The 79 AD Vesuvius eruption revisited: the pyroclastic density currents

Abstract: Seventeen pyroclastic density currents (PDCs) were produced before, during and after the Plinian phase of the 79 AD eruption of Vesuvius. Their deposits were correlated using the proportions of components together with the recognition of distinctive intercalated regionally traceable fall marker layers, revealing sectoral and distance-dependent variations. During an extensive field analysis 27 lithostratigraphic units were detected and mapped, and the lateral and vertical variations of 15 lithofacies were documented, described and interpreted. The total volume of PDC units is 1.25 km³. We consider the early PDCs were generated by partial collapses from the sustained Plinian eruption column, whereas subsequent post-Plinian PDCs were generated by more sustained pyroclastic fountaining. New facies mapping revealed lateral migration and extended travel distances for some currents. Most of the lobed PDCs remained relatively uniform, while radial PDCs exhibited fluctuating waxing and waning pulses and changed gradationally downcurrent. Re-evaluation of the timing of the caldera collapse and the transition from a magmatic to a phreatomagmatic eruption style suggests that substrate fracturing during incremental caldera subsidence may have allowed gradual ingress of groundwater to the erupting magma. The results provide a new chronology of the eruption, revealing that the post-Plinian collapse phase lasted approximately 12 hours.