Vesicularity of various types of pyroclastic deposits of Campi Flegrei volcanic field: evidence of analogies in magma rise and vesiculation mechanisms

In order to investigate the factors controlling the style of explosive eruption in Campi Flegrei volcanic field, image analyses of ash collected in all types of magmatic and hydromagmatic deposits have been carried out. Our results indicate that bubble shape, size distribution and number density are not correlated with the eruptive styles. Moderate ash vesicularity and relatively high bubble number density suggest that in most cases magma rose with moderate gas oversaturation and in critical conditions, near the transition between viscosity-controlled and diffusivity-controlled regimes. In these conditions, melt viscosity limited the bubble growth causing fragmentation to occur under the typical limits of gas volume fraction (75–85%) for foam rupture. The scarce deviation in vesicularity between pure magmatic and phreatomagmatic ashes suggests that the bubble growth was always arrested, although not completed, before the ultimate occurrence of magma–water interaction and eruption. Thus, magma flow rate and upper conduit events rather than conspicuous differences in magma vesiculation and ascent rate background likely controlled the eruptive style.