Pompeii to Stabiae: downwind versus substrate-induced variations of the AD 79 Vesuvius fall deposits and their impact on human settlements

The AD 79 Vesuvius eruption destroyed the famous towns of Pompeii and Herculaneum located 5 and 10 km from the vent, respectively. A more distant town, Stabiae, where Pliny the Elder found his death, was also buried by pyroclastic material. Recent excavations carried out in collaboration with the Archaeological Park of Pompeii, both in Pompeii and in the imposing Stabian villas, have shown the products of the AD 79 eruptive sequence that covered these Roman settlements. During the excavation phases, ephemeral sections are exposed and then removed as the excavation proceeds. The presence on the excavation sites of a team of volcanologists allows the acquisition and evaluation of all stratigraphic and sedimentological data. The discovery of thick sequences of reworked material accumulated during previous excavations, testifies for the presence of underground tunnels dug for the Royal House of Bourbon. The deposit of pumice lapilli which forms the lower part of the pyroclastic succession, was studied in detail to define the downwind variations of its sedimentological features and how these were influenced by urban structures. At Pompeii, the AD 79 fall deposit consists of a lower white to grey pumice lapilli bed (units A and B) showing a remarkable thickness variation ranging from 0 to 4.5 metres. Two ash layers (units C1 and C2) are interstratified at the top of unit B. The internal structure of the pumice lapilli fall deposit is weakly stratified, showing sub-horizontal layering when observed in open areas (peristyles), or appearing strongly stratified with coarse to fine layering when the pumice lapilli deposit forms piles with steep layers. At Stabiae, this deposit ranges in thickness from 0 to 2 metres and is not interstratified by ash horizons. Its internal structure shows the same types of stratification observed at Pompeii. Several roofing-tiles, either intact or in fragments, were recovered at various stratigraphic heights in the lapilli deposit at Pompeii and Stabiae. This study shows that downwind variations in lapilli fall deposits are strongly influenced by an articulated substrate like that of urban structures. During the first phase of the AD 79 eruption, several roof collapses occurred, as evidenced by abundant debris and tiles found in the lapilli fall deposit. The presence of steep roofs allowed the falling pumice clasts to roll and slide down and then accumulate in the impluvium areas and in the alleys, attaining greater thicknesses with respect to deposits accumulated in open areas. This rolling produced a selection by size and density of the pyroclasts, thus forming a well-stratified deposit. At the same time, under the canopies the fall deposit thins dramatically towards the sheltered corridors. It is evident 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.