The AD 79 tsunami in Naples Bay

Submarine vicinities of volcanoes are an important source of data for the study of the physical interaction between volcanic flows and seawater and for the evaluation of the coastal hazards. A traditional example of this interaction is from the Krakatau volcano where throughout the 1883 volcanic eruption, in a period of 2 days, a series of pyroclastic currents entered the sea and deposited hot massive tuff on the sea floor at a current water depth of about 40 m; as a consequence, shallow marine tsunami deposits formed. Vesuvius, located in southern Italy is a stratovolcano that developed within the breached crater of Monte Somma volcano. The most famous Vesuvius eruption was the Plinian event of AD 79, which buried the major Roman cities of Herculaneum and Pompeii and killed thousands of people. This event was the first volcanic eruption in human history described by an eyewitness, Pliny the Younger, who gave a detailed account of the eruptive phenomena in his famous letters (epistolae) to the Roman historian Tacitus. It is noteworthy that the Pliny letters report a tsunami in the Bay of Naples in the morning of the second day of eruption. Our paper concentrates on the record of pyroclastic flow sea entries on the continental shelf adjacent to the Vesuvius coastal volcano. The material available for this study includes a grid of high resolution single-channel seismic-reflection profiles and a set of gravity cores. The stratigraphic framework was reconstructed using a sequence stratigraphy approach and seismic reflection data have been calibrated using gravity cores. The AD 79 Plinian eruption of Vesuvius that buried Pompeii and Herculaneum began with pumice falls deposited towards the SE of the volcano followed by pyroclastic currents directed towards the SW. These currents reached Herculaneum and rapidly entered the sea, forming a fan. The interpretation of seismic reflection profiles and gravity cores collected off Herculaneum documents a submarine fan-shaped pyroclastic body at 10-140 m water depth that we interpret as the submarine counterpart of the onshore pyroclastic current deposits. This fan, c. 0.3 km3 in volume, displays a chaotic seismic facies that changes seaward to parallel reflectors and then to wavy reflectors. Gravity cores reveal a succession consisting of centimetre-thick sand- or silt-sized ash couplets followed by a graded gravelly sand-sized bed up to 180 cm thick, containing shell fragments and beach-derived pebbles, overlain by centimetre-thick graded and laminated sandy ash layers. The depositional textures and sedimentary structures of the submarine pyroclastic fan have been interpreted as the product of the interactions between pyroclasts, water waves and tsunamis induced by the AD 79 pyroclastic density currents into the Bay of Naples. Despite published case histories, in which subaerial pyroclastic flows entering seawater are either massively deposited on the shoreface (e.g., Krakatau volcano) or mostly evacuated by gravity flows to a deep basin (e.g. Soufriere volcano), the Herculaneum Submarine Fan represents the first documented record of pyroclastic current deposits delivered by a coastal volcano on the continental shelf, where shallow-water conditions and significant syndepositional reworking by wave action occurred. The results of this research may be relevant for studies of eruptive events in close proximity of coastlines and imply that pyroclastic flow-generated tsunamis need to be taken into account for hazard evaluation in the management of the coastal zones.