Numerical modelling of tephra fallout associated with dome collapses and Vulcanian explosions: application to hazard assessment on Montserrat C. BONADONNA~, G. MACEDONIO* & R. S. J. SPARKS' Department of Earth Sciences, University of Bristol, Bristol BS8 IRJ, UK (e-mail: steve.sparks@bristol.ac.uk) Osservatorio Vesuviano, Via Diocleziano 328, 80 124 Napoli, Italy Abstract: Hazardous effects of tephra fallout on Montserrat include roof collapse, aviation threats, health hazards from respirable crystalline silica, crop pollution, road safety and lahar generation. An advection-diffusion model was developed to investigate tephra dispersal from dome collapses and Vulcanian explosions, which generated most of the fallout tephra during the 1995-1999 eruptive period of Soufritre Hills Volcano. Wind field, atmospheric diffusion, gravity settling, aggregation and elutriation processes are considered. Computed isomass maps compare well with field observations and require aggregation of fine ash for good agreement. Probability maps were also compiled. Individual probability maps (for individual dome collapses and Vulcanian explosions) are based on the statistics of wind profiles and show that fallout tephra generated by individual erup- tive events on a Montserrat scale do not cause serious damage in any area on Montserrat. Cumulative probability maps (for a given scenario of activity) are generated by sampling statistical distributionsof wind profiles and eruptive events over an extended period of time. They show that persistent tephra fallout can accumulate enough material to cause roof collapses and serious damage to vegetation in the SW part of the island, and minor damage to vegetation in the north, as also confirmed by field data. The study of tephra fallout from Soufrikre Hills Volcano, Mont- serrat, became a priority for hazards assessment when tephra fallout started to have a substantial effect on the quality of life of people living and working close to the volcano. There are several hazardous effects of fallout tephra: roof collapse, aviation threats, health hazards from respirable crystalline silica, crop pollution, road safety and lahar generation (Blong 1984; Baxter et a/. 1999). In this paper the term tephra is used in the original sense of Thorarinsson (1944), as a collective term for all particles ejected from volcanoes irrespective of size, shape and composition, whereas tephrafallout indicates the process of particle fallout. Processes leading to significant tephra fallout on Montserrat during the 1995-1999 period were mainly Vulcanian explosions and elutriation of fines from dome-collapse pyroclastic flows. Vulcanian explosions produced more fallout tephra in about five weeks in 1997 than did dome collapses in about one year from June 1996 to June 1997 (Bonadonna et al. 2002). However, the ash associated with dome-collapse pyroclastic flows seems to be more hazardous to health than Vulcanian tephra, as it contains more crystalline silica and is very fine-grained (Baxter et a/. 1999; Moore et al. 2002). Furthermore, large dome collapses also occurred once dome growth had stopped in March 1998, being purely gravitational and not needing any magmatic input (Norton et a[. 2002). In this paper an advection-diffusion model for dispersal of tephra from discrete sources is presented. This model is aimed a t improving understanding of particle fallout from multiple plumes generated by dome collapses and Vulcanian explosions. Diffusion, advection by wind transport and particle sedimentation are described using a physical model, which is a two-dimensional modification of that presented by Armienti et al. (1988). Previous models (e.g. Suzuki 1983; Armienti et a/. 1988; Glaze & Self 1991) considered dispersal of tephra from point-source plumes, in particular Plinian eruptions. Here we adapt these models to consider also weaker, fine-grained ash plumes from distributed sources in the areas inundated by pyro- clastic flows. The results are compared with data gathered at Montserrat Vol- cano Observatory (MVO) throughout the Soufrikre Hills Volcano 1995-1999 eruptive period. This model (HAZMAP) was developed as part of the emergency response programme on the effects of vol- canic ash, which started in July 1997. Elutriation from pyroclastic flows Tephra-fallout deposits from pyroclastic flows were first recognized and described by Lacroix (1904) and Hay (1959) in studies of the eruption of Mont Pelke on Martinique and Soufrihre of St Vincent in 1902. Sparks & Walker (1977) recognized that many deposits are commonly enriched in glass particles, and are complementary to the deposits of coevally emplaced pyroclastic flows, which are often enriched in crystals. They called this kind of tephra co-ignimbrite ash. Co-ignimbrite ash is very fine grained (typically <I mm), resulting from the combination of progressive fragmentation of material within the pyroclastic flows and elutriation of fines by expanding gases. However, pyroclastic-flow deposits produced by dome collapses are not strictly ignimbrites, if ignimbrite is defined as a deposit formed from pumiceous pyroclastic flows (Sparks et a/. 1973). In this paper, ash plumes from dome-collapse and fountain- collapse pyroclastic flows will be termed co-pyroclastic-flow plumes, or co-PFplumes. Experimental studies (Huppert et al. 1986; Carey et al. 1988; Woods & Caulfield 1992; Sparks et al. 1993; Woods & Bursik 1994), combined with observations and modelling (Sparks et a/. 1986; Dobran et a/. 1993; Hoblitt 1986; Woods & Kienle 1994; Calder el al. 1997), have helped in understanding the mechanisms lead- ing to the formation of co-PF plumes. Such plumes form by release and expansion of gases as the juvenile fragments disintegrate, and by expansion of air entrained into the flow. Large flows can generate very high ash plumes by buoyant lift-off, where the whole upper part of the flow ascends buoyantly due to entrainment and heating of air and sedimentation (Sparks et al. 1993). Studies at Mount St Helens show that topography plays an important role in the formation of such co-PF plumes (Hoblitt 1986; Levine & Kieffer 1991; Calder et al. 1997). Breaks in slope, bends and jumps cause enhanced mix- ing with air and produce more vigorous pulses of plume buoyancy, aiding the formation of discrete plumes, which merge as they ascend. Calder et a/. (1997) showed that the behaviour of ash plumes from pyroclastic flows at Mount St Helens (1980) were intermediate between that of a discrete thermal and a steady flux source. Eruption summary SoufriGre Hills Volcano is an andesitic dome complex, which started to erupt on 18 July 1995 (Robertson et al. 2000) after being dormant for at least 350 years (Wadge & Isaacs 1988; Harford et al. 2002). From November 1995, a new lava dome started to grow inside English's Crater (Fig. 1) and the first substantial pyroclastic flow was produced on 31 March 1996 by collapse of the new dome. Pyroclastic flows were initially confined to the Tar River valley, progressively building a delta in the sea. In March 1997 pyroclastic flows started to move down the White River valley on the southern flanks (Fig. 1). In May and June 1997, pyroclastic flows started DRUITT, T. H. & KOKELAAR, B. P. (eds) 2002. The Eruption of SoufriGre Hills Volcano, Montserrat, from 1995 to 1999. Geological Society, London, Memoirs, 21, 517-537. 0435-4052/02/$15 0 The Geological Society of London 2002.