Controls on variations in cristobalite abundance in ash generated by the Soufriere Hills Volcano, Montserrat in the period 1997 to 2010

Horwell, C.J.; Hillman, S.E.; Cole, P.D.; Loughlin, S.C.; Llewellin, E.W.; Damby, D.E.; Christopher, T.E.. 2014 Controls on variations in cristobalite abundance in ash generated by the Soufriere Hills Volcano, Montserrat in the period 1997 to 2010. In: Wadge, G.; Robertson, R,E.A.; Voight, B., (eds.) The eruption of Soufriere Hills Volcano, Montserrat from 2000 to 2010. Geological Society of London, 399-406. (Memoir, 39, 39).

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Abstract/Summary

The Soufrière Hills Volcano (SHV) crystallizes cristobalite (crystalline silica) in its lava domes, and inhalation of cristobalite-rich ash may pose a chronic respiratory hazard. We investigate the causes of variation in cristobalite abundance (measured by X-ray diffraction) in ash from dome collapses, explosions and ash venting from 1997 to 2010. Cristobalite abundance in bulk dome-collapse ash varies between 4 and 23 wt%. During periods of slow lava extrusion (<5 m3 s−1), cristobalite is abundant (7–23 wt%), which we attribute to extensive devitrification in slow-cooling lava; it can also form rapidly (15 wt% in 2 months), but we find no correlation between cristobalite abundance and dome residence time (DRT). By contrast, during rapid extrusion (>5 m3 s−1), cristobalite abundance is low (4–7 wt%, similar to that associated with Vulcanian explosions), and correlates strongly with DRT. We attribute this correlation to progressive vapour-phase mineralization or devitrification, and the lack of contamination by older lava. Cristobalite abundance is expected to be >7 wt% for collapse of slowly extruded lava, for ash venting through a dome or for incorporation of hydrothermally altered edifice during explosions; cristobalite abundance is expected to be <7 wt% for collapse of rapidly extruded lava, for ash venting without dome incorporation and from Vulcanian explosions at SHV.

Item Type:	Publication - Book Section
Digital Object Identifier (DOI):	10.1144/M39.21
ISSN:	0435-4052
Date made live:	05 Oct 2015 14:21 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/511953

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