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 , 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: Wehave used a digital model of thetopography 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 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 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

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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 pyroclasticfall 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. Volcano. The cliffed eastern flanks expose a sequence of Soufriere Hills Volcano deposits up to 150 m thick with no base exposed. The onshore boundaries of the flank deposits Mudflowdeposits. The best exposed mudflows on arequite well defined buttheir extension offshoreis Soufriere Hills Volcano are up to 6 m thick and overlie the unknown. Approximate estimates of the volume of material latest block-and-ash flow deposits and fill channels cut into constituting Soufriere Hills Volcano have been made by Rea them (e.g. Fort Ghaut, 98-102; Pea Ghaut, 1,4,5). These (1974): 3.5 km3, and Wadge (1984): 2.2 km3. These figures two particular valleys were probably also the sites of earlier do notaccount for volumes at seaand are therefore valleys that were only partially infilled by the block-and-ash minimum estimates. If we assume a total volume of 5 km3 flow deposits. Evidence for this can be seen in upper Pea produced during the volcano’s lifetime of about 100 000 a Ghaut (143) where rivergravel deposits underlie the

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pyroclastic flow deposits. Elsewhere onthe eastern and (about 19 600 f 100 a BP) for separate units and one slightly north-eastem flanks, and particularly in the White’s Bottom anomalousone (18890a BP). Smith et al. (1985) Ghaut region, thinner (1-3 m) deposits of a tan to orange independently obtained another determination for a sample breccia,not incised intothe underlying deposits,are thought to be from this section that falls within the same common beneath the present soil. Some of the fragments errorrange as Baker’s (19970f340a BP).The two are hydrothermally altered and give the appearance of a lag 24-23000aBP ages may represent an earlier phase of deposit butelsewhere appear to be incorporatedinto eruptions within the 24-16000 a BP period. There is also mudflow deposits. evidence of weakly-develaped soil horizons that adds weight to the idea of a series of different eruptions rather than a Airfall deposits. Airfall depositsform only a minor single episode with a wide scatter of ages. The lack of a proportion of the recent depositsof Soufriere Hills Volcano. dated sequence spanning the 24-16 000 a BP range at any There is no evidence of recent airfall ash deposits mantling one locality is probably mainly due to the rarity of charcoal the surface of Centre Hills or South Soufriere Hills. From samples. It is clear that the whole of the current subaerial this we deduce that the past behaviour of the volcano has extent of the volcano was covered at least once and in many not involved Plinian-type eruptions. On the east coast near places several times by deposits during the 24-16000a BP Mulcares (35,36) three ash beds underlie the soil horizon period. though these cannot be traced far around the volcano. The The age of320 f 54a BP for sample SRU (1959)is predominantpresent-day winds onMontserrat are the important because of its youthfulness and its location in easterly Trades which extend up to at least 4 km above the deposits immediately downslopefrom the Castle Peak surface (Roobol et al. 1985). Thusfor relatively small dome. However, this age does overlap with the European eruption columns expected wind dispersal would send ash colonization in 1632 and thus is suspect. The exact location over the south-west coast. of this sample is not known but a nearby sample (SSR-3183) collected by usyielded amodern ageand is Radicarbon chronology probably producta of forest burning. Despite these Table 1 and Fig. 3, present all the available radiocarbon age reservations the 320 f 54 a BP age sample may represent an determinationsfor charcoal from Soufriere Hills Volcano eruption associated with the lastactivity at Castle Peak including three previously unpublished determinations. . stratigraphic positions of thesamples analysed are shown (whereknown) in Fig. 2.All but two of the age determinations lie in the range 24-16000 a BP. The Fort Nature of previous eruptions Ghaut section from which Baker (1985) obtained four ages The combination of a preponderance of andesitic block-and- within a 9 m succession gave three consistently similar ages ashflow deposits on the flanks anda series of domes of

