The ad 79 Vesuvius eruption revisited: the pyroclastic density currents

Abstract

Seventeen pyroclastic density currents (PDCs) were produced before, during and after the Plinian phase of the ad 79 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 km 3 . We consider that the early PDCs were generated by partial collapses from the sustained Plinian eruption column, whereas the subsequent post-Plinian PDCs were generated by more sustained pyroclastic fountaining. New facies mapping revealed lateral migration and extended travel distances for some of the currents. Most of the lobed PDCs remained relatively uniform, whereas the radial PDCs exhibited fluctuating waxing and waning pulses and changed gradationally down-current. Re-evaluation of the timing of caldera collapse and the transition from a magmatic to a phreatomagmatic eruption style suggests that substrate fracturing during incremental caldera subsidence may have allowed the gradual ingress of groundwater into the erupting magma. These results provide a new chronology of the eruption, revealing that the post-Plinian collapse phase lasted c. 12 h.