Pompeii to Stabiae: downcurrent versus substrate-induced variations of the AD 79 Veusvius pyroclastic current deposits and their impact on human settlements

The AD 79 Vesuvius eruption buried the Roman towns around the volcano under several metres of pyroclastic materials. The destruction of these Roman towns allows volcanologists to build models that can provide valuable information on the extent and type of damage that a future Plinian eruption could cause in urbanized areas. In order to fully understand these phenomena, volcanologists need to observe the sequence of volcanic layers (stratigraphic reconstruction) that buried the city during the AD 79 eruption and which of these layers are associated with damage and victims. This study reports the results of a collaboration between the Archaeological Park of Pompeii and the University of Naples Federico II to document the stratigraphic sequence and the distribution of damage and victims unearthed by new excavations in the archaeological sites of Pompeii and Stabiae. A systematic survey of all exposed pyroclastic sequences allowed us to study in detail the distribution and lateral facies variations of the different stratigraphic units. The deposit of stratified ash forming the upper part of the pyroclastic succession, was studied in detail to define the downcurrent variations of its sedimentological features and how these were influenced by urban structures. Pronounced lateral variations are observed in the upper part of the sequence at Pompeii, mainly consisting of a pyroclastic density current (PDC), stratified ash deposit, that ranges in thickness from few tens of centimetres to two metres. In this case, thin, massive ash layers can be traced laterally into thick, poorly sorted, ash and lapilli layers, with well-developed sedimentary structures. All PDC layers, except the lowermost, are dispersed across the entire Pompeii area, although some are missing locally as a result of the erosive action of the following PDC. The layer associated with the most destructive impact on the Roman buildings shows a strong lateral variation in thickness (0 to 330 cm) and sedimentary structures. Where it is less than 30 cm thick, the deposit is fine-grained and thinly stratified. Where it thickens, the lower part is rich in coarse pumice lapilli and locally shows well-developed stratifications, while the upper part shows an internal arrangement of alternating layers of fine and coarse ash forming progressive dunes. Upwards, ash deposits show rare pumice lapilli clasts and diffuse accretionary lapilli. This ash sequence is interstratified with four well-sorted, thin lithic-rich layers that exhibit mantling structures of fall deposits. At Stabiae, the ash PDC deposit ranges in thickness from 70 to 160 centimetres. Its internal structure shows the same types of stratification observed at Pompeii. Ash layers thicken and show lateral lithofacies variations where the pumice deposit thins and close to standing walls. It is proposed that the urban structures affect the structure of the deposit much more than the variations induced by the increase in the distance from the eruptive vent.