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Pre-and post-fragmentation conditions during the 1999 Vulcanian activity at Guagua Pichincha volcano (Ecuador) revealed by textural analysis of breadcrust bombs

Abstract:

Breadcrust bombs are typical products of Vulcanian eruptions. The exteriors of such pyroclasts experience rapid quenching after fragmentation and hence preserve a dense crust with no or limited textural change. The core of breadcrust bombs can instead stay above the glass transition temperature for a long time and evolve texturally as vesicles nucleate and grow. Breadcrust bombs are thus key pyroclasts that provide information about (i) pre-eruptive textural, chemical, and pressure conditions in the conduit at the time of fragmentation,(ii) the link between radial cooling history of the bomb and dynamics of bubble formation and (iii) transition from closed-to open-system degassing in natural magma at near-atmospheric pressure and in the presence of variable amounts of crystals. We analyzed two breadcrust bombs from 1999 Vulcanian activity at Guagua Pichincha volcano, Ecuador, previously described by …