Small-volume, highly mobile pyroclastic flows formed by rapid sedimentation from pyroclastic surgesat Soufriere 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), Universite Blaise Pascal, 5 rue Kessler,63038 Clermont-Ferrand, France (e-mail: T.Druitt@opgc.univ-bpclermont.fr)

2 Department of Earth Sciences, University of Bristol, Queens Road, Bristol BS8 1RJ, UK3 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, UK6 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

1 Department of Geosciences, Penn State University, University Park, PA 16802, USA

Abstract: Gravitational collapses of the lava dome at Soufriere Hills Volcano on 25 June and 26 December 1997 generatedpyroclastic surges that spread out over broad sectors of the landscape and laid down thin, bipartite deposits. In each case, partof the settling material continued to move upon reaching the ground and drained into valleys as high-concentration granularflows of hot (120-410°C) ash and lapilli. These surge-derived pyroclastic flows travelled at no more than 10ms - 1 but extendedsignificantly beyond the limits of the parent surge clouds (by 3 km on 25 June and by 1 km on 26 December). The front of the25 June flow terminated in a valley about 50m below a small town that was occupied at the time. Despite their small depositvolumes (5-9 x 104m3), the surge-derived pyroclastic flows travelled as far as many of the Soufriere Hills block-and-ash flowson slopes as low as a few degrees, reflecting a high degree of mobility. An analysis of the deposits from 26 December suggeststhat sediment accumulation rates of at least several millimetres per second were sufficient to generate pyroclastic flows bysuspended-load fallout from pyroclastic surges on Montserrat. Surge-derived pyroclastic flows are an important, and hithertounderestimated, hazard around active lava domes. At Montserrat they formed by sedimentation over large catchment areas anddrained into valleys different from those affected by the primary block-and-ash flows and pyroclastic surges, thereby impactingareas not anticipated to be vulnerable in prior hazards analyses. The deposits are finer-grained than those of other types ofpyroclastic flow at Soufriere Hills Volcano; this may aid their recognition in ancient volcanic successions but, along with valley-bottom confinement, reduces the preservation potential.

Pyroclastic surges are turbulent density currents of volcanicparticles and gas that can travel at high velocities. They arecommonly associated with block-and-ash pyroclastic flows formedby gravitational collapses of lava domes. Pyroclastic surges arehazardous due to their ability to traverse topographic barriers andinterfluves, and are a serious concern during all lava domeeruptions (Yamamoto et al. 1993; Fisher 1995; Fujii & Nakada1999). The high speeds and commonly high temperatures ofpyroclastic surges render them devastating to life, vegetation andbuildings up to several kilometres from the dome (Valentine 1998;Kelfoun et al. 2000; Voight & Davis 2000). In 1902, pyroclasticsurges at Mont Pelee (Martinique) swept over the town of St Pierre,killing 28 000 people. More recently, they claimed 43 lives at MountUnzen (Japan) in 1991 and 65 at Merapi Volcano (Java) in 1994(Yamamoto et al. 1993; Abdurachman et al. 2000). Nineteen peoplewere killed, and seven seriously injured, by block-and-ash flows andassociated pyroclastic surges on Montserrat on 25 June 1997.

The eruption of Soufriere Hills Volcano has highlighted anadditional hazard from pyroclastic surges. Hundreds of domecollapses have occurred during the eruption due to retrogressivegravitational failures involving 104 to 108 m3 of andesite and lastingfor periods of several minutes to hours (Calder et al. 1999, 2002;Cole et al. 1998, 2002). These collapses generated block-and-ashflows with accompanying pyroclastic surges. The most voluminousand fastest-moving surges formed during large collapses, involvingyoung, pressurized parts of the dome and rapid decompression ofentrapped gases, and spread out over broad sectors of southernMontserrat. Collapses of relatively old, degassed dome andesite,on the other hand, generated block-and-ash flows with relativelyweak surge components. During two of the largest dome collapses(25 June and 26 December 1997) suspended-load fallout frompyroclastic surges generated high-concentration, granular flows ofash and lapilli, which in the former case travelled 3km beyondthe limit of the parent surge. Another example occurred during a

collapse on 12 May 1996 but was much smaller, travelling onlya few hundred metres (Cole et al. 2002). An important featureof these surge-derived pyroclastic flows is that they formed bysedimentation over large catchment areas and were able to draininto different valleys from those impacted by the main block-and-ash flows and pyroclastic surges. They thus affected areas notanticipated to be vulnerable in prior hazards analyses, and in onecase terminated close to an occupied town. Given their smallvolumes compared with typical block-and-ash flows of SoufriereHills Volcano, the hot surge-derived flows were notable for theirlong runout distances and for their ability to travel along drainageswith gradients of only a few degrees. This combination ofproperties makes surge-derived pyroclastic flows a potentiallyimportant hazard around active lava domes.

The aim of this paper is to describe the deposits and effects of thesurge-derived pyroclastic flows, to use field measurements andcalculations to constrain the processes of formation and transport,and to discuss hazard implications. Special attention is paid to oneof the surge-derived flows generated on 26 December 1997, SE of thelava dome. This example was studied in most detail because the areawas not threatened by later activity and fieldwork could be carriedout in relative safety. In contrast, the example of 25 June 1997was less well studied, as the deposits were rapidly buried and/oreroded by subsequent pyroclastic flows and lahars, and fieldworkwas more hazardous.

Summary of the eruption during 1995-1999

The eruption of Soufriere Hills Volcano began with a phreaticphase on 18 July 1995 (Robertson et al. 2000; Gardner & White2002). The lava dome first appeared on 15 November 1995, situatedon an old dome in an ancient sector-collapse scar 1 km across called

DRUITT, T. H. & KOKELAAR, B. P. (eds) 2002. The Eruption of Soufriere Hills Volcano, Montserrat, from 1995 to 1999.Geological Society, London, Memoirs, 21, 263-279. 0435-4052/02/S15 © The Geological Society of London 2002. 263

264 T. H. DRUITT ET AL.

Fig. 1. Map of southern Montserrat showing place names and the total area of impact by pyroclastic flows and pyroclastic surges during the1995-1999 eruptive period.

English's Crater (Fig. 1). Throughout 1996 and the first part of1997 the dome was contained within the collapse scar, and dome-collapse block-and-ash flows were channelled down the Tar Rivervalley to the sea, where they built a delta. On the night of 17 Sep-tember 1996, a large dome collapse triggered an explosive eruption(Robertson el al. 1998). In late March 1997, block-and-ash flowsbegan to be shed southwards into the White River valley, and thenin June westwards into Gages valley and northwards into Mosquitoand Tuitfs Ghauts (Fig. 1). On 25 June, part of the dome collapseddown the northern flank of the volcano, and the resulting pyro-clastic flows and surges killed 19 people and generated one of thesurge-derived pyroclastic flows described below. Between 4 and12 August there occurred 13 Vulcanian explosions and, followinga large dome collapse to the north on 21 September 1997, another75 explosions between 22 September and 21 October (Druitt el al.2002). Large collapses occurred down the White River valley on4 and 6 November. On 26 December 1997, the southern wall ofEnglish's Crater failed gravitationally, forming a debris avalanche(Sparks el al. 2002; Voight el al. 2002). At the same time a pyro-clastic surge swept out across a 70 sector, devastating 10km2 ofsouthern Montserrat (Ritchie el al. 2002). Sedimentation from thissurge generated surge-derived pyroclastic flows in several drainages.

