G. Mastrolorenzo et al., Vesicularity of various types of pyroclastic deposits of Campi Flegrei volcanic field: evidence of analogies in magma rise and vesiculation mechanisms, J VOLCANOL, 109(1-3), 2001, pp. 41-53
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.