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 erup tion in Campi Flegrei volcanic field, image analyses of ash collected in al l types of magmatic and hydromagmatic deposits have been carried out. Our r esults indicate that bubble shape, size distribution and number density are not correlated with the eruptive styles. Moderate ash vesicularity and rel atively high bubble number density suggest that in most cases magma rose wi th moderate gas oversaturation and in critical conditions, near the transit ion between viscosity-controlled and diffusivity-controlled regimes. In the se conditions, melt viscosity limited the bubble growth causing fragmentati on to occur under the typical limits of gas volume fraction (75-85%) for fo am 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 inter action and eruption. Thus, magma flow rate and upper conduit events rather than conspicuous differences in magma vesiculation and ascent rate backgrou nd likely controlled the eruptive style. (C) 2001 Elsevier Science B.V. All rights reserved.