Table 1. Radiocarbon age determinations

~ ~ Sam ple Reference TypeReference Sample deposit of Locality’ BP)Age (a

[SRU 19591 Robson & Tomblin (1966) Thin, pyroclastic flow Tar River (lO?) 320f 54 Birm-52 Rea (1970) Pyroclastic flow Pea Ghaut/ 23566 f 886 Farm River (5?) Birm-156 Rea (1970) Pumice flow White’s Bottom Ghaut 18390f 360 (originally described as Tuitt’s Ghaut) (72?) MO 83 Baker (1985) Pyroclastic flow Fort Ghaut 19580f 100 (Fox and Grapes) (101/2?) MO 80 Baker (1985) Pyroclastic flow Fort Ghaut (101/2?) 19600f 100 MO 75 Baker (1985) Pyroclastic flow Fort Ghaut (101/2?) 18890f 160 MO 71 Baker (1985) Pyroclastic flow Fort Ghaut (101/2?) 19630f 100 MO 28 Baker (1985) Pyroclastic flow Pea Ghaut (bridge) >l1 OOO MO 38 Baker (1985) Pyroclastic flow Galway’s Soufriere >l5 OOO road MO-IR Smith et al. (1985) Block-and-ash flow Fort Ghatu (101/2?) 19970f 340 MO-l0 Smith er al. (1985) Block-and ash flow Dyers (117?) 17 670 MO-l3 Smith et al. (1985) Dense, andesite surge White’s Estate (113?) 18450f 450 MO-l5 Smith er d. (1985) Dense, andesite surge Galway’s Soufriere 20800f 650 road (75?) MO-l7 Smith et al. (1985) Dense, andesite surge Lower Fort Ghaut (92?) 18600f 550 SSR3183 This paper Pyroclastic flow/soil Ghaut Mefraimie road modern contact (89) SRR3184 This paper Andesitic surge Lower Fort Ghaut (92) 16630f 150 SSR3185 This paper Block-and-ash flow Dyers road (115) 23 200

~ ______~ Number in parentheses refers to section shown in Fig. 2. A question mark indicates uncertainty in the location within this section.

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0- Crater prior to the emplacement of Castle Peak dome. It is 1- possible that English’s Crater represents alandslide-scar from a Bezymianny-type eruption (Siebert et al. 1987). The 2- evidence for this will mainly lie offshore to the east but may 3- also exist in the little-studied cliffs on the east coast. At the 4- least there has been the explosive destruction of part (Chance’s ‘Peak or Galway’s Mt) or all of a summit dome 5- complex of hornblende-hypersthene andesite, the formation 6- of a horseshoe-shaped craterand the subsequent partial 7- fillingby another andesite dome. The substantial mudflow deposits that incise some of the block-and-ash flow deposits 8- may be only slightly younger than the underlying deposits. 9- Evidence for much younger pyroclastic deposits isonly 10 - found tothe east of English’s Crater suggesting thatthe post-24000 to 16 000a BP activityhas been on amuch - Years 11 smaller scale. x103 12 -

B.P. 13 ~ Volcano-seismic crises 14 - Three times during the last ninety years: 1897-8?, 1933-7 15 - and 1966-7, there have been volcano-seismic crises beneath southern Montserrat. The maineffects of these crises are 16 - summarized in Fig. 4. Th: 1966-7 crisis was studied in detail 17 - (Shepherd etal. 1971) and the mean depths of the 18 - earthquakes decreased during the first 9 months and subsequently increased to the end of the crisis. Shepherd et 19 - al. (1971) interpreted this as the result of magma movement 20 - to within 4-5 km of the surfacebelow Soufriere Hills 21 - Volcano during 1966-7, a conclusion that is probably also xxx - P valid for the other crises. From this we deduce that magma 22 is currently available within the crust beneath the volcano 23- and that any eruption will probably involve a considerable 24- i i precursory swarm of local seismicity. 25 L Fig. 3. Radiocarbon age determinations for Soufriere HillsVolcano Volcanic hazard assessment with error bars. The data arefrom the following sources: + , Soufriere Hills Volcano, although only moderately active in Robson & Tomblin (1966); A, Rea (1970); X , Baker (1985); 0, the geological past, poses a considerable potential threat to Smith et al. (1985); *, this paper. the inhabitants of southern Montserrat. With no previous experience to rely on the Montserrat government authorities similar composition higher up allowsus to infer thatthe need to have a full assessment of possible hazards from the characteristic behaviour of the volcano is the emplacement next eruption. of acid andesite domes preceded, and perhaps accompanied Baker (1985) presented a hazard map of Soufriere Hills by, the of a more volatile-rich fraction of Volcano based on an qualitative evaluation of the magma to produce the pyroclastic flow deposits. A close recurrence interval between major flank-covering eruptions. analogy to this type of behaviour, both geographically and A quantitative statistical measure of such recurrence is given volcanologically, is the 1902 eruption of Peke in by: (Lacroix 1904). The composition of the 1902 Pekean Probability of recurrence = X - e-”“‘)% magma and the nature of the resultant block-and-ash flow 100 (1 deposits and surge deposits (Fisher & Heiken 1982) are very where t is the future period of concern and m is the mean similar to the prehistoric deposits of Soufriere Hills return period of the hazard (Dibble et al. 1985). Using the Volcano. The heights of fall (about 700-1000 m) and flow radiocarbon-dated sections and assuming that each pyro- distances (5-8 km) of the flows are probably similar. There is clastic flow unit represents a different unit separated in time, also evidence from Soufriere Hills Volcano for the two flow recurrence probabilities of l-2% per 100 a can be calculated facies:channel-filling block-and-ash andmore dispersed for most of the flanks of the volcano, rising to >10% per surges that Fisher & Heiken (1982) discuss in detail from 100 a for English’s Crater and theregion to theeast. Baker’s Pelte . map istypical of many of the ‘traditional’ methods of The radiocarbon ages indicate a period of major cartographically representing hazards (Crandell et al. 1984). pyroclasticflow formation from 24000 to 16000 a BP, It has a number of drawbacks that limit its usefulness: though we are unable to distinguish the details within this period. Castle Peak dome undoubtedly postdates this but by (1) For a volcano dominated by pyroclasticflow deposits how long is uncertain. Rea (1974) considered thatthe there is great difficulty in erecting a general stratigraphy pumice flows from the 24 000-16 OOO a BP period indicated upon which to base local spatial variations of the open vent conditions indicating the existence of English’s recurrence interval statistic.

