Variability of the eruption dynamics during a Subplinian event: the Greenish Pumice eruption of Somma-Vesuvius (Italy)

Subplinian eruptions are generally characterized by unsteadiness in magma discharge, which is reflected in the formation of strongly oscillating convective columns. As a result, the pyroclastic fall deposits show clear grain size stratification, and are frequently interlayered with small-volume pyroclastic density currents. The causes of this pulsatory behavior are at present not well understood, and represent one of the most interesting topics for future research in explosive volcanism. The Greenish Pumice (16 020 +/- 130 yr BP) is one of the higher magnitude Subplinian eruptions of Somma-Vesuvius and shows an alternation of pyroclastic fall and flow deposits. Given their complexity, the deposits were subdivided into seven lapilli beds, five ash beds, and deposits derived from pyroclastic density currents. The stratigraphic sequence records the transition from an initial phase of quasi-steady discharge resulting in a Subplinian convective column to phases of more discontinuous, pulsating activity, with the formation of Vulcanian to Subplinian plumes. Juvenile clasts have low phenocryst contents and exhibit a broad range in color, vesicularity, and groundmass crystallinity throughout the eruptive sequence. The density of juvenile fragments shows a broad range within each single layer, ranging from 0.4 to 2 g/cm(3). Maximum lithics in the lapilli layers suggest column heights between 17 and 20 km (mass discharge rate from 1.5 to 3 x 10(7) kg/s). Volume estimates based on proximal and medial deposits give a very conservative value of 0.47 km(3) that, based on thickness data at two distal sites, can be expanded to a volume up to three times larger, greater than the typical values of mid-intensity eruptions at Somma-Vesuvius. We suggest that the complex dynamics of the Greenish Pumice (GP) eruption are related to syn-eruptive viscosity gradients in the rising magma, induced by an important, degassing related, groundmass crystallization during ascent along the conduit. Phreatomagmatic activity did not play a major role in the eruption dynamics. (C) 2003 Elsevier Science B.V. All rights reserved.