Impact of 79 AD explosive eruption on Pompeii I: relations amongst the depositional mechanisms of the pyroclastic products, the framework of the buildings and the associated destructive events.

A quantitative and qualitative evaluation of the damage caused by the products of explosive eruptions to buildings provides an excellent contribution to the understanding of the various eruptive processes during such dramatic events. To this end, the impact of the products of the two main phases (pumice fallout and pyroclastic density currents) of the Vesuvius AD 79 explosive eruption onto the Pompeii buildings has been evaluated. Based on different sources of data, such as photographs and documents referring to the archaeological excavations of Pompeii, the stratigraphy of the pyroclastic deposits, and in situ inspection of the damage suffered by the buildings, the present study has enabled the reconstruction of the events that occurred inside the city when the eruption was in progress. In particular, we present new data related to the C.J. Polibius’ house, a large building located inside Pompeii. From a comparison of all of the above data sets, it has been possible to reconstruct, in considerable detail, the stratigraphy of the pyroclastic deposits accumulated in the city, to understand the direction of collapse of the destroyed walls, and to evaluate the stratigraphic level at which the walls collapsed. Finally, the distribution and style of the damage allow us to discuss how the emplacement mechanisms of the pyroclastic currents are influenced by their interaction with the urban centre. All the data suggest that both structure and shape of the town buildings affected the transport and deposition of the erupted products. For instance, sloping roofs ‘drained’ a huge amount of fall pumice into the ‘impluvia’ (a rectangular basin in the centre of the hall with the function to collect the rain water coming from a hole in the centre of the roof), thus producing anomalous deposit thicknesses. On the other hand, flat and low-sloping roofs collapsed under the weight of the pyroclastic material produced during the first phase of the eruption (pumice fall). In addition, it is evident that the walls that happened to be parallel to the direction of the pyroclastic density currents produced during the second eruptive phase were minimally damaged in comparison to those walls oriented perpendicular to the flow direction. We suggest that the lower depositional parts of the pyroclastic currents were partially blocked (locally reflected) and slowed down because of recurring encounters with the closely spaced walls within buildings. Locally, the percentage of demolished walls decreases down-current, which has been interpreted as a loss in kinetic energy within the depositional system of the flow. However, it seems that the upper transport system by-passed these obstacles, then supplied new pyroclasts to the depositional system that restored its physical characteristics and restored enough kinetic energy to demolish the next walls and buildings further along its path.