The Bridge River Volcanic Assemblage comprises the eruptive products of a 2400 BP eruption of Mount Meager including airfall pumice, pyroclastic flows, lahars, and lava flow. There is also a unique form of welded block and ash breccia derived from collapsing fronts of the lava flow. Rock avalanches comprising mainly blocks of Plinth Assemblage volcanic rocks are found underlying and overlying Bridge River Volcanic Assemblage deposits, but appear to be unrelated to the eruptive events. This report presents new units that were not previously recognized, new stratigraphic relationships, and new origins for some of the deposits. Rocks of the Bridge River Volcanic Assemblage are dacitic with phenocrysts of plagioclase, orthopyroxene, amphibole, biotite and minor oxides in a glassy groundmass. Sieve-textured plagioclase is pervasive in all Bridge River Volcanic Assemblage rock types. The presence of banded pumices suggests mingling of mafic and dacitic magma prior to eruption and perhaps represents the trigger mechanism for Recent volcanism. Pearce element ratio diagrams demonstrate that the Bridge River Volcanic Assemblage rocks record a chemical variation that correlates with mode of eruption. Pumices from airfall deposits and pumice blocks from pyroclastic flows are richer in water and tend to be more differentiated than do the Bridge River Volcanic Assemblage lavas. These slight chemical variations correlate with a change in eruptive style from explosive to extrusive.
Differentiation of magmas, as argued by [Bowen (1928)][1] and tested and refined by petrologists over the subsequent 50 years (cf. [Carmichael, et al. 1974][2]; [Yoder, 1979][3]; [Cox, et al., 1979][4]; [Hargraves, 1980][5]), contributes to the chemical, mineralogical, and textural diversity of
Explosive volcanic eruptions are destructive geological phenomena that pose hazards of significant socioeconomic impact and potential loss of life. Effective risk mitigation and decision making prior to and during volcanic crises require real-time monitoring of gas overpressure – the single most important driving force for explosive eruptions. Development and release of gas overpressure are regulated by gas loss through permeable pathways that are inherently transient. Here, we use geometry-dependent conductive cooling models, in concert with the most up-to-date welding and permeability models, to assess the potential for "freezing in" permeability within (1) conduit-filling pyroclastic deposits and (2) tuffisite veins within the edifice. We find that both geometry and dimension of each deposit dictate its thermal evolution and, with that, its transient outgassing capacity. Rapid cooling of thin sheet-like tuffisite veins preserves high porosities and permeabilities. In contrast, wide cylindrical conduit-filling deposits cool slowly and permeability is annihilated over a period of minutes to hours. This highlights that conduit-filling deposits lose their outgassing capacity through welding, while tuffisite veins (previously thought to rapidly seal) can form long-lived outgassing features. We use the model results to calculate the time dependent gas flow partitioning between both degassing lithologies. Based on the reconstructed outgassing pattern we outline the potential to use the gas flow balance between the central conduit and distal fumaroles fed by tuffisite veins as a simple tool to monitor gas overpressure within a volcanic edifice.
Abstract Outgassing of volcanic systems is a ubiquitous phenomenon. Yet the mechanisms facilitating gas escape from vesiculating magmas and lavas remain poorly understood. Pervasive outgassing is thought to depend on the efficient and abundant formation of permeable pathways. Here we present results from experiments designed to identify the conditions and mechanisms needed to form such permeable pathways. We use a foamed silicate melt (FOAMGLAS®) in our experiments as a proxy for natural vesicular melts. FOAMGLAS® cores are compressed under a range of temperatures and strain rates, and results are evaluated against the state of melt relaxation (parameterized using the Deborah number). We find that foam microstructure and rheological and outgassing behaviors evolve with strain and as a function of melt relaxation state. Relaxed melt foams harden during deformation but remain impermeable. As foams become less relaxed at higher strain rate and/or melt viscosity, they show complex responses to deformation (strain weakening and hardening) yet remain impermeable. Strain localization and formation of high‐permeability bands occur only in highly strained, unrelaxed foams. However, these bands are thin and oriented perpendicular to the principal stress, resulting in limited outgassing. Permeable pathways do not readily form in foams; rather, high‐porosity melts are “persistently impermeable.” Our results imply that vesicular silicic lavas may not outgas as efficiently as previously thought. Instead, sustained impermeability allows the lava to maintain low effective viscosities, and to flow extended distances. In addition, pore pressures may rise in deforming impermeable lavas, perhaps priming the lava for later explosive behavior.
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