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Investigating transitions in eruption style at ocean island volcanoes using XCT analysis of bubble textures

Amy Kember 1, Margaret Hartley1, Margherita Polacci1, Elizabeth Evans2, Lucia Mancini3

  • Affiliations: 1Department of Earth and Environmental Sciences, The University of Manchester, M13 9PY, U.K. 2Henry Moseley X-ray Imaging Facility, Alan Turning Building, Manchester, M13 9PY, U.K. 3Slovenian National Building and Civil Engineering Institute, 1000 Ljubljana, Slovenia 

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 201

  • Programme No: 3.6.8

  • Theme 3 > Session 6

Abstract

Transitions in eruption style can be driven by the behaviour of magmatic volatiles in the shallow volcanic conduit. Although volatile exsolution and degassing dynamics cannot be observed directly, records of these processes may be discerned through quantitative study of vesicle textures within tephra samples. X-ray Computed Tomography (XCT) offers a micron-scale, non-destructive means for analysing vesicle morphologies and size distributions within pyroclastic rocks. The 2021 Fagradalsfjall eruption in southwest Iceland exhibited significant changes in eruptive style. Initial stages consisted of near-continuous effusive activity with mild, intermittent lava fountaining from multiple fissures. By 28 April sustained lava fountaining had been confined to a single vent. After 2 May, activity became cyclical where periods of inactivity alternated with intense lava fountaining. This eruption provides an ideal opportunity to test if the vesicle textures within samples acquired from contrasting and well-constrained phases of activity can be indicative of different degassing mechanisms which may have determined the eruption style. Five lapilli clasts from representative samples acquired during distinct eruption phases have been analysed by XCT. We present and compare bubble size distributions (BSDs) obtained for each clast. We discuss how the BSDs may be interpreted according to models which can be applied to basaltic pyroclasts to elucidate the modes of bubble nucleation and growth. Consideration is given to post-eruptive processes which may overprint the signals from the conduit. We correlate these outcomes with surface observations to propose which degassing regimes may have been operative during each eruptive phase.