Impact of Diluted Pyroclastic Density Currents on Distal Settlements: A Case Study From the Bronze Age Eruption of Avellino, Somma-Vesuvius, Italy

During the ancient Bronze Age (Palma Campania Facies) a violent plinian eruption, known as the Avellino eruption, occurred at the Somma-Vesuvius, Italy. The eruption was characterised by two main phases: plinian and phreatomagmatic, respectively. The plinian phase dispersed fallout products across the Italian peninsula in a NE direction, while the phreatomagmatic one generated dilute, turbulent pyroclastic density currents (PDCs). The latter phase impacted the plain NNW of Vesuvius, from Naples to Nola, extending tens of kilometers from the volcano. This territory was densely occupied by human settlements. These currents emplaced dune-bedded and thinly stratified deposits that reached a maximum thickness of 8-10 m in the vent area located in the western sector of the edifice. These deposits thin with distance downstream across the northwestern plain, reaching a maximum runout of up to 25 km. The onset of the phreatomagmatic phase of the eruption, during which highly efficient magma-water interaction triggered highly energetic PDCs is marked by lithic rich, fine grained ash deposits. Across the northwestern and northeastern sectors several human settlements were covered by these deposits. Volcanological field investigations integrated with a detailed facies analysis revealed that the presence of village huts (of wood and straw) affected the distribution and accumulation of these dilute PDCs. A multidisciplinary approach was applied to constrain a number of parameters for the PDCs cropping out in the excavated villages. These include PDC temperature (from the Thermal Remnant Magnetization of the lithics and pottery fragments found within the deposits) and the flow directions (from the magnetic fabric of the fine matrix). These data show that the currents, even if diluted and distal, were still hot, with temperatures of at-least 240-280 °C. The first PDC was able to engulf the village, entering huts, as well as to being locally diverted by these structures. Small vortexes probably formed down flow of obstacles as revealed by the upstream orientations found at some sites. However, people survived this first PDC, as indicated by the discovery of thousands of human and animal footprints left on the surface of these deposits. These reveal a surviving population escaping toward N, away from the volcano. The presence of the footprints, observed at the top of this first PDC deposit indicate that this thin ash layer cooled rapidily to allow people to walk on it bare footed. In addition, a short time break occurred before emplacement of the following PDC which then covered these footprints. The pause between the two PDCs was long enough to allow people to feel sufficiently secure to leave their shelter and escape.