2D and 3D Textural Analysis of Vesicles in Breadcrust Bombs From the 1999 Vulcanian Activity at Guagua Pichincha Volcano, Ecuador

Breadcrust bombs formed during Vulcanian eruptions are assumed to originate from the shallow plug or dome. Their rim to core texture reflects the competition between cooling and degassing timescales, which results in a dense crust with isolated vesicles contrasting with a highly vesicular vesicle network in the interior. Due to relatively fast quenching, the crust can shed light on pre-eruptive conditions prior to or during fragmentation, whereas the interior allows us to explore post-fragmentation vesiculation. We investigate these pre- to post-fragmentation processes in breadcrust bombs from the 1999 Vulcanian activity at Guagua Pichincha, Ecuador. Via 2D and 3D textural analysis, we reveal a complex vesiculation history, with multiple, spatially localized nucleation and growth events. Large vesicles (Type 1), present in low number density in the crust, are interpreted as pre-eruptive bubbles formed by collapse of a permeable bubble network during ascent or stalling in the plug. Halos of small, syn-fragmentation vesicles (Type 2), distributed about large vesicles, are formed by pressurization and enrichment in volatiles in these haloes. The nature of the pressurization process in the plug is discussed in light of seismicity and ground deformation signals, and previous textural and chemical studies. A third population (Type 3) of post-fragmentation small vesicles appears in the interior of the bomb, and growth and coalescence of Type 2 and 3 vesicles causes the transition from isolated to interconnected bubble network in the interior. We model the evolution of viscosity, bubble growth rate, diffusion timescales, bubble radius and porosity during fragmentation and cooling.