Bubble nucleation, growth and coalescence during the 1997 Vulcanian explosions of Soufrière Hills Volcano, Montserrat

Abstract

Soufrière Hills Volcano had two periods of repetitive Vulcanian activity in 1997. Each explosion discharged the contents of the upper 0.5–2 km of the conduit as pyroclastic flows and fallout: frothy pumices from a deep, gas-rich zone, lava and breadcrust bombs from a degassed lava plug, and dense pumices from a transition zone. Vesicles constitute 1–66 vol.% of breadcrust bombs and 24–79% of pumices, all those larger than a few tens of µm being interconnected. Small vesicles (< few tens of µm) in all pyroclasts are interpreted as having formed syn-explosively, as shown by their presence in breadcrust bombs formed from originally non-vesicular magma. Most large vesicles (> few hundreds of µm) in pumices are interpreted as pre-dating explosion, implying pre-explosive conduit porosities up to 55%. About a sixth of large vesicles in pumices, and all those in breadcrust bombs, are angular voids formed by syn-explosive fracturing of amphibole phenocrysts. An intermediate-sized vesicle population formed by coalescence of the small syn-explosive bubbles. Bubble nucleation took place heterogeneously on titanomagnetite, number densities of which greatly exceed those of vesicles, and growth took place mainly by decompression. Development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion–decompression and surface quench in contact with air. Lava-plug fragments entered the air quickly after fragmentation (∼ 10 s), so the interiors continued to vesiculate once the rinds had quenched, forming breadcrust bombs. Deeper, gas-rich magma took longer (∼ 50 s) to reach the surface, and vesiculation of resulting pumice clasts was essentially complete prior to surface quench. This accounts for the absence of breadcrusting on pumice clasts, and for the textural similarity between pyroclastic flow and fallout pumices, despite different thermal histories after leaving the vent. It also allowed syn-explosive coalescence to proceed further in the pumices than in the breadcrust bombs. Uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn-explosive vesiculation even prior to magma fragmentation, probably in a zone of steep pressure gradient beneath the descending fragmentation front. Syn-explosive decompression rates estimated from vesicle number densities (> 0.3–6.5 MPa s − 1 ) are consistent with those predicted by previously published numerical models.