Journal ofthe Geological Sociefy, London, Vol. 145, 1988, pp. 541-551, 9 figs, 2 tables. Printed in Northern Ireland Mapping the volcanic hazards from Soufriere Hills Volcano, Montserrat, West Indies using an image processor G.WADGE' & M. C.ISAACS' 1 NERC Unit for Thematic Information Systems, Department of Geography, University of Reading, Whiteknights, Reading RG6 2AB, UK 'Sehmic Research Unit, University of the West Indies, St Augustine, Trinidad and Tobago Abslnct: We have used a digital model of the topography of Montserrat, a simple mathematical model of gravitational flow and some assumptions of the way in which the next eruption will develop to create a map of the volcanic hazards from Soufriere Hills Volcano. This has been done using an image processing computer to simulate the deposits producedby pyroclastic flows. This technique has the advantages over more traditional cartographic methods of spatial precision, rapid computation of multiple eruption models and the explicit nature of the physical model used. Soufriere Hills Volcano is a small andesitic volcano characterized by a cluster of summit domes and an apron of pyroclastic flow deposits and mudflows upon which several thousand people now live. Most of the flanks were covered by deposits from a series of eruptions from 24000 to 16000 a BP, though there is some evidencethat dome growth and smallpyroclastic flows have occurred since. The modellingis constrained by field evidence from the deposits of previous eruptions. Although the evidence is not good enough to model individual flow units, the cumulative deposits can be used. From the eruption deposit modelswe havecreated a new type of map specifically foremergency planning. This sequential hazard zone map attempts to portray the regions that would be at hazard from pyroclastic flows during successive stages from the start of an eruption whose energy release was increasing with time. Soufriere Hills Volcano is a small, andesitic central volcano the Raspberry Hill domes at the summit are little eroded located on southern Montserrat in the Lesser Antilles island and are probably no older than afew hundred thousand arc. It has not erupted in historicaltimes and has shown years. Although it is conceivable that South Soufriere Hills only moderate levels of activity in the past. Several Volcano could erupt againthis is thought to be very thousand people live within a fewkilometres of the volcano's unlikely. summit and they would be at considerable risk from any Soufriere Hills Volcano is of moderate size by Lesser future activity. In this paper we present some new evidence Antilles standards with an area of 35 km2 above sea-level onthe nature of the previous eruptions and suggest the though its deposits also extend beneath sea-level. The shape likeliest course of events during the next eruption. Our main of the volcano is quite complex, consisting of a series of five concern is to illustrate how image processing techniques can central andesitic lava domes: Gage's Mt, Chance's Mt, be used to simulate the deposits resulting from future Galway's Mt, Perche's Mt and Castle Peak. The last of these eruptions andgenerate maps of hazard from them. In occupiesa crater, English's Crater, whichis 1 km in particular, we present anew type of hazard map-the diameter with walls 100-150m high but open to the ENE. sequential hazard zone map-thatwe believe to be an These domes are from 800-1200m in diameter and up to improvement on existing methods of informing the 500 m high forming slopes of 32"-22". Surrounding them is authorities of short-term hazard. an apron of fragmental deposits with slopes ranging from10" to 2". The slopes to the NE of English's Crater are steeper Volcanology of Soufriere Hills Volcano and range from 16" to V. The domes are alignedalong a zone trending ESE Setting (115'). This trend alsoincludes the partiallyhidden older Montserrat is over 16 kmlong (N-S) and 10 kmwide dome at Roche's Bluff to the ESE (Rea 1974, Fig. 3) and (E-W) and consists of four main mountain massifs: Silver two other small parasitic centres to the WNW; St George's Hill, Centre Hills, Soufriere Hills and South Soufriere Hills Hills and Garibaldi Hill (Fig. 1). We interpret this alignment (Fig. 1). Chance's Peak on Soufriere Hillsis the highest as a deep-seated zone of crustalweakness along which point at (3003) feet 915m asl. Geological mapping by magma has risen during the growth of Soufriere Hills MacGregor (1938) and Rea (1970,1974) supplemented by Volcano. This zone must be regarded as a preferential zone isotopic dating (Briden et al. 1979; Le Gall et al. 1983) has for future eruptions. established an outline geologicalhistory. Silver Hill and The absolute and relative ages of the domes are not well Centre Hills are old volcanic centres active between 4.4 and known. One K-Ar age date from