Pyroclastic flows and surges generated by the 25 June 1997 dome collapse, Soufrière Hills Volcano, Montserrat

Abstract

On 25 June 1997, an unsteady, retrogressive, partial collapse of the lava dome at Soufrière Hills Volcano lasted 25 minutes and generated a major pulsatory block-and-ash flow, associated pyroclastic surges and a surge-derived pyroclastic flow that inundated an area of 4 km2 on the north and NE flanks of the volcano. Three main pulses are estimated to have involved 0.78, 2.36 and 2.36 x 106m3 of debris and the average velocities of the fronts of the related block-and-ash flow pulses were calculated to be 15 ms-1, 16.1 ms-1 and 21.9 ms-1 respectively.Deposits of block-and-ash flow pulses 1 and 2 partially filled the main drainage channel so that material of the third pulse spilled out of the channel at several places, inundating villages on the eastern coastal plain. Bends and constrictions in the main drainage channel, together with depositional filling of the channel, assisted detachment of pyroclastic surges from the pulsatory block-and-ash flow. The most extensive pyroclastic surge detached at an early stage from the third block-and-ash flow pulse, swept down the north flank of the volcano and then climbed 70 m in elevation before dissipating. Rapid sedimentation from this surge generated a high-concentration granular flow (surge-derived pyroclastic flow) that drained westwards into a valley not anticipated to be at high risk.Observations support the hypothesis that the interior of the Soufrière Hills Volcano lava dome was pressurized and that pyroclastic surge development became more substantial as deeper, more highly pressurized parts of the dome were incorporated into the pyroclastic flow. Surge development was at times so violent that expanded clouds detached from the block-and-ash flow within a few tens of metres of the lava dome.

Chapter 10 The effect of topography on ash-cloud surge generation and propagation

“…Ashcloud surges have also been known to overspill narrow channels, especially around valley bends, as at Merapi (Bourdier & Abdurachman 2001;Charbonnier & Gertisser 2008Lube et al 2011;Charbonnier et al 2013;Jenkins et al 2013;Komorowski et al 2013), Unzen (Yamamoto et al 1993;Fujii & Nakada 1999) and SHV (Loughlin et al 2002b). For the 25 June 1997 SHV BAF described in detail by Loughlin et al (2002b), detachment of ash-cloud surges was found to occur at valley bends and constrictions, and was related to the slope of the channel, and was enhanced by depositional filling. Cole et al (2002), analysing data collated from 14 SHV flows, found Komorowski et al (2010).…”

“…Surges may separate from valley-confined flows when steep topography or barriers are encountered, as was (Charbonnier & Gertisser 2011;Lube et al 2011;Charbonnier et al 2013;Jenkins et al 2013;Komorowski et al 2013). Ashcloud surges have also been known to overspill narrow channels, especially around valley bends, as at Merapi (Bourdier & Abdurachman 2001;Charbonnier & Gertisser 2008Lube et al 2011;Charbonnier et al 2013;Jenkins et al 2013;Komorowski et al 2013), Unzen (Yamamoto et al 1993;Fujii & Nakada 1999) and SHV (Loughlin et al 2002b). For the 25 June 1997 SHV BAF described in detail by Loughlin et al (2002b), detachment of ash-cloud surges was found to occur at valley bends and constrictions, and was related to the slope of the channel, and was enhanced by depositional filling.…”

“…A number of field studies investigate the morphology and granulometry of single events in great detail (e.g. Loughlin et al 2002b;Charbonnier & Gertisser 2008;Komorowski et al 2010;Lube et al 2011;Komorowski et al 2013), but few aggregate many events in order to extract general information and investigate similarities and differences between PDCs. Complex field relationships that result from the spatial and temporal variability of PDCs need to be reliably distilled in order to understand key physical processes which apply generically and which can be incorporated into dense-dilute coupled models.…”

Modelling the lava dome extruded at Soufrière Hills Volcano, Montserrat, August 2005–May 2006

“…Deposit volumes are typically in the order of 10 4 -10 6 m 3 for small to medium-sized (1-3 km run out) pyroclastic flows (Calder et al, 1999;Cole et al, 2002). Large dome collapses, in excess of 1 x 10 6 m 3 , have occurred on 47 occasions throughout the eruption to date (Calder and Bernstein, 2007).…”

“…Furthermore variations in amplitude can occur as a result of the pulselike nature of flow generation, due to retrogressive collapse initiation (Loughlin et al, 2002), but also as a result of the pyroclastic flows changing proximity to the geophones as they approach and subsequently pass-by the stations (Jolly et al, 2002). Some pyroclastic flows, and most rockfalls, are only observed seismically and thus the seismic data, irrespective of its complexities, provides one of the most important and complete records for the collapse history of the dome.…”