Collapse of 25 June 1997

Chronology and events

The dome collapse of 25 June 1997 generated block-and-ash flowand pyroclastic surge deposits with a total volume of 6.4 x 106m3,or about 4.9 x 106m3 dense-rock equivalent (DRE; Calder el al.2002). The collapse and ensuing events are described in detail byLoughlin el al. (2002 a ,b ) , and the deposits and effects are describedby Cole el al. (1998. 2002). Prior to collapse, the dome had avolume of 68 x 106m3 (Calder el al. 2002). Strong cyclic swarmseismicity and edifice deformation began on 22 June and increasedin duration and intensity until, on 24 and 25 June, they merged intoalmost continuous tremor at peak edifice inflations (Voight el al.1999). The main collapse and associated seismic signal on 25 Junecommenced during rapid deflation of the dome at about 12:55 localtime (LT, four hours behind universal time). The seismic signallasted about 25 minutes, with three particularly intense pulses start-ing at 12:57, 13:00 and 13:08 LT (Loughlin el al. 2002b). These threeseismic pulses are believed to have coincided with the generationof three block-and-ash flows observed on time-lapse video and byeyewitnesses. The flows travelled down the deeply entrenched

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Fig. 2. Map of the area impacted by pyroclastic flows and surges generated by the dome collapse of 25 June 1997.

Mosquito Ghaut and the Paradise River valley, almost reaching thesea and the third spilling into Spanish Point. Pyroclastic surgesassociated with flows 2 and 3 swept out successively across Farrell'splain and that of flow 3 passed over the village of Streatham andclimbed a vertical height of 70m up Windy Hill (Fig. 2). Surgemovement was mainly to the north and NW, as shown by tree-blowdown directions, the orientation of charred surfaces, shadowzones behind houses, and the transport of a water tank (Fig. 2).Nineteen people died from the pyroclastic flows and surges, andseven others were seriously injured. The ash plume formed by loftingof the pyroclastic surges and by elutriation from the pyroclasticflows rose to a height of 10km within a few minutes.

Surge-derived pyroclastic flow and its parent pyroclastic surge

The 25 June 1997 surge-derived pyroclastic flow was generated bysuspended-load fallout from the pyroclastic surges associated withpyroclastic flows 2 and 3 (Fig. 2). Surge-derived pyroclastic flowdeposits formed in all the main distributory valleys feeding into theupper part of Dyer's River valley, indicating that the flow was fedfrom a wide area. High-concentration granular flows of ash andlapilli are inferred to have drained in a fluid-like manner off Farrell'splain westwards into Tyre's Ghaut and northwestwards into theheadwaters of Dyer's River valley. Eyewitnesses on the road a fewhundred metres east of Streatham saw ground-hugging flows of ashtravelling downhill to the west along the main road almost at 90° tothe original flow direction of the surge. They commented on how theash was confined to the road, was hugging the ground, was 'boiling'and how it 'moved round the bends on the road like a vehicle'(Loughlin et al. 2002a). Once in Dyer's River valley, the multipleflows of ash and lapilli merged into a single surge-derived pyroclastic

flow, which then travelled 3km down Dyer's River and BelhamRiver valleys, around a series of tight meanders, before terminat-ing about 50m below the small town of Cork Hill (Figs 2 and 3b).The volume of the surge-derived flow deposit was estimated as90000m3 (non-DRE), or about 1.5% of the entire 25 June depositvolume of 6.4 x 106 m3 (Calder et al. 2002).

The pyroclastic surge deposit, typically 5-20 cm thick, wasexamined on Farrell's plain one month after emplacement. A repre-sentative section (location 1, Fig. 2) is shown in Figure 3A. Twolayers of similar thickness were present. The lower one (layer 1)consisted of grey, friable, fines-poor ash and lapilli that thickenedinto depressions and thinned onto highs, maintaining an even uppersurface. Where emplaced onto furrowed fields, layer 1 thickenedinto the furrows and was commonly absent on the ridges. Clots ofsheared soil and vegetation were common in layer 1 and rare blocksup to 10cm were present. Layer 2 consisted of brown ash richer infine-grained components than layer 1 and tinged orange at the topby in situ oxidation. The thickness of layer 2 was approximatelyconstant in any section, and there were segregation pipes rooted onfragments of vegetation. The whole deposit thickened and coarsenedinto drainages and topographic depressions. Near location 1 thedeposit on the floor of one drainage several metres deep had a lower,fines-poor layer of blocks and lapilli and an upper layer of lapilli andash. Trenching at this site suggested that the two layers of the surgedeposit were continuous between the plain and drainage facies. It isinferred that both layers were laid down by a single pyroclasticsurge, probably that associated with pyroclastic flow 3. Decriptionsof the 25 June 1997 surge deposits at other locations are given byLoughlin et al. (2002b).

The deposit of the surge-derived pyroclastic flow was exam-ined in July at two locations: Dyer's bridge (location 2, Fig. 2) andthe terminus below Cork Hill (location 3, Fig. 2). At Dyer's bridge

266 T. H. DRUITT ET AL.

Fig. 3. (a) The 25 June 1997 pyroclastic surge deposit on Farrell's plain atlocation 1 (Fig. 2). The deposit has a lower fines-poor layer (layer 1), and anupper layer (layer 2) richer in fine components. Knife for scale, (b) Aerialview of the Belham River valley, looking east, showing the 25 June 1997surge-derived pyroclastic flow deposit (f) and associated singe zone (s). Thedeposit had already been partially eroded when this photo was taken.Gages Mountain (g) and the village of Dyers (d) are also marked.

(Figs 4 and 5) the surge-derived flow deposit was generally 0.5-1 mthick and draped the valley to a vertical height of 6m (Fig. 5). Only10cm of flow deposit occurred on the bridge top surface. The depo-sit was pink to pale grey and had little internal structure (Fig. 4a).It consisted mainly of ash with subordinate lapilli of dense, juvenileandesite, and was noticeably finer grained than typical block-and-ash flow deposits of Soufriere Hills Volcano (Cole et al. 1998, 2002).

Blocks up to 10cm diameter made up a few per cent of the deposit,but these were heterolithologic with many altered and weatheredrocks, suggesting that they were picked up by the flow furtherupstream. Concentrations of outsized blocks occurred at the side ofthe valley upstream of Dyer's bridge (Fig. 4b).

