Transport and deposition of pyroclastic density currents over an inhabited area: the deposits of the AD 79 eruption of Vesuvius at Herculaneum, Italy

Geological and volcanological studies were performed in the Herculaneum excavations, 7 km west of Vesuvius, Italy, to reconstruct the main features of the pyroclastic density currents and the temporal sequence of the AD 79 eruptive events that destroyed and buried the town. The identification of two distinctive marker beds allows correlation of these deposits with the better-known sequences to the south of Vesuvius, along the dispersal axis of the Plinian fall deposit. Detailed observations from stratigraphic sections show that the pyroclastic density current deposits are characterized by several sedimentary facies, each recording different depositional and emplacement mechanisms. Facies analysis reveals both lateral and vertical variations from massive to stratified deposits, which can be related to the combined effects of flow dynamics and local irregularities of the substratum at centimetre or metre scales. These topographic irregularities enhanced turbulence and allowed rapid transition from non-turbulent to turbulent transport within the flow. Fabric data from these deposits, both from roof tile orientations and anisotropy magnetic susceptibility (AMS) analyses carried out on some of the pyroclastic deposits, suggest that the pyroclastic density currents were strongly affected by the presence of buildings. These obstacles probably caused deflection and separation of flows into multiple lobes that moved in different directions.