Chance's Peak of 1.6Ma but are now substantially eroded. The southern half 1.1 f 0.25 Ma is probably much too old (le Gall et al. 1983), of the island consists of younger volcanoes. K-Ar ages of a common failing of attempts to datesuch lava domes. There basalticlava flows from South Soufriere Hills indicate is some evidence that Gage's and Perche's Mts, which are activity between 1.8 and 0.9 Ma. However, the youngest composed of two-pyroxene andesite, are the oldest of the deposits from this centre have not been dated. In particular, domes. Fragmental deposits of this rock type on the flanks 541 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/541/4889357/gsjgs.145.4.0541.pdf by guest on 01 October 2021 542 WADGEISAACS G. C. & M. I WlOW this gives an order-of-magnitude estimate of the long-term effusion rate of about 0.002 m3S-’, which is moderate to low compared toother Lesser Antillean volcanoes(Wadge 1984). Silver Hill Stratigraphy Block-and-ash flow andsurge deposits. The flank deposits are dominated by pyroclastic flow deposits. Mudflow deposits are next in importance followed by pyroclastic fall deposits (Fig. 2). There are no lavaflows. The block-and-ash flow deposits consist of 3-15 m of ash Cent re and lapilli with variable amounts of larger blocks (up to3 m) of lavawithin them. The compositions of the blocks and matrix are very similar and consist of acid andesite (58-62% silica)with crystals of plagioclase (anorthite = 85-40%), hypersthene (enstatite = 60%) and augite or hornblende together with minor quartzand magnetite. There is a general spectrum of varieties from these block-and-ash flow deposits through ash flow deposits to pumic flow deposits. Intimately associated with this ‘family’ of block-and-ash flow deposits are surge deposits, typicallyless than 1 m thick, that are the best source of carbonized twigs and branches used for radiocarbon dating. They are usually seen in one of three contexts: on top of inversely graded block-and-ash flow deposits, below very thin beds of airfall ash or laterally juxtaposed to thick, valley-fillingblock-and-ash flow deposits. Most of the exposures studied are of the younger Fig. 1. Map of Montserrat showing theextent of Soufriere Hills deposits of the volcano, no deeper than 20-25 m below the Volcano and the older volcanic massifs. The two parallel bold lines present surface. Correlation of deposits among the studied define an ESE-trending zoneof preferential magma intrusion. The sections (Fig. 2) is hampered by the lack of accessible domes are: G, Gage’s; Ch, Chance’s; C, Castle Peak; Ga, exposures andthe fact that individual flow deposits are Galway’s; P, Perche’s; R, Raspberry; Rb, Roche’s Bluff. typically restricted in their area1 distribution to sectors of the volcano. Also assessments of deposit thickness variations with distance from the vent are limited by the narrow range are overlain by later deposits of hornblende-pyroxene of available data (90% of the measured sections lie between andesite, which is the composition of the other three domes 2.5 and 4.5 km from Castle Peak). (Rea 1974). Castle Peak is undoubtedly the youngest of the Almost all the pyroclasticflow deposits studied are of domes. It occupies English’s Crater whose present form hornblende-hypersthene andesite. Rea (1974) claimed to be would have been destroyed if Chance’s Peak or Galway’s Mt able to distinguish between an older and a younger group of domes hadbeen emplaced subsequently. The surface of deposits of this type based on the existence of a pumice flow Castle Peak isvery rugged with summit pinnacles that deposit atthe bottom of the younger group and larger probably represent extrusive spines that have been little hornblende phenocrysts within that group, which eroded, unlike themore roundedshape of the earlier correspond to the rocks of the youngest, Castle Peak, dome. domes. A reasonable relative summit chronologywas These criteria did not prove to be very useful in the field. In proposed by Rea (1974): Gage’s Mt, Perche’s Mt, Chance’s particular, the pumiceous character of some of the flow Peak, Galway’s Mt, English’s Crater and Castle Peak. deposits is highly variable over short distances. However, in Soufriere Hills Volcano has grown on the northern flank general terms our fieldwork does reinforce Rea’s contention of South Soufriere Hills Volcano. In stream sections on the thatthere is a pumice flow deposit at an intermediate south-western flanks the andesitic deposits of Soufriere Hills position between the lesspumiceous block-and-ash flow Volcano, which are 60-75 m at a distance of 2.5 km from deposits on both the north-eastern and south-western flanks the top of the volcano, can be seen to be underlain by the of the volcano, though this is not found on the eastern and distinctivebasaltic deposits of South Soufriere Hills western flanks.