Chapter 21 Controls on variations in cristobalite abundance in ash generated by the Soufrière Hills Volcano, Montserrat in the period 1997 to 2010

Abstract

Abstract:The Soufrière Hills Volcano (SHV) crystallizes cristobalite (crystalline silica) in its lava domes, and inhalation of cristobaliterich 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 m 3 s 21 ), 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 m 3 s 21), 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. Gold Open Access:This article is published under the terms of the CC-BY 3.0 license.Cristobalite is a high-temperature, low-pressure crystalline silica polymorph that may crystallize as a meta-stable phase in dome lavas, and persists at ambient conditions. In industrial settings, the silica polymorphs of quartz, cristobalite and tridymite are capable of causing silicosis, a fibrotic lung disease. Crystalline silica is also classed as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC 1997). The discovery of cristobalite in the volcanic ash from the 18 May 1980 eruption of Mount St Helens, USA, prompted intensive research to determine the silicosis risk from inhaling the ash (e.g. Dollberg et al. 1986), but the evidence from a series of toxicological, epidemiological and clinical studies at the time was inconclusive on this point (see Horwell & Baxter 2006 for a review). In the event, exposure was short-lived and this substantially reduced public concern. Today, the chronic pathogenicity of cristobalite in volcanic ash is still under debate .The Soufrière Hills Volcano (SHV), Montserrat began its current eruption in July 1995. Lava dome growth started in late 1995 and has continued, intermittently, in a series of five phases (Wadge et al. 2014). Lava domes are inherently unstable, and are prone to partial or full collapses that generate pyroclastic density currents (PDCs) and associated co-PDC ash plumes. There have also been more than 100 Vulcanian explosions during the eruption. Baxter et al. (1999) observed that cristobalite is generated within the Soufrière Hills dome and is abundant in the co-P...

The nature and formation of cristobalite at the Soufrière Hills volcano, Montserrat: implications for the petrology and stability of silicic lava domes

“…This reconstructive transformation will hence be very slow, or will not occur if there is insufficient energy to break and re-form the bonds in the silica structure. The presence of quartz in totally devitrified samples (such as MVO1236) indicates that, in some environments, cristobalite formed by devitrification of glass can undergo reconstructive transformations to quartz very rapidly (collected from a total dome collapse on 12 July 2003, this sample was derived from dome material <24 months old; Horwell et al 2013). We infer this from textures which indicate that the sub-hedral quartz was once part of a larger fish-scale crystal (Fig.…”

“…It is possible that MVO288 contained devitrification cristobalite nuclei beyond the resolution of the SEM. Scott et al (2013) and Horwell et al (2013) propose that the extent of devitrification in dome rock is related to the rate of lava extrusion, with slow extrusion promoting greater devitrification as the lava is held at suitable pressure/temperature conditions for longer.…”

“…Dome-collapse events of SHV produce volcanic ash rich in cristobalite (up to~25 wt.%; Baxter et al 1999;Horwell et al 2003;Baxter et al Accepted;Horwell et al 2013). The ash plumes produced by pyroclastic density currents (PDCs) resulting from these domecollapse events (known as co-PDC ash fall) show notable cristobalite enrichment in the very-fine particle fraction (sub-2 μm; Horwell et al 2003), probably as a result of size and density fractionation during fragmentation of dome rock within PDCs .…”

The α–β phase transition in volcanic cristobalite

“…However, in scaling up the enthalpy associated with the transition in sample MBA5/6/99, which has $11 wt% cristobalite on the basis of an XRD quantification by Horwell et al (2014), the heat flow for a 'pure-phase' sample of volcanic cristobalite would be approximately 5400 J mol…”

“…We propose that the breadth of the transition reflects heterogeneity amongst cristobalite crystal grains, resulting from the presence of substituted ions and associated structural defects. Cristobalite crystallization can commence within hours or days of a magma package entering the dome environment (Williamson et al, 2010;Horwell et al, 2014), and crystals will be subjected to variable temperatures, pressures and interaction with hydrothermal fluids throughout their growth and residence. As a result of this dynamic thermochemical environment, volcanic cristobalite probably undergoes multiple phases of disordered growth and ion substitution.…”