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Origins of a public health hazard: Crystalline silica in respirable volcanic ash from the May 18, 1980 Mount St Helens eruption

David Damby 1, Claire J Horwell2, Julia Eychenne3,4

  • Affiliations: 1Volcano Science Center, U.S. Geological Survey, Moffett Field, California, USA 2Institute of Hazard, Risk & Resilience, Department of Earth Sciences, Durham University, Durham, UK 3Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France 4Université Clermont Auvergne, CNRS, INSERM, Institut Génétique, Reproduction et Développement, F-63000 Clermont-Ferrand, France

  • Presentation type: Poster

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

  • Poster Board Number: 24

  • Programme No: 6.6.12

  • Theme 6 > Session 6

Abstract

Abundant cristobalite, a potentially toxic form of crystalline silica, in respirable volcanic ash was an unforeseen hazard prior to the 1980 eruption of Mount St Helens. Its discovery prompted widespread concern of ash inhalation owing to the known hazard of respirable crystalline silica in occupational settings. Here, we revisit the respirable cristobalite hazard in ash from the 1980 eruption through retrospective analysis of variations in deposit characteristics. Leading up to the May 18 eruption, cristobalite crystallized in the cryptodome through preferential devitrification of 'black' dacite compared to 'grey'. This bimodality aligns with assertions that the grey dacite was sufficiently ductile to undergo secondary vesiculation upon depressurization. Cristobalite is also present as a secondary mineral in samples of the pre-existing edifice. Consequently, cristobalite would be concentrated in ash derived from fragmentation of the cryptodome and edifice, produced in the early blast phase of the eruption. The proportion of respirable ash in the fall deposit increases with downwind distance from vent, following the general trend of deposits fining with distance. However, there is pronounced crosswind asymmetry in the particle size distributions and textural componentry: towards the north, enrichment in fine ash from the early explosive phase; towards the south, coarser grain sizes and particles dominantly from the late, high-intensity Plinian phase. These retrospective constraints on the origin and distribution of cristobalite in the 1980 tephra advance our spatiotemporal understanding of ash hazards during eruptions and can inform syn-eruptive hazard assessments and sampling strategies during future eruptions, at Mount St Helens and globally.