The surface of the surge-derived flow deposit was uneven,reflecting the underlying topography. There were fresh exposures of

SMALL PYROCLASTIC FLOWS: A HAZARD 267

Fig. 4. (a) Surge-derived pyroclasticflow deposit (f) of 25 June 1997 nearDyers bridge (location 2, Fig. 2),showing the characteristically fine-grained nature with sparse lapilli. Thedeposit is overlain by a thin lahardeposit (1). The scale is in inches(1 inch = 2.54 cm), (b) Concentration ofblocks at the margin of the 25 June 1997surge-derived pyroclastic flow deposit(f) at Dyers bridge. Person for scale.

the deposit in scar faces with dips of 30-60° towards the drainageaxis. The scars were interpreted as the headwalls of breakawayavalanches formed immediately after deposition, as reported frompyroclastic flow deposits at Mount St Helens (Rowley et al. 1985)and Mount Pinatubo (Torres et al. 1997). Corrugated iron roofingmaterial was wrapped around parts of the bridge and around treesat the valley sides, showing that the active flow had occupied thewhole valley to a depth of 5-6 m (Fig. 5). The surge-derived flow atDyer's bridge did not break branches off trees more than 1 cm indiameter. Many burnt logs and branches in the flow must have beenderived from further upstream where the flow was more energetic.

The associated pyroclastic surge deposit at this location reachedat least 15m higher up the valley sides than the parent surge-derived flow deposit and was 5-15 cm thick. It was composed offine ash and contained carbonized twigs. The surge had singed thevegetation to heights of 10-15m above the valley floor in this area,but quite delicate twigs and leaves remained on the trees, suggestingthat the surge cloud was travelling very slowly. There was a sharpgrain-size break between the surge-derived flow deposit and thefiner-grained pyroclastic surge deposit at 5-6 m above the valley

floor, showing that the surge-derived flow had a well defined uppersurface. The trees were strongly burnt up to the level of the grain-size break. Leaves had been stripped from the trees to only 1 mabove the top surface of the surge-derived flow deposit.

At the distal limit of the surge-derived flow, in the valley belowCork Hill (location 3, Fig. 2), the deposit was flat-topped and0.5-1 m thick, smoothing over the boulder-strewn valley bottom.Vegetation and trees were not singed above the top of the flow andleaves were not removed. The deposit at the terminus was fine-grained, with sparse lapilli up to 1 cm in diameter. The singe zonefrom the associated surge cloud extended only about 3 m above thesurface of the flow deposit. The terminus of the surge-derived flowwas marked by a log jam oriented perpendicular to the valley axis.The temperature measured using a thermocouple at 30-3 5 cm depthin the surge-derived flow deposit was 410°C at Dyer's bridge and350°C at the terminus, both five days after emplacement.

The observations at Dyer's bridge (location 2) and at Cork Hill(location 3) imply the existence of a hot, high-concentrationpyroclastic flow with a well defined upper surface and a weakoverriding pyroclastic surge cloud. The surge at these locations is

268 T. H. DRUITT ET AL.

Fig. 5. Schematic cross-section throughthe surge-derived pyroclastic flow depositof 25 June at Dyers bridge (location 2.Fig. 2).

attributed mainly to ash elutriation from the surge-derived flow,but may also have been in part a remnant of the more powerfulparental surges further upstream. Both the surge-derived flow andaccompanying surge cloud are inferred to have travelled slowly, asthere were no significant superelevation effects on bends. Most treesremained standing, even where their bases were immersed in thedeposit, and the flow left two concrete bridges intact. At peakdischarge, the flow moved down the valley as a wave up to 6mdeep, as shown from the relationships at Dyer's bridge, leavingbehind it a deposit 0.5 to 1.0m thick draping the valley floor andsides as it spread out downstream. As the flow drained the valleyaxis, immediate remobilization of the flow deposit generated smallbreakaway scars. The fine-grained character of the surge-derived

pyroclastic flow deposit is attributed to its origin by suspended-loadfallout from pyroclastic surges.

Related volcanic hazards

Pyroclastic flows in the Belham River valley were not expected fromhazards analyses carried out prior to the 25 June 1997 collapse. Sincethe dome had most recently been growing on its northern side, andsince block-and-ash flows throughout June had travelled north-eastwards, towards the airport, the areas to the north and NE of thevolcano were considered to be those at greatest risk. The town ofCork Hill had been considered relatively safe from impact and was

Fig. 6. Map of the area impacted by the pyroclastic density currents generated following the sector collapse of 26 December 1997.

SMALL PYROCLASTIC FLOWS: A HAZARD 269

still inhabited at the time of the 25 June collapse. As it turned out,the surge-derived pyroclastic flow terminated in the Belham Rivervalley about 50m below Cork Hill and within about 400m of thevillage school with an estimated 200 children in it. A child was burntwhile approaching the new deposits (Fig. 2). This incident highlightsthe fact that surge-derived pyroclastic flows can move in unexpecteddirections, well outside the area of the parent surge cloud and anyassociated block-and-ash flows.

Collapse of 26 December 1997

Chronology and events

The collapse of 26 December began at 03:01 LT and the peakseismic signal lasted 11.6 minutes (Sparks et al. 2002; Voight et ai2002). Failure of the southern flanks of the volcano generated adebris avalanche that travelled down the White River valley to thesouth coast (Fig. 6). The collapse was followed by a lateral blastand pyroclastic density current, which swept out across a 70°,10km2 sector south of the volcano and into the sea. The pyroclasticdensity current occurred in two pulses: a first, very powerful pulseand a second, less powerful one, forming a deposit with twonormally graded layers (Ritchie et al. 2002). In total, the collapseand lateral blast involved about 46 x 106m3 of hydrothermallyaltered flank rocks and dome talus and about 35-45 x 106 m3 of thelava dome (Sparks et al. 2002; Voight et al. 2002).

In many respects the 26 December pyroclastic density currentresembled those generated by other lateral blasts, such as MontPelee in 1902 (Boudon & Lajoie 1989), Mount St Helens in 1980(Hoblitt et al. 1981; Fisher 1990; Druitt 1992) and Bezymianny in1956 (Belousov 1996). The general term pyroclastic density currentis used to describe the flow generated by the 26 December 1997lateral blast in order to emphasize the existence within it of strongvertical and flow-transverse gradients in grain size and particleconcentration (Sparks et al. 2002; Ritchie et al. 2002). It differedfrom the pyroclastic surges from other collapses such as 25 June1997 in that it transported large quantities of decimetre- or evenmetre-sized debris, particularly in the higher-concentration lowerpart. The events on 26 December 1997 have been reconstructed asfollows. Gravitational collapse of the southern wall of English'sCrater, with failure in hydrothermally altered rocks associated withGalway's Soufriere, generated a debris avalanche that travelled4km to the south coast. Decompression of the dome interior thencaused explosive expansion of high-pressure gases and productionof a fast-moving, density-stratified pyroclastic current (Woods et al.2002). Much of the basal, higher-concentration part of the current

was channelled down the White River valley to the sea, while theupper, more dilute part spread out across the landscape. In thispaper we refer to this upper, more dilute part as a pyroclastic surge.

