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Geological Society, London, Memoirs; 2002; v. 21; p. 263-279;
DOI: 10.1144/GSL.MEM.2002.021.01.12
© 2002 Geological Society of London

Volcanic processes, products and hazards

Small-volume, highly mobile pyroclastic flows formed by rapid sedimentation from pyroclastic surges at Soufrière Hills Volcano, Montserrat: an important volcanic hazard

T. H. Druitt1, E. S. Calder2, P. D. Cole3, R. P. Hoblitt4, S. C. Loughlin5, G. E. Norton5, L. J. Ritchie3, R. S. J. Sparks2 & B. Voight7

1 Laboratoire Magmas et Volcans (UMR 6524 & CNRS), Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France (T.Druitt{at}opgc.univ-bpclermont.fr)
2 Department of Earth Sciences, University of Bristol, Queens Road, Bristol BS8 1RJ, UK
3 Centre for Volcanic Studies, University of Luton, Park Square, Luton LU1 3JU, UK
4 David A. Johnston Cascades Volcano Observatory, US Geological Survey, 5400 Mac Arthur Boulevard, Vancouver, WA 98661, USA
5 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LE, UK
6 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
7 Department of Geosciences, Penn State University, University Park, PA 16802, USA

Gravitational collapses of the lava dome at Soufrière Hills Volcano on 25 June and 26 December 1997 generated pyroclastic surges that spread out over broad sectors of the landscape and laid down thin, bipartite deposits. In each case, part of the settling material continued to move upon reaching the ground and drained into valleys as high-concentration granular flows of hot (120-410°C) ash and lapilli. These surge-derived pyroclastic flows travelled at no more than 10 m s-1 but extended significantly beyond the limits of the parent surge clouds (by 3 km on 25 June and by 1 km on 26 December). The front of the 25 June flow terminated in a valley about 50 m below a small town that was occupied at the time. Despite their small deposit volumes (5-9 x 104m3), the surge-derived pyroclastic flows travelled as far as many of the Soufrière Hills block-and-ash flows on slopes as low as a few degrees, reflecting a high degree of mobility. An analysis of the deposits from 26 December suggests that sediment accumulation rates of at least several millimetres per second were sufficient to generate pyroclastic flows by suspended-load fallout from pyroclastic surges on Montserrat. Surge-derived pyroclastic flows are an important, and hitherto underestimated, hazard around active lava domes. At Montserrat they formed by sedimentation over large catchment areas and drained into valleys different from those affected by the primary block-and-ash flows and pyroclastic surges, thereby impacting areas not anticipated to be vulnerable in prior hazards analyses. The deposits are finer-grained than those of other types of pyroclastic flow at Soufrière Hills Volcano; this may aid their recognition in ancient volcanic successions but, along with valley-bottom confinement, reduces the preservation potential.