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VOLCANO-SEISMIC CRISES : MONTSERRAT

EARTHQUAKES: WNW belt beneath SoufriereHills EARTHQUAKES : SoufriereHills 3 - 13 km deep St.Georgei Hill max : 4.0 M,., EARTHQUAKES : Soufriere 1-2km deep ? Hills? max: Vlll TILT : max: 0.3 mrad/day max: VIII FUMAROLIC : Gage’sSoufriere FUMAROLIC:Increased increased H,S,CO, FUMAROLIC : Galway’s Soufriere heatflow doubles 1 1

Fig. 4. Synopsis of the - I I I 1 I 1 1 volcano-seismicI historical 1890 1900 1910 19’20 1930 1940 1950 1960 1970 1980 19901980 1970 1960 1950 1940 1930 19’20 1910 1900 1890 crises.

(2) The spatial accuracy of hazard boundaries is low and in (3) Relatively non-explosive dome growth, with occasional some cases may be entirely subjective. pyroclastic flows, that gradually diminishes over months (3) Although a hazard map based on a recurrence interval to years. statistic may be of use for long-term planning it is of Two variations onthe above behaviour are worth much less relevance to government authorities faced considering. Firstly, the eruption may not develop beyond with the prospect of an imminent eruption and needing stage 1 (e.g. Soufriere, Guadeloupe, 1976-7; Feuillard et al. to know the immediate hazards posed. The approach we havetaken to hazard mapping is to simulate the deposits produced by a series of model eruptionsthat most closely match the evidence from the geological record and where this is lacking, evidence ‘from observed eruptions of similar character. We have used an image processor to do this. From these model simulations we areable to produce maps of hazard zones thatare spatially much more precise than is traditionally possible. The modelling also forces the assumptions of the hazard map to be made explicit though the accuracy of the results will depend onthe physical and geological assumptions made.