Surge-derived pyroclastic flows formed in all drainages inun-dated by the upper, more dilute part of the density current,including substantial ones down Germans and Gingoes Ghauts(Fig. 6). The deposits are referred to as block-poor pyroclasticdensity current facies by Ritchie et al. (2002). The surge-derivedflow we consider below formed where part of the expanding surgeswept over the saddle separating the White River valley from DryGhaut (Fig. 6). This isolated it from the main impact zone,simplifying field relationships and making the deposit and floweffects amenable to study.

The Dry Ghaut surge-derived pyroclastic flow and itsparent pyroclastic surge

As the pyroclastic surge descended Dry Ghaut, rapid suspended-load fallout formed a high-concentration granular flow of ash andlapilli. Figure 7 shows the area of surge impact and the surge-derived pyroclastic flow deposit. The volume (non-DRE) of thesurge deposit in Dry Ghaut was estimated to be about 1.5 x 105 m3

and that of the surge-derived flow about 0.5 x 105 m3. The map alsodistinguishes a zone of tree blowdown in which many trees weretoppled and/or removed by the surge, and a singe zone in whichmost trees were killed but remained standing. The height of the topof the singe zone above the valley floor decreased progressivelydownstream (Fig. 8). For the purposes of discussion the ghaut isdivided into three regions: upper (<0.6km from the head of theghaut), middle (0.6 to 2km) and lower (>2km).

Upper region of Dry Ghaut (0-0.6km). The upper region of DryGhaut consists of a broad valley that becomes progressivelyconstricted downstream (Figs 7 and 9). The top of the singe zone inthis region lay about 100m above the ghaut floor, reaching a higherelevation on the south side than on the north, probably due tocentrifugal effects as the pyroclastic surge swept from the northover the saddle separating the White River valley from Dry Ghaut.

The pyroclastic surge deposit draped the ghaut sides to athickness of a few centimetres to a few tens of centimetres. Grainsize and thickness were greatest at the head of the ghaut anddecreased rapidly downstream. At most locations the deposit over-lay, in erosive contact, a few centimetres of buff-coloured ash withscattered pumice lapilli produced by the Vulcanian explosions ofAugust, September and October 1997. Like the deposit of the

Fig. 7. Map of Dry Ghaut showingthe deposits and effects of thepyroclastic surge and surge-derivedpyroclastic flow of 26 December1997. Locations 1 to 3 are referred toin the text.

270 T. H. DRUITT ET AL.

Fig. 8. Profile down Dry Ghaut showinghow the pyroclastic surge singe line on thenorthern and southern ghaut walls falldownstream due to a combination of surgedeceleration, decrease in surge bulk densitythrough sedimentation, buoyant lofting andinflow of ambient air. The pyroclastic surgecloud was very weak beyond about 2 kmdownstream of the ghaut entrance.Superelevation effects are observed wherethe pyroclastic surge turned bends in theghaut. The sketches depict schematically aninterpretation of the development of thepyroclastic surge (grey) and surge-derivedpyroclastic flow (black) down the valley.Rapid suspended-load fallout in the upperghaut caused the surge to become densitystratified. By the middle ghaut, segregationof the lower, more concentrated part of thepyroclastic surge had generated a well-defined surge-derived pyroclastic flow.

25 June 1997 pyroclastic surge, the surge deposit in Dry Ghaut hadtwo principal layers of comparable thickness. Layer 1 consisted ofcoarse, fines-poor lapilli and blocks (the latter being most abundantnear the head of the ghaut, where blocks up to 15 cm occurred), andwas laterally discontinuous, being thickest in depressions and in thelee of obstacles. The base of layer 1 was commonly marked by a zoneof intense shearing, with surge deposit, clots of soil and Vulcanianash, and charred vegetation mixed together. Overall, layer 1 wasnormally or inverse-to-normally graded. Layer 2 consisted of brownash with scattered lapilli. The contact between the two layers wastypically very sharp, although locally a more gradational contactshowed that both layers were the product of a single depositionalevent. A third layer of thin fallout ash up to few millimetres thickcapped the surge deposit.

At some sites in the upper ghaut, two discrete surge units wereobserved, each with bipartite layering. Two surge units were alsopreserved widely across the main impact zone south of the dome(termed Units I and II by Ritchie el al 2002). Unit I was thicker,coarser-grained, and more extensive than Unit II, from which weinfer that the main surge unit in Dry Ghaut was probably Unit I.Some repetition of layering in Dry Ghaut may also record localsplitting of the surge around topographic irregularities, as occurredat Mount St Helens (Kieffer 1981; Druitt 1992).

The surge deposit in upper Dry Ghaut thickened to 1 m or morein pre-existing drainage channels on the ghaut floor. The same two-layer stratigraphy was present in the channel facies, but both layerswere thicker and coarser-grained than outside the channels. A typi-cal section through channel fills consisted of a grey to mauve, fines-poor layer of blocks and lapilli a few decimetres thick overlain by afew decimetres of brown ash and lapilli with matrix-supportedblocks. Trenching confirmed that these layers passed without breakinto layers 1 and 2 respectively of the thin surge deposit outside thedrainage channels. Locally, clast-supported accumulations of blocksand lapilli were observed at the bases of the drainage fills. Althoughwe have mapped the drainage facies in the upper ghaut as surge-derived flow deposit (Fig. 7), it is essentially overthickened surgedeposit, which passes downstream into the true surge-derived flowdeposit in a manner that could not be studied in detail during ourfieldwork, owing to practical reasons of safety and limited helicopteraccess. The field relationships suggest that suspended-load fallout inthe upper ghaut concentrated coarse debris towards the base of thepyroclastic surge, and that this accumulated preferentially in pre-existing gullies and topographic lows. Abundant segregation pipeswere present in layer 2 near the top of the drain-age fills.

The pyroclastic surge deposit, traced up the valley sides fromthe ghaut floor to the top of the singe zone, showed essentiallycontinuous decreases in grain size and thickness, indicating that thepyroclastic surge was vertically stratified in grain size, and probably

concentration, in the upper ghaut. No deposits resembling those ofblock-and-ash flows were observed in the upper region of DryGhaut, showing that the surge-derived pyroclastic flow furtherdownstream formed by sedimentation from the pyroclastic surge,and not from any primary, high-concentration material swashedinto the ghaut from the White River valley.

Many small trees on the floor of the upper ghaut were felledand/or removed by the surge (Fig. 9). The width of the blowdownzone decreased rapidly down the ghaut, probably because the surgewas decelerating and or becoming less dense as it went forward(Fig. 7). Felled trees were oriented obliquely inwards towards theghaut axis, and in some small tributaries were reoriented directlydownslope due to local formation and drainback of dense under-flows, as also observed at Mount St Helens (Hoblitt & Miller 1984).At a given site, mature trees were felled more readily than small,younger trees, probably because the latter were more supple and,being smaller, offered less resistance to the surge. Many young treesremained standing in the blowdown zone, bent over in the directionof flow. Some young trees remained standing on the ghaut flooreven where partly immersed in metre-thick surge deposits.

