Influences of urban fabric on pyroclastic density currents at Pompeii (Italy), part II: temperature of the deposits and hazard implications

Abstract

During the AD 79 eruption of Vesuvius, Italy, the Roman town of Pompeii, was covered by 2.5 m fallout pumice and then partially destroyed by pyroclastic density currents (PDCs). Thermal remanent magnetization (TRM) measurements performed on the lithic and roof tile fragments embedded in the PDC deposits allow us to quantify the variations in the temperature (Tdep) of the deposits within and around Pompeii. These results reveal that the presence of buildings strongly influenced the deposition temperature of the erupted products. The first two currents which entered Pompeii at a temperature around 300-360 ºC, show drastic decreases in the Tdep, with minima of 100-140 ºC found in the deposits within the town. We interpret these decreases in temperature as being the result of localized interactions between the PDCs and the city structures, which were only able to affect the lower part of the currents. Down flow of Pompeii, the lowermost portion of the PDCs regained its original physical characteristics, emplacing hot deposits once more. The final, dilute PDCs entered a town that was already partially destroyed by the previous currents. These PDCs left thin ash deposits which mantled the previous ones. The lack of interaction with the urban fabric is indicated by their uniform temperature everywhere. However, the relatively high temperature of the deposits, between 140 and 300 ºC, indicates that even these distal, thin ash layers, capped by their accretionary lapilli bed, were associated with PDCs that were still hot enough to cause problems for unsheltered people.