Nature of future eruptions The next major eruption will probably involve the explosive emplacement of pyroclastic flows andthe building of a summit dome of acid andesite. The fiveexisting domes define the known area of venting of magma on the volcano. In Fig. 5 a zone is drawnthat includes these vents and encloses the probable site of the next eruption., Based on the observed development of eruptions from similar volcanoes wemight expect a three stage develop- ment to the next eruption from Soufriere Hills Volcano: Fig. 5. Map showing the zone within which the next eruptive (1) Initial, relativelysmall explosive eruptions that are of vent is likely to be sited (diagonal lining). Thepart of phreatic or phreatomagmatic origin. English’s Crater that lies within this zone is consideredto have an (2) Major explosive activity sending pyroclastic flows down even higher probability of containing the vent and is shown the flanks as a dome begins to build over the vent. double-ruled. A, B, C and D are the ventsof the models discussed.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/541/4889357/gsjgs.145.4.0541.pdf by guest on 01 October 2021 Because we do not have an identifiable example of the complete area1 distribution of one of the pyroclastic flows { Co",';;tve we refer again to those of the 8 and 20 May 1902 eruption of Mont Peke.The volume of these twoflows is about 100-150 106 X m3 (assuming the main block-and-ash flow deposits average 15m thick and extrapolating the Fisher & Column Heiken (1982) mapped isopachs out to sea). The flow units height of the24000-16000 a BP eruptions of Soufriere Hills Volcano are of similar thickness and approximately similar length (5-8 km) and volume. The directions taken by the PelCean flows were largely determined by a 'notch' in the southwestern wall of the summit crater (Lacroix 1904) andthe gravitational instability of the vent region in this direction (Westercamp 1987). Thus only a sector of about 60"of the volcano was involved. This shows theimportance of topography in controlling the distribution of flow paths. The equivalent to such a direction of preferential flow at Soufriere Hills Volcano is theopen ENE side of English's Crater. However, the Crater walls themselves are only 100-150 m high in this case, and could be overtopped by a sufficiently high eruption column. Another, secondary component of the next eruption should be thegeneration of mudflows.We would expect these mudflows to be produced intwo ways. Firstly, by Q. 6. The energy cone model of pyroclastic flow formation. inundation of the existing drainage systems with pyroclastic Above is a sectional view of a collapsing eruption column debrisand its mobilization byhigh rainfall. Secondly, as generating pyroclastic flows. The two energy lines form angles of magma rises it may expel large volumes of hot groundwater theta with the horizontal.In three dimensions this is the cone slope from fumaroles which develop into mudflows downstream. angle. At pointion the surface of localslope, /3, the velocity of the flow is proportionalto the square root of Hi. Computer simulation of eruption deposits 1983). Secondly, the explosive eruptions of stage 2 may To assess the hazards posed by the next eruption we need to involve lateral blasts (Crandell & Hoblitt 1986). Diagnostic know the detailed spatial distribution of the deposits from features of this type of eruption suchas coarse ballistic the expected pyroclastic flowsand mudflows. Pyroclastic deposits have not been identified at SoufriereHills Volcano. flow is primarily controlled by gravitational forces. Malin & It is conceivable that English's Craterrepresents the Sheridan (1982) adopted the concept of an energy line, used remnants of such alateral eruption but muchof any by Heim (1932) and Hsu(1975) to explain landslides, to depositional evidence must lie beneath the sea to the ENE. model energy losses dueto pyroclasticflows. In three Although this hazard is relevant we do not consider it dimensions these energy loss computations canbe further here. performed using an image processing computer and mapped as an image of theresultant deposits. This approach of Malin & Sheridan (1982) is that used here. Figure 6 illustrates the principle involved. An eruption

Table 2. Parameters of eruption modeh thatmight represent future eruptions of Soufriere Hills Volcano

Vent Coordinates LocationCoordinates Vent

71-600 A 315,185 Castle Peak dome B 330,175 B Upper Tar River valley Lipari - U=I M.Guardia 282,197 C ESE of Gage's Mt D 320,162 Between Chance's and Galway's r=4mt ?= Mtns I M Angle, 0 (degrees)

Height 01 I I I I I I (m) 7.5 1025 12.520 15 0 5 10 15 20 25 30 ANGLE (") 400 X Fig. 7. Plot of the cone slope angle against height of eruption 300 X column for the modelsof Sheridan & Malin (1983). The field of 200 X X X diagonal striping represents the range of these two model l00 xxx parameters used to model the eruptions of Soufriere Hills Volcano.

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(4 Fig. 8. Images of four (a-d) eruption deposit modelswhose parameters are shown in the bottom left-hand corner of each image. Vent locations are given in horizontal and vertical pixel units (corresponding to 25 m) from the bottomleft-hand corner of the image and by the white cross. The marginal grid lines are the UTM 1 km grid. Model deposits are shownin five units of increasing hue of blue to magenta corresponding to five unscaied, equal unitsof thickness. Potential zones for mudflows are shown in red in the appropriate valleys. The base maps are thedigital terrain model that has beenprocessed to appear asthough illuminated from thenorth-west together with cultural information: major roads (red), minor roads(yellow), houses (magenta), public buildings (blue) and airport (white).