Bark was abraded or removed on the upstream sides of trees,and that remaining was commonly charred and blistered. The charon young trees and bushes was commonly very thin (1 mm or less)and the wood inside unaffected, showing that the burn was intense,but short-lived. The boundary between the charred, upstream-facing sides of branches and their uncharred downstream sides wasvery sharp (Fig. lOa), as also observed at Mount Lamington(Taylor 1958). The degree of tree damage decreased gradually fromthe blowdown zone upwards to the top of the singe zone.

The evidence from the upper ghaut implies that, during this partof its passage, the pyroclastic surge was vertically stratified in grainsize and density and was actively segregating into an upper, moredilute part and a lower, more concentrated part that was beingconcentrated into channels and depressions. As the surge sweptthrough the upper ghaut, it decelerated and or decreased in densityrapidly, as shown by the rapid downstream fall-off of grain size andextent of tree blowdown.

Middle region of Dry Ghaut (0.6-2.0 km). The middle region ofthe ghaut is dominated by a steep-sided canyon. When the surgeentered the middle ghaut it was travelling too slowly, and or was toodilute, to fell many trees. The valley sides here were covered by asurge deposit, commonly consisting of just a single layer less than adecimetre thick. The second surge unit (probably Unit II of Ritchieel al. 2002) was not observed outside the upper ghaut. On the ghautfloor a massive surge-derived pyroclastic flow deposit formeda continuous drape up to several decimetres thick over stream

Fig. 9. Panorama looking up (west) into the upper reaches of Dry Ghaut. The pyroclastic surge overtopped the saddle on the skyline and travelled towards the observer before being deflected towards the lower right ofthe field of view. The surge-derived pyroclastic flow deposit (f) occurs in the pre-existing drainages. The surrounding slopes are covered by the deposit from the pyroclastic surge. Areas in which trees are mostly blowndown and/or removed (b) and those in which many trees remain standing (s) are distinguished. The field of view of the skyline is about 1 km wide (see Fig. 7).

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Fig. 10. Effects of the 26 December 1997 pyroclastic surge and surge-derived pyroclastic flow in Dry Ghaut, (a) Branch (held vertical) charred by thepyroclastic surge as it passed through the upper reaches of the ghaut. The extremely sharp limit to the char suggests that the heat was very intense and short-lived. Location 1 on Figure 7. (b) Log-jam generated by the surge-derived pyroclastic flow7 as it passed through the lower reaches of the ghaut. The boulder waspicked up upstream and rammed into the accumulated logs. Location 2 on Figure 7.

boulders. In the old stream channels and in narrow gorges the flowdeposit thickened to 1 m or more of massive ash with matrix-supported blocks and lapilli and lacking the bipartite layering of thesurge deposit.

Most trees in the middle ghaut remained standing right down tothe ghaut floor and only where the progressively forming surge-derived pyroclastic flow swept up and over the inside corners ofbends were small trees on the valley sides systematically felled. Thesurge-derived flow had a maximum thickness no more than that ofthe resulting deposit. On the ghaut floor there was a clear boundarybetween where small trees had been pushed over by the high-concentration flow, and where trees remained standing; thisboundary corresponded with the upper surface of the flow deposit.A particularly convincing example occurred 1.5km down theghaut, where the flow travelled through a narrow gorge. In thisgorge young trees with delicate branches remained upright andintact immediately above the top of the flow deposit, which wasnearly 2m thick.

The observations suggest that by the time the pyroclastic surgeentered the middle ghaut, a high-concentration granular underflow(the surge-derived pyroclastic flow) was already developed, and thatthe two were separated vertically by a sharp concentration andrheological interface.

Lower region of Dry Ghaut (>2km). By the time the pyroclasticsurge reached 2 km down the ghaut it had become very weak and isinferred to have mostly lofted; most of the moving mass was by then

concentrated in the surge-derived pyroclastic flow. The singe zone inthe lower ghaut was less than 10m high, and only 2-3 m high at theflow7 terminus. Figure 11 is a map of one stretch of the ghaut wherethe surge-derived flow travelled round two bends. The surge-derivedflow deposit on the ghaut floor was typically flat-topped and con-sisted of massive ash and lapilli with abundant charred wood. Woodfragments were commonly concentrated at the top of the depositdue to flotation in the high-concentration flow. Traced to the sidesof the ghaut, the flow deposit had a very sharp limit, beyond which itpassed abruptly into a few centimetres of fine-grained grey ash fromthe overriding ash cloud (Fig. 11). showing that the moving flow hada very well defined upper surface. Where the flow travelled aroundbends there were clear superelevation effects. w?ith a rising and fall-ing strandline of massive flow deposit where the flow had swashedup on the outer side of the bend. On the outside curve of oneparticularly sharp bend, part of the flow ran up out of the streamchannel, dumped a pile of charred logs, and spread out over thegently sloping forest floor as a sheet about 40cm thick (location A.Fig. 11).

Most trees remained standing in the lower ghaut, and manyretained their leaves at heights greater than a few metres above theghaut floor. This showed that the pyroclastic surge component wasvery weak by the time it reached this part of the ghaut and wasprobably sustained largely by elutriation of ash from the high-concentration surge-derived pyroclastic flow. Trees impacted by thesurge-derived flow were commonly felled, and the bark stripped offtheir upstream sides. Older trees snapped, but younger ones weresimply flattened down parallel to the flow direction. In several

SMALL PYROCLASTIC FLOWS: A HAZARD 273

Fig. 11. Interpretative map of part of the lower region of Dry Ghaut, where the surge-derived pyroclastic flow encountered two bends in the stream valley. Thelocation is marked on Figure 7.

places the surge-derived flow left jams of partially charred logs piledup against trees or boulders (Fig. 10B). Some large trees remainedstanding in the flow deposit, presumably because the flow was tooshallow and/or too slow to fell them.

The pyroclastic flow terminated behind a pile of boulders 200 mfrom the sea. Near the terminus the deposit had an almost flatsurface (Figs 12A and 13), with just 1-2 cm of fine ash on theadjacent valley sides. The singe zone here was only 2-3 m high.Trees in the singe zone retained very delicate twigs and many stillhad charred leaves on them, so the surge cloud was very weak nearthe flow terminus. Many trees remained standing in the flow andhad suffered only minor bark loss and charring to a few tens ofcentimetres above the top of the deposit. There were no noticeablesuperelevation effects on bends near the terminus, showing that thespeed of the surge-derived flow was low. The distal flow deposit wastypically massive or (coarse-tail) normally graded, with a crudeupstream-inclined fabric of coarser clasts, and was about 1 m thick(Figs 12B and 13). The gradient of the stream bed where the flowfront came to rest was 7°. Temperatures of 120-140°C were mea-sured in the flow deposit at depths of 25-35 cm four days after theeruption, accounting for the weak thermal effects on vegetation.Another 26 December 1997 surge-derived pyroclastic flow depositproduced near St Patrick's was 190°C nine days after emplacement.

