Degassing mechanisms during silicic eruptions

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Silicic volcanoes can produce the most devastating eruptions on Earth. The high-viscosity magmas involved may erupt explosively, affecting large areas and producing a global impact on climate, or effusively, producing relatively short-range lavas, with only local impacts. This project seeks to identify the principal causes for that fundamental difference in eruptive behaviour at the Earth's surface. Recent detailed observations of large rhyolitic eruptions demonstrate that these two eruption styles may occur simultaneously at the same vent location (Figure 1). These observations show that explosive eruptions often penetrate otherwise effusing lavas through fracture networks, and leave behind a welded remnant of their opening and closing, that is then rafted down-flow. Importantly, these venting and welding processes demonstrate that the erupting system is variably open- and closed- to the atmosphere. This has a wide range of physical and geochemical implications and yet is only poorly understood. The main goals of this project are to: (1) combine field observations at a range of scales, with novel laboratory experiments to reconstruct the range of physical processes controlling degassing through fractures at silicic volcanoes; and (2) use argon and iron in volcanic glass as geochemical markers of the degree to which the uppermost parts of the silicic volcanic plumbing system is open to mixing with air during eruption. While fracture networks are open, atmospheric air can enter the uppermost magmatic conduit, and change the oxidation state of iron, as well as diffusing into the melt. These processes are underexplored, but will provide key insights into the depths to which the shallow volcanic system is 'chemically open'. This information will help to better pose experimental and numerical studies of vent opening and sealing, which are currently over-simplified.