Textural and chemical features of a “soft” plug emitted during Strombolian explosions: A case study from Stromboli volcano

Between May 2009 and March 2010, six small scale paroxysms were recorded at Stromboli volcano (Aeolian Islands, Italy). The small scale paroxysm of 21 January 2010 was the only one characterized by a SSE to SW dispersal direction, which allowed access for sampling of the associated bomb-dominated deposit. The quenched marginal portions of twelve bombs were used to perform density, textural and chemical analyses to define the mechanisms operating in the shallow conduit during the explosion. Whole-rock density values span a range of 1100 to 2300 kg/m3 which, using a dense rock equivalent density of 2850 kg/m3, converts to a vesicularity of 20 to 61%. The vesicle volume distribution (VVD) is unimodal, with a mode at 1.8 mm, consistent with a single bubble nucleation event followed by growth, coalescence expansion and/or densification. Crystallinity ranges from 30 to 62 vol%. Vesicle and crystal contents, however, show considerable variation, consistent with the presence of an extremely dense and degassed component in the fragmented magma. Both groundmass glasses and melt inclusions are chemically homogenous, with CaO/Al2O3 in the range 0.40-0.60. Melt inclusion volatile contents (H2O up to 0.47 wt%) are consistent with fragmentation of a shallow magma residing at a depth of about 480 m. We suggest that dense, degassed and crystal-rich magma formed a “soft” rheological plug at the top of the conduit. Under such a condition, bubbles can accumulate under the plug to slowly build the pressure to a threshold point, after which the pressure is enough to cause the fragmentation of the plug.