Granulometry

The grain-size compositions of deposits from the Belham Rivervalley and Dry Ghaut pyroclastic surges and surge-derivedpyroclastic flows are shown in Figure 14 on a standard r Mdplot. Samples from the upper, middle and lower regions of Dry

Ghaut are distinguished on these graphs. In each case samples fromdifferent heights in a single section are connected by tielines.

The plots show two important features of the surge-derived flowdeposits. First, they are systematically finer grained than typicalblock-and-ash flow deposits of Soufriere Hills Volcano, but havegrain-size compositions within the range of those of the associatedsurge deposits. This is consistent with derivation of the flows bysedimentation from the pyroclastic surges, as deduced from fieldrelationships. Second, the grain-size compositions of the surge-derived flow deposits are rather uniform. Those from the lower andmiddle regions of Dry Ghaut are very similar granulometrically,and in turn resemble the surge deposit in the upper region of DryGhaut. The observations suggest: (1) that the Dry Ghaut surge-derived flow formed mainly by suspended-load fallout in the upperregion of the ghaut (top few hundred metres); and (2) that the flowtravelled through the middle reaches of the ghaut withoutsignificantly changing its grain-size composition.

Physical parameters

We have used field measurements and calculations to estimate thephysical properties of the surge-derived pyroclastic flows andparent pyroclastic surges. Emphasis is placed on Dry Ghaut, sinceit is where our most detailed fieldwork was carried out.

The parent pyroclastic surges

The surge associated with block-and-ash flow pulse 3 of 25 June1997 ramped 70m up the side of Windy Hill. Simple conversion of

274 T. H. DRUITT ET AL.

Fig. 12. Surge-derived pyroclastic flow deposit from 26 December 1997 in Dry Ghaut. Location 3 on Figure 7. (a) About 200m from the terminus, showing theflow deposit (f) (cut by a later gully) and associated singe zone (s). (b) Close-up of the surge-derived flow deposit (0 showing its fine-grained nature withscattered, matrix-supported lapilli. Note also the crude upstream (right) inclined fabric of the coarser clasts. The topmost layer (1) is a lahar deposit of unknownlater date. The light and dark patches on the left are surface effects.

kinetic to potential energy implies a speed of about 35ms l (seealso Loughlin et al. 2002&). The frontal velocity of the 26 December1997 pyroclastic surge across the main impact area south of thedome has been constrained by to have been in the range 55-85ms - 1 , corresponding to an inferred internal velocity of perhaps80-120ms-1 (Sparks et al. 2002).

We now consider the upper, dilute part of the 26 December1997 pyroclastic density current (surge) that entered Dry Ghaut.The velocity was estimated crudely using measured superelevationeffects on bends. This exploits the observation that at several placesdown the ghaut the singe line of the surge in Dry Ghaut rises on theoutsides of bends and falls on the insides (Fig. 8), and this is

SMALL PYROCLASTIC FLOWS: A HAZARD 275

Fig. 13. Section through the surge-derivedpyroclastic flow deposit about 200 m fromits terminus in Dry Ghaut (reconstructed,pre-gullying). The boulders on the bed ofthe stream channel followed by the surge-derived flow pre-dated the flow. Thelocation is marked on Figure 7.

attributed to centrifugal effects. The balance of centrifugal andgravity forces on a bend generates a slope 9 on the surface of acurrent, given by:

where R is the mean radius of curvature of the bend, U is thecurrent velocity, and 0 is the channel slope parallel to flow. Tan 0was estimated at five bends of different radii by measuring theelevation difference AH of the upper limit of the singe zone acrossthe valley, along with the valley width W (tan 0 = AH/W}. In all

Fig. 14. Plot of median grain size Mdo versus sorting (Inman 1952),showing data for the 25 June 1997 and 26 December 1997 pyroclastic surgedeposits (a) and surge-derived pyroclastic flows (b) in the Belham Rivervalley and Dry Ghaut. The fields of Montserrat dome-collapse block-and-ash flows are shown for comparison.

five cases a significant superelevation effect was present. The calcu-lated velocities range from 13 to 2 2 m s - 1 (Fig. 7, Table 1).

The velocities can in turn be used to constrain the mean particleconcentration in the surge. Britter & Linden (1980) found experi-mentally that the frontal velocity Uf of a turbulent density currenton a slope in the range 5-90° is insensitive to the slope angle (due tobalance of buoyancy and entrainment-related drag forces) and isgiven approximately by:

where Q is the internal volumetric flux per unit width of the current,p is the density of the ambient fluid, and Ap is the excess density ofthe current. Q is given by Q = Uih, where Ui is the internal cur-rent velocity and h is the current thickness. In experimental den-sity currents, Uf is found to be about 60% of Ui Assuming thatthe velocities derived above from superelevation effects are esti-mates of the internal velocity of the surge, substituting Uf — 0.6Ui,Ui= 13-22ms -1, p = 1.3kgm - 3 (density of air near sea level) andh = 50-100 m shows that the average bulk density of the surge can-not have exceeded about 1.4kgm - 3 , otherwise its velocity wouldhave been greater. Similarly, the particle concentration in the surgecannot have exceeded about 0.1 vol% irrespective of what gastemperature (<850°C), gas density and solids density are assumed.This crude calculation ignores sedimentation and unsteadinesseffects, but probably provides the right order of magnitude. Notethat this reasoning applies only to the middle region of the ghaut,where it was possible to quantify the superelevation effects. In alllikelihood the surge had a higher mean density and particleconcentration in the upper ghaut. Moreover, the surge would havebeen density-stratified, so that density and concentration near theghaut floor were higher than these mean values (Valentine 1987).

Table 1. Velocity estimates for the Dry Ghaut surge and flow componentsfrom superelevation effects on bends

Distance (km) A// (m) W (m) R (m) o (°) U (ms - 1 )

Pyroclastic surge0.75 ±0.201.05 ±0.201.20 ±0.201.75±0.101.85±0.10

Surge-derived2.10±0.052.33 ±0.052.60 ±0.05

3025101842

pyroclastic flow3.51.53.6

350210250150150

101010

30540554015090

302510

12101088

776

1622151316

1066

AH is the height difference of the top of the pyroclastic surge or flowdeposit across the valley, width W; R is the mean radius of curvature of thebend; o is the mean valley gradient; and U is the estimated pyroclastic surgeor flow velocity.

