Magmatic volatiles provide the driving force for volcanic eruptions through the positive feedbacks between gas exsolution and bubble growth during magma ascent. However, volatile exsolution and degassing in magmatic conduits are not directly observable at the surface, and have proved difficult to understand from geophysical signals and geochemical studies of gases emitted at active volcanoes. Vesicles (gas-filled cavities) in erupted volcanic rocks preserve a textural record of volatile exsolution, and provide critical insights into the mechanisms of exsolution and degassing in volcanic conduits.
This project will investigate the processes of magma degassing, vesiculation and fragmentation operating during the 2021 Fagradalsfjall eruption in southwest Iceland, and the 2021 eruption at Cumbre Vieja on La Palma, Canary Islands. The Fagradalsfjall eruption underwent several changes in eruptive style. The earliest stages of the eruption were characterized by continuous, mild Hawaiian fire-fountaining, but after a month the activity became pulsatory, where periods of inactivity alternated with intense fire fountaining ejecting jets of magma up to 450 m in height. The Cumbre Vieja eruption likewise underwent several changes in eruptive style, with eruption intensity strongly correlated with increased gas fluxes measured at the surface. In both cases, the changes in eruption style are likely driven by the behaviour of magmatic volatiles in the volcanic conduit. This project aims to quantify the efficiency of volatile exsolution, vesiculation and gas escape that drove these different eruptive styles. What were the original dissolved volatile concentrations prior to eruption? At what pressures did volatile exsolution commence? How did vesiculation and fragmentation in the conduit affect degassing efficiency? What role did crystallization play in controlling vesiculation and gas escape? To what extent can an improved understanding of subsurface degassing in magmatic conduits be used to predict the local and global environmental impacts of eruptions at ocean island volcanoes?
The project will involve using a range of analytical techniques to examine the 3-dimensional (3D) properties of volcanic samples. A series of X-ray computed microtomography experiments will be conducted to reconstruct and visualise the textures of erupted tephra samples directly in 3D, and to quantify the vesicle proportions, shapes, size distributions and interconnected porosity. Permeability measurements on the tephra products will be used ascertain how gas was transported within and outside the magmatic conduit systems during the eruption. The dissolved volatile contents of the erupted products will be measured using electron microprobe analysis, FTIR and Raman spectroscopy. Diffused volatile concentrations measured in profiles perpendicular to the vesicle walls will be used to calculate timescales of volatile release, and investigate degassing efficiency under different eruptive regimes.
The broader impacts of this project are myriad. Depending on the student’s interest, the project could include an exploration of the links between textural and geochemical data with gas flux measurements from satellite and ground-based instruments; numerical modelling of crystallization and degassing during magma ascent; and/or reconstructions of the eruption styles and volatile emissions from large basaltic eruptions at ocean islands that pre-date human observations.
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