columncollapses down the flanks of the volcano and the comes to rest. The values of 0 depends on the nature of the acceleration, a, of the flow at point i isgiven by: flow and its inertia and internal friction. For largeflows that travel great distances 0 tends to be small. The ‘energy line’ = g[sin(/3) - tan(0) cos(j3)] ai and in three dimensions the ‘energy cone’ have no physical where g is gravitational acceleration, /3 is the slope of the meaning but can be thought of as a measure of the rate of landsurface atthe point and 0 is the angle between the dissipation of potential energy. Although thisdissipation is horizontal and the ‘energyline’ joining thetop of the assumed to be linear in realityit may not be. At each point collapsing eruption column and theend of the flowwhen itthe heightdifference between the ground and the energy

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/541/4889357/gsjgs.145.4.0541.pdf by guest on 01 October 2021 Fig. 9. Sequential hazard zoneof Soufriere Hills Volcano. Zone 1 is the earliest hazard zoneaffected by mudflows derived from the major fumaroles. Zones2-7 represent those areas that would be coveredby deposits from an eruption centredon Castle peak dome thatincreased sequentially in strength by five increments.

line Hiprovides a measure of the amount of deposition at buildings was also captured in digital form fromthis map for thatpoint. The informationthat is neededfor the use in the final display images. The position of the eruptive three-dimensionalcomputation is thetopography around vent within the zone identified (Fig. 5) can be selected by the volcano, the location of the vent on that surface, the moving a cursor on an image of the DEM. The other two height of the collapsing eruption column above the vent, parameters, column height and angle of collapse, must be and the angle the energy cone makesto the horizontal. specified before each run of the model. Sheridan & Malin The topography of Soufriere Hills Volcano was (1983,fig. 1) plottedthese two values fora number of converted to digital form from the 1:25 OOO scale map of observed and model eruptions. Figure 7shows this plot with Montserrat (E803 (DOS 359) 7DOS/1983). This digital grid the field that we propose to be applicable to eruptions of or digital elevation model (DEM) has a spacing of 1mm on Soufriere Hills Volcano. Phreaticand phreatomagmatic the map or 25 m on the ground and a vertical interval of eruptions have small heights and large angles whilst highly about 10m. Culturalinformation such as roads and energetic and mobile surge eruptionshave large heights and