276 T. H. DRUITT ET AL.

The ranges of velocity and density of the pyroclastic surgeappear to be consistent with the limited extent of tree blowdown inDry Ghaut. Once the surge left the upper region, it was incapable offelling significant numbers of trees (Fig. 7). The ability of a surge tofell trees depends on the dynamic pressure exerted. This scales asP 3Uf, where 3 is the bulk density and Ui the internal velocity.In a review of nuclear weapon effects, Valentine (1998) gives acritical dynamic pressure of about 500-800 Pa for light (<30%) treeblowdown. This will depend on tree size and species, but is consis-tent with measured wind speeds of up to c. 40ms -1 (P = c. 800 Pa)during Hurricane Hugo, which felled a considerable number of treeson Montserrat in 1989. For a surge of density 1 .4kgm - 3 travellingdown the middle region of Dry Ghaut at 2 0 m s - 1 , we haveP = c. 280 Pa, which is indeed below the threshold for blowdown.

The velocity and density of the surge at the top of the ghautcannot be constrained, but it is likely that both were higher thanfurther downstream. We estimate that on average about 30%, andlocally as much as 90%, of trees were felled in the upper ghaut. Thismay require dynamic pressures as high as 2500 Pa (Valentine 1998).For a flow density of 2 k g m - 3 the calculated internal speedrequired to generate a dynamic pressure of 2500 Pa is 50m s - 1 , andfor 5 kgm - 3 it is 30m s - 1 . These examples give the correct order ofmagnitude for a mean current density and average internal velocityover the height of a felled object.

The internal velocities calculated for the pyroclastic surge in DryGhaut (<50 ms-1) are lower than those estimated for the pyroclasticdensity current across the main impact zone south of the lava dome(80-120ms-1; Sparks et al. 2002). This is probably because: (1) DryGhaut is oriented perpendicular to the main current axis, so that thecurrent impacted the head of the valley obliquely and the initialdownstream velocity component was not very high; (2) only theupper, most dilute part of the current (pyroclastic surge) spilled overinto Dry Ghaut, having a lower density (and thus lower velocity)than that along the main axis; (3) the velocities estimated for mainimpact zone were enhanced by gravitational acceleration down theflanks of the volcano south and SW of the lava dome.

We can estimate very approximately the rate of sedimentaccumulation from the surge as it flowed down the ghaut. The rateof sediment accumulation from a turbulent suspension current is(Bursik & Woods 1996):

where {3 is the bulk density of the surge, n is the mass fraction ofsolids (c. 0.5, for a particle concentration of c. 0.1 vol%), C is thedeposit density (c. 1000 kg m - 3), and w is the mean particle settlingvelocity . The settling velocity W of a spherical particle in gas can beexpressed approximately as (Kunii & Levenspiel 1991):

where u is the fluid viscosity, d is the particle diameter, p is the fluiddensity, a (>>p) is the particle density, and g is gravitationalacceleration. The mean particle size in the surge was about 300-500um (Fig. 14), so w is a few metres per second. Taking a surgedensity of 1.4kgm - 3 , appropriate for the middle stretch of theghaut, yields an average sediment accumulation rate of a few milli-metres per second. This implies a sedimentation duration of a fewtens of seconds to generate the observed deposit, typically a fewdecimetres in thickness. Higher accumulation rates were likely alongthe valley axis due to effects of density stratification (Valentine1987), and in the headwaters of the ghaut where the surge densityand particle concentration were probably higher.

The surge-derived pyroclastic flows

Once formed, the surge-derived pyroclastic flows of 25 June and26 December 1997 travelled slowly down their respective drainagesunder gravity. The Belham River valley flow moved as a wave of

material up to 6m high, which spread out longitudinally and thin-ned as it went. The speed of the flow must have been very low,probably a few metres per second, since many trees remained stand-ing, even where partially immersed in the flow deposit, and the flowwas unable to destroy a bridge over which it passed.

The flow in Dry Ghaut was thinner during transport (<l-2m)than the Belham River valley example. It moved fast enough to felltrees with trunks 10cm or less in diameter, but most larger treesremained standing. Velocity estimates for the Dry Ghaut flow wereobtained using superelevation effects around bends. We used a tapeand abney level to survey swash-up heights at three locations inthe lower ghaut. In each case there was a clear trim line between thesurge-derived flow deposit and the finer-grained deposit from theoverriding pyroclastic surge. The results (Fig. 7 and Table 1) showthat the surge-derived flow was moving at only a few metres persecond, which seems consistent with the tree evidence on the ghautfloor and with the weak nature of the associated ash cloud in thelower ghaut.

The surge-derived flows were extremely 'mobile' in the sensethat, given their very small volumes compared with typical Sou-friere Hills block-and-ash flows, they had remarkably long runoutsbeyond the main pyroclastic surge impact zones (3 km and 1 kmrespectively for the Belham River valley and Dry Ghaut examples).Calder et al. (1999) compared the mobilities of the three types ofMontserrat pyroclastic flow with those of cold-rock avalanches,using both L/H (the inverse of the conventional parameter H\L\e.g. Hayashi & Self 1992) and a parameter A : V 2 / 3 based on a recentscaling analysis of avalanche dynamics (Dade & Huppert 1998):H is the maximum vertical height drop and L is the horizontaldistance to the flow terminus: A and V are the areas and volumesrespectively of the resulting deposits. Calder et al. (1999) showedthat the surge-derived pyroclastic flows had mobilities (at a givenvolume) significantly greater than block-and-ash flows or cold-rockavalanches, and comparable to, or greater than, those of pumice-and-ash flows formed by fountain collapse on Montserrat inAugust, September and October 1997. For example. L/H andA / V 2 / for the surge-derived flows are 6-15 and 130-360 respec-tively, whereas for pumice-and-ash flows they are 3.3 ±0.2 and135±18 and for block-and-ash flows 4.5±0.5 and 34±4. Theirhigh mobility is also reflected in the low gradients (2 and 72 forBelham River valley and Dry Ghaut, respectively) on which theflow fronts came to rest. Note that these angles of rest are broadlyconsistent with the values of L H given above. Both the tangent ofthe angle of rest at the terminus (0.04-0.12) and the inverse of L/H(0.06-0.16) are crude measures of apparent friction coefficient inthat part of the moving debris that reached the distal limit (Pariseau& Voight 1979) and give similar ranges of values for the surge-derived pyroclastic flows.

Discussion

Pyroclastic flows formed by three different mechanisms duringthe 1995-1999 eruptive period of Soufriere Hills Volcano: lava-dome collapse, fountain collapse during Vulcanian explosions, andsuspended-load fallout from pyroclastic surges generated duringdome-collapse events (Calder et al. 1999: Cole et al. 2002; Druittet al. 2002). The first two mechanisms have been described frommany volcanoes and are relatively well understood (e.g. Fisher &Schmincke 1984; Cas & Wright 1997; Carey 1991; Druitt 1998).In this paper we have described the evidence for the third mech-anism in some detail and have constrained approximately some ofthe relevant physical parameters. The fact that surge-derived pyro-clastic flows were generated on at least two occasions, and at severallocations, on Montserrat suggests that they may be a commonthreat around lava domes. Although we have stressed the DryGhaut example in this paper, surge-derived flows also formedin several other drainages along the south coast of Montserrat on26 December 1997, with particularly large ones in Gingoes andGermans Ghauts (Fig. 6). Another, smaller, example also occurredduring a collapse on 12 May 1996 (Cole et al. 2002).

