Cristobalite in a rhyolitic lava dome: evolution of ash hazard
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Prolonged and heavy exposure to particles of respirable, crystalline silica-rich volcanic ash could potentially cause chronic, fibrotic disease, such as silicosis, in individuals living in areas of frequent ash fall. Here, we show that the rhyolitic ash erupted from Chaitén volcano, Chile, in its dome-forming phase, contains increased levels of the silica polymorph cristobalite, compared to its initial plinian eruption. Ash erupted during the initial, explosive phase (2–5 May 2008) contained approximately 2 wt.% cristobalite, whereas ash generated after dome growth began (from 21 May 2008) contains 13–19 wt.%. The work suggests that active obsidian domes crystallise substantial quantities of cristobalite on time-scales of days to months, probably through vapour-phase crystallisation on the walls of degassing pathways, rather than through spherulitic growth in glassy obsidian. The ash is fine-grained (9.7–17.7 vol.% <4 µm in diameter, the respirable range) and the particles are mostly angular. Sparse, fibre-like particles were confirmed to be feldspar or glass.
KeywordsRhyolite Dome Cristobalite Ash Health Hazard Obsidian
Thanks to Chris Rolfe, University of Cambridge, UK for grain size analyses and Nick Marsh, University of Leicester, UK for XRF analyses. Thanks to all those who were kind enough to supply fresh ash samples so rapidly following eruption. We are grateful to Luis Lara for advice on the Chaitén dome obsidian. Horwell acknowledges a Natural Environment Research Council (NERC) Urgency Grant NE/G001561/1 and a NERC Post-doctoral Fellowship NE/C518081/2. JSL's work is funded by an NERC studentship NER/S/A/2006/14107. Particular thanks to P. Baxter, A. Bernard, G. Plumlee and S. Hillier for useful reviews of the paper before and after submission.
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