Abstract
Abstract 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 m 3 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 m 3 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.
Highlights
The Soufriere Hills Volcano (SHV), Montserrat began its current eruption in July 1995
There is no correlation between cristobalite abundance and dome residence time (DRT) (Pearson’s product –moment correlation coefficient 1⁄4 20.169; two-tailed p-value 1⁄4 0.619) for samples from collapses of dome material extruded during periods with normal, slow extrusion rates ( 5 m3 s21) (Fig. 21.1, Table 21.1)
There is a strong, positive correlation between cristobalite abundance and DRT (Pearson’s product –moment correlation coefficient 1⁄4 0.954; two-tailed p-value 1⁄4 2.28 Â 1024) (Fig. 21.1) for samples generated during periods of rapid extrusion ( 5 m3 s21)
Summary
The Soufriere Hills Volcano (SHV), Montserrat began its current eruption in July 1995. Baxter et al (1999) observed that cristobalite is generated within the Soufriere Hills dome and is abundant in the co-PDC ashfall. The current study was devised to quantify cristobalite abundance in samples spanning the entire eruption, using a single XRD technique for consistency, in order to determine whether this decrease in cristobalite abundance was real or an artefact of technique progression over time. Studies of Mount St Helens and Soufriere Hills ash addressed this problem by employing the Talvitie method (Talvitie 1951), in which all minerals are dissolved except crystalline silica (Baxter et al 1999) This method, which involves boiling the ash in phosphoric acid for 8 min, is difficult to perform consistently since the final mineral. Throughout this pause in extrusion there were dome collapses, explosions and ash venting (Norton et al 2002). The east and NE parts of the dome were extruded between January and November 1997, the small portion removed in the dome collapse on 5 June 1999 probably included material from the 17 May 1997 and 22 October 1997 lobes. 5A large dome collapse on 20 March 2000 removed 95% of the dome, lava extrusion began again almost immediately at 3– 5 m3 s21 (Carn et al 2004) and persisted almost continuously for 16 months (Matthews et al 2002)
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