostratigraphic analysis of bore holes drilled by private companies and local authorities and by the reinterpretation of data collected previously (D’Erasmo, 1931; Bellucci, 1998) (Fig. 2). Continuous seismic reflection profiles were acquired in the Somma-Vesuvius offshore area in September 2000 (Fig. 2). The survey was conducted using a Multi-tip Sparker system equipped with multi-electrode arrays. A Sparker source of 200 J with a broad

Results

A stratigraphic reconstruction—from the proximal to distal parts—of the 3.5 ka Avellino eruption products in the region west of Vesuvius volcano is presented here. Some of the most important outcrops of the 3.5 ka Avellino eruption products are located at Novelle and Herculaneum excavation sites (Rolandi, 1993b, Rolandi, 2004a; Bellucci, 1998). The Novelle deposits represent the most proximal section of these products and crop out in two quarries located to the west of the Somma crater (Rolandi,

The 3.5 ka B.P. Avellino eruption: reconstruction of the events and impact on the surrounding physical environment

The 3.5 ka B.P. Avellino eruption was the most powerful and important prehistoric Plinian event produced by the Somma-Vesuvius volcano (Lirer et al., 1973; Vogel et al., 1990; Rolandi et al., 1993b) and had a profound effect on the surrounding territory and its inhabitants (e.g., Albore Livadie et al., 1998).

Considering the distribution and facies variations of the related products, the vent area of the Avellino eruption was most likely located on the western slope of the volcano (e.g., Cioni et

Final remarks

This study allowed mapping the onshore–offshore distribution of the 3.5 ka B.P. Avellino eruption deposits. In the Volla plain the products of this eruption are buried and correspond to approximately 15 m thick and widespread volcanic tuffs.

Offshore, in the western sector of the volcano these products show a thickness of 50–70 m and form a mound that deeply modified the landscape, inducing localized coastal progradation. The erosional and depositional features characterizing the top of the

Acknowledgements

We are grateful for the constructive reviews of two anonymous reviewers and the editor. Financial support was given by the Italian “Ministero dell’Università e della Ricerca Scientifica e Tecnologica” (FAR 2005, 2006, M.M. Torrente).

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