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small angles. The position of the low energy end of the Although this is observed in real pyroclastic surges on a proposed Montserratfield isplaced close to theLa Soufriere, local scale (e.g. Fisher & Heiken 1982) it would probably Guadeloupe 1976 model and would be appropriate forinitial not occur at this scale because of inertial effects. phreatic explosions. The position of the high energy end was Mudflows cannot be modelled directly by this method. chosen becausethe Vesuvius, MtSt Helensand Lipari However, in Fig. 8 we have shown in red the valleys that (MonteGuardia) eruptions all hadaccompanying Plinian would be susceptible to mudflow hazard by the two criteria ash fall deposits associated with them. Evidence forthis type (existing fumaroles andinundation by pyroclastic flow of deposit is lacking on Soufriere Hills Volcano and so the deposits) discussed earlier. more energetic models required by these eruptions are not The following general conclusions can be drawn from the considered. 32 models that were examined systematically: There are three major limitations to the application of this method of eruption simulation. Firstly, the underlying Low energy eruptions (100 m/20"-25") do not pose a physical assumptionsare very simple. The energy lossis direct threat of pyroclastic flows to any community. assumed to be linear downflow, corresponding to theflow of Models are most sensitive to angular changes in the cohesionless grains with no change in local rheological range 15"-10". This is because the apron of fragmental parameters.There is also no allowance for local deposits surrounding the volcano is largely at an angle topography-local rheology interactions such as the forma- of 10". tion of channel flow as in the case of mudflows. This can be The most energeticeruptions considered (400 m/7.5" overcome only by more sophisticated algorithms to describe and 300 m/lOo) would deposit material over most of the physics of flow. Secondly, radial flow from the vent is southern and central Montserrat. There is no geological assumed. This is generally observed not to be the case for evidence for such deposits over Centre Hills and hence some pyroclastic flows, particularly block-and-ash flows. eruptions of this magnitude have probably not occurred Thirdly, in this case we do not have sufficiently detailed field in the past. observations, such as vertical and distal limits (Sheridan & Theeruption model that best fits the cumulative Malin1983) or regression on observed thicknesses mapped distribution of the 24 000-16 000 a BP eruption (Armienti & Pareschi 1987), to enableus to constraintightly deposits is one with its vent at A and a column collapse the model parameters for individual eruptions. These last height of 200m and an angle of collapse of 12.5" to the two limitationstend to cancel oneanother in this case. horizontal (Fig. 8b). This model fitswell themapped Because of the lack of information on individual flow units northern extent of these deposits. we areforced to consider seriesa of deposits(the Several objections could be raised to conclusion 4. Castle 24 000-16 OOO a BP deposits).The cumulative distribution of Peak dome itself is thought to be younger than 24-16 000 a this series is essentially radial although individual flows were BP and hence the morphology of the summit region during probably sectoral. theemplacement of the flankflows must have been different. Also the model does not extend pyroclastic flow deposits far enough to the NNE which suggests there may Results of the modelling have been an additional laterally-directed component of Thereare thousands of different models that could be energy in this direction. The 24 000-16 000 a BP deposits are created using the restricted range of parameters discussed in composed of several depositional units not one. Neverthe- the last section.Table 2 narrows this choice down to 32 less we think that this model gives a reasonable basis on representative models: eight combinations of height and which to base hazard assessment for the inhabited areas on angle multiplied by fourvent positions. Vent A is the the lower flanks of the volcano. middle of Castle Peak dome the probable site of the last eruption, and B is placed to the south-east within English's Crater to simulate magma rising beneath Castle Peak dome Sequential hazard zone map but breaking out to the sideof the dome at lower elevation. Although precursory seismicity might enable the location of The other two vent sites are likewise located between the the vent of the next eruptionto be determinedbefore sites of olderdomes. The models were created on an eruptive activity begins we have no way of constraining the International Imaging Systems Model 75 image processor, other two model parameters (height and angle 0) than those each model taking about 2-3 min to compute and display. discussed already. How then can this range of possibilities Figure 8 shows four examples. Theeruption deposits are be communicated to non-specialist government authorities? shown inhues of blueand magenta, deeper hues Major destructive pyroclastic flows rarely occur without representing increasing thickness, each increment being one some warning symptoms.From the perspective of fifth of thetotal, unscaled, depthrange. The roads and government authorities this lead time is a most important buildings have also been digitized andare shownin dimension in that it allowswarnings to be issued and red, yellow and blue fororientation. Figure Sa shows a resources to be mobilizedin an orderlyand pre-planned small, low-energy eruptionfrom vent A whose deposits way. Similarly, if short-term hazards can be presented in would be largely confined to English's Crater. Such a model sequence (hazard A will occur before hazard B) the planner may represent the 320 a BP-dated deposits.Figure Sb, c and will be ableto assign priorities in the same manner. We d shows threemodels with constant height and angle have attempted to introduce such a temporal dimension to parametersat vents A, B and C. Thedeposits of these ourpresentation of the hazards from Soufriere Hills models cover much of the flanks of the volcano but the Volcano. details are quite sensitive to vent position. One artefact of We assume that the eruption will increase in vigour with the simple physical model used is the apparent lateral flow time frominitiation tothe time of the most destructive behind hills (e.g. south of South Soufriere Hills in Fig. 8c). pyroclastic flow. This assumption can be supported by

