The consequences of pyroclastic fallout on the dynamics of mountain catchments: geomorphic events in the Rivo d’Arco basin (Sorrento peninsula, Italy) after theplinian eruption of the Vesuvius in 79AD

This paper deals with the results of a study carried out on the lowest alluvial terrace of the Rivo d'Arco stream (Sorrento Peninsula), which terminates with an active sea cliff in the Marina di Equa area. This cliff exposes a very interesting geo-archaeological section which allowed us to conduct important observations about the geomorphic events which occurred in the area soon after the Vesuvius Plinian eruption of 79 AD and during the following couple of centuries. Said events consisted of a dramatic phase of valley floor aggradation (up to 20 m) and coastal progradation (over 400 m) that were fed by debris flow and alluvial events reworking the 79 AD pyroclastic cover from the steepest parts of the catchment. This intense and short (few decades) period of deposition was then followed by one of erosion that included both the longitudinal dissection and the frontal truncation of the fan-deltaic body the previous sedimentary events had created. Due to this coastal erosion the shoreline retreated up to almost its original (i.e. pre-eruption) position in less than two centuries. This study demonstrates that the stronge Plinian eruptions of the Vesuvius may very well have dangerous consequences on the mountainous areas surrounding the Campana Plain, not only because of damage deriving directly from the distal fallout, but also because of subsequent events of rapid downwasting of the pyroclastic mantles. This kind of hazard is particularly high in the foothill areas and especially with regard to those human settlements that are located along valley floors and on the quiescent alluvial fans of the mountain front.