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evidence from some but not all andesitic and basaltic from the original and subsequent applications (e.g. Sheridan andesiticvolcanoes that have generated pyroclasticflows & Malin1983). Firstly, we have notattempted to match after a period of dormancy. At Mt Lamington in 1951 closely the models with field data forspecific eruptions, This during the 7 days between the start of the eruption and the becauseis the field evidence to produce accurately first devastating pyroclasticflows there werepossibly two correlated stratigraphic sections or isopachmaps of minor flows as theeruption column grew(Taylor 1958). individual flow units is not available. The distal portions of Mayon Volcano in 1968 experienced 6 days of increasingly flowsmay be under the sea and on land erosion and violent eruptions before pyroclasticflows reached their vegetation cover hide much of the morphological and maximum flow distance (Moore & Melson 1969). The initial stratigraphical evidence. This situtation is common on the phases of the 1953Bezymianny eruption, many months flanks of many volcanoes. The result of this is to defocus before the famous directed blast, gradually grew in intensity attention from individual deposits to a succession of deposits over a period of weeks (Gorshkov 1959), as did the 1929-32 that must represent varietya of eruptions. From the eruption of PelCe (Roobol & Smith 1975). However, during standpoint of hazard assessmentthis is not necessarilya the 1902 PelCean eruption the first pyroclastic flow on May 8 disadvantage. Herethe interest lies in placing safe limits was also the most devastating although mudflows and debris based on the integrated evidence from previous eruptions. flows had preceded it (Lacroix1904). Sigurdsson et al. These limits should correspond to the maximum extents of (1984)discussed some of the physical reasons that may the series flowof deposits whichleasta is partly explainsuch general behaviour in relation tothe 1982 recognizable on Soufriere Hills Volcano. eruption of El Chichon. There thefirst major eruption was a The second difference in our approach is that we have non-collapsing sub-Plinian ash column followed 6 days later sought to produce a type of hazard map, based onthe by phreatomagmatic, cold debris flows and surges and later eruption models, that is of practicaluse to government by major column-collapsepyroclastic flows. They inter- authorities during the few days to weeks immediately before preted this sequence in terms of widening of thevent, and immediately after the start of the eruption. Framing the reduction of magma column pressure and gas content with hazards in this map in a temporal context, albeit contrived, time.Such sequential behaviour is probably generally should help planners to focus attentionon aseries of applicable to many andesitic volcanoes in the first few days priorities for action. Such a map is only one example of its to weeks of eruption. type and would be only one form of recommendation from Sucha gradually increasing vigour of eruption can be volcanologists to the authorities. The social aspects of any simulated by a series of modelswith height parameter eruption contingencyplanning would need to be a major increments, angle parameter decrements and a constant vent additional factor in modifying conclusions to be drawn from position. In Fig. 9 such a series of models with the vent at A such a map. Any form of assessment that falls short of the (Castle Peak dome), the height increasing from 100 to 200 m worst-case scenario will be open tothe criticism that it in 20 m increments and the angle decreasing from 17" to 12" underestimates the potential hazard. Against thismust be in 1" decrements, has been computed. These are mapped as weighed the economic and social disadvantages and practical a series of coloured zones each colour representing the area difficulties of wholesale evacuation (Blong 1984). We see the that would be newly covered by deposits from an eruption volcanologist's earliest responsibility here to be the clearest one increment more energetic than the previous one. The demonstration of the range of hazards, weighted, if possible resulting map is thus apseudo-time sequence map of the according to their probability of occurrence. areasthat would be covered by an eruption becoming increasingly more energetic. We do not intend these zones J. F. Tomblin of UNDRO gave initial support to the idea of this to represent the expected actual deposition of flows, rather study and A. Wayson and the staff of the Pan Caribbean Disaster they represent the potential for inundation by pyroclastic Preparedness and Prevention Project helpedorganize the field work flows. For example, on day 1 of the eruption the potential of GW. We were treated with great hospitality and cooperation by for flows may be represented by zone 2 whilst by day 7 it members of the Government of Montserrat. P. E. Baker and W. J. may be zone 5. We cannot quantify such a time-scale but the Rea,A. L. Smithand M. J. Roobolwere helpful indiscussing relative sequence is what matters. We have also added the radiocarbon age determinations on Montserrat. We thankD. mudflow hazard zones as before except that this time the Harkness of the NERC Radiocarbon laboratory for his help with three valleys affected by the fumarole-generated mudflows our own samples and J. Townshend and J. B. Shepherd for their are assigned to an initial zone 1. The most energetic continuing support.The referees' comments helped improvethe component of the sequence isalmost the same as that final form of the paper. This work was carried out under NERC discussed as being the best representative of the integrated contract F60/G6/12. 24-16 000 a BP eruptions. The sequential hazard zone map can be used to suggest References priorities for evacuation of communities at risk atthe ARMIENTI,P. & PAFSSCHI, M. T. 1987. Automaticreconstruction of surge earliest period of an eruption. It couldalso supply useful depositthicknesses. Application to someItalian volcanoes. Journal of information on the viability of evacuation routes at different Volcanology and Geothermal Research, 31, 313-20. BAKER,P. E. 1985. Volcanichazards on St. Kittsand Montserrat, West stages of the eruption. For example, a community lying in Indies. 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ReceiveG 28 September 1987; revised typescript accepted 2 February 1988.

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