SMALL PYROCLASTIC FLOWS: A HAZARD 277

Small-volume, high-concentration flows were also producedlocally by suspended-load fallout from the 18 May 1980 lateralblast pyroclastic surge at Mount St Helens (Hoblitt et al. 1981;Hoblitt & Miller 1984; Brantley & Waitt 1988; Fisher 1990). Thesetravelled down local slopes at low speeds, sometimes at right anglesto the main surge transport direction, but were largely trapped bylocal topography. During the 1902 eruption of Mont Pelee, settlingof particles from energetic surges generated pyroclastic flows thatwere finer grained than the associated primary block-and-ash flowsof the Riviere Blanche (Fisher et al. 1980; Fisher & Heiken 1982).

The term 'secondary' has been used to describe these flows(Hoblitt & Miller 1984; Brantley & Waitt 1988; Fisher et al. 1987);however, it is important to distinguish them from secondarypyroclastic flows formed by the remobilization of ignimbrite, suchas followed the 1991 eruption at Mount Pinatubo (Torres et al.1997). Moreover the term 'secondary' implies a time gap betweensurge emplacement and remobilization to form concentrated flows.This was not the case at Montserrat, where the emplacement of thepyroclastic surge and its derivative pyroclastic flow were essentiallysimultaneous. It is for this reason that we propose the term surge-derived pyroclastic flow for this phenomenon.

The main features of the Belham River valley and Dry Ghautflows are summarized in Table 2. Both formed by suspended-loadfallout from pyroclastic surges, which generated granular flows ofash and lapilli that then drained gravitationally into neighbouringvalleys and topographic depressions. The flows were highlyconcentrated with well defined upper surfaces, they travelled at afew metres per second and the associated overriding surge cloudsnear the distal limits were very weak (in contrast to the parentalsurges that generated the surge-derived flows in the first place). TheBelham River valley flow had a broader catchment area and alarger volume than that in Dry Ghaut. The former consequentlytravelled as a wave of debris up to 6 m high, reaching 3 km from thelimit of the parent surge, whereas the Dry Ghaut flow nowhereexceeded a couple of metres in thickness and outran its parent surgeby only 1 km.

The Dry Ghaut example has been studied in some detail. As thedome collapsed, a powerful pyroclastic density current sweptsouthwards, and part of the upper, dilute levels of it (pyroclasticsurge) spilled over into Dry Ghaut. As it entered the ghaut, thissurge was travelling at a few tens of metres per second and wascapable of felling trees. However, it decelerated rapidly and beganto segregate into a more concentrated lower part and an uppermore dilute part. Evidence for deceleration is provided by the rapiddecrease in grain size of the surge deposit, and narrowing of thetree-blowdown zone, over the first few hundred metres from the top

of the ghaut. As the surge decelerated, suspended-load falloutgenerated a high-concentration granular flow that cascaded down-stream through the steep-sided gorge of the middle section of theghaut. As it did so, it was fed by further fallout from the surge,which by now was travelling at no more than 20 m s-1 and was verydilute (0.1% or less of solids), although this was not sufficient togreatly modify the grain-size composition of the flow. The down-stream reduction in height above the valley floor of the surge singeline probably records a combination of progressive surge decelera-tion, decrease in bulk density through sedimentation, drawing inof the sides of the surge cloud by advective indrafts of air, andbuoyant lofting (Fig. 8). By the time the surge-derived pyroclasticflow reached 2km down the ghaut it was travelling at only a fewmetres per second and the accompanying pyroclastic surge was veryweak. The front of the flow then continued a further kilometrebefore coming to rest 200m from the sea. In total, the surge-derivedflow accounts for about a quarter of the material that entered DryGhaut (Table 2).

Given their small volumes relative to many block-and-ash flows,an important feature of the surge-derived pyroclastic flows wastheir large runout distances compared to the height drop (i.e. highmobility in the sense of Calder et al. 1999). This was not a tem-perature effect, since the emplacement temperatures (120-410°C)were lower than those of less 'mobile' block-and-ash flows (up to650°C). If the flows were partially fluidized by escaping gas, the gascannot have been derived by exsolution because the diffusion rateof gas in volcanic glass is negligible at temperatures as low as 410°Cor less. Nor can the gas have contained a significant component ofexternally derived steam since, to our knowledge, both the BelhamRiver valley and Dry Ghaut were largely free of water prior to the25 June and 26 December 1997 events. Two possible gas sourcesthat cannot be ruled out are juvenile clast attrition and rupture ofgas-filled vesicles during transport, and incorporation and combus-tion of vegetation. Calder et al. (1999) tentatively attributed thehigh mobility of the surge-derived flows to rapid sedimentation ofmaterial from turbulent suspension and the formation of a poorlysorted granular flow with low frictional resistance, perhaps due tothe generation of high transient pore pressures.

Recent studies have recognized different regimes of sedi-mentation from turbulent pyroclastic suspensions (Druitt 1998and references therein). In some cases, particles settle directly toform a deposit. The deposit can be planar or cross-stratified if par-ticles undergo late-stage traction, or massive if deposition occursdirectly from suspension. The deposits of pyroclastic surges are laiddown principally in this manner. In other cases, the settlingparticles continue to move upon reaching the ground, forming

Table 2. Comparison of the surge-derived pyroclastic flows of 25 June and 26 December 1997

Feature

SimilaritiesOrigin of pyroclastic flowNature of flowFlow velocityNature of flow deposit

DifferencesVol. of surge deposit (m3)Vol. of surge-derived flow deposit (m3)Flow thickness in transport (m)Runout beyond surge (km)Ground slope at terminus (°)Mobility ratio, L/HTemperature of flow deposit (°C)

26 December (Dry Ghaut) 25 June (Belham valley)

Rapid suspended-load fallout from pyroclastic surgeConcentrated granular flow with well-defined upper surface10ms - 1 or less, with weak accompanying ash cloudHummocky veneer (upstream), flat-topped drainage fill(downstream); massive, poorly sorted, weak normal grading;unusually fine-grained for pyroclastic flow deposit

150000* SOOOOOf50000 90000

<l-2 <61 37 26±1.5 15±5120-140 310-400

* Volume of surge deposit within Dry Ghaut, not DRE.Volume of the whole pyroclastic surge deposit from that event, not DRE.

278 T. H. DRUITT ET AL.

high-concentration pyroclastic flows. These decouple from the pyro-clastic surge and drain gravitationally into lows, sometimes out-running the surge (Fisher 1990, 1995).

The present study offers new insight into the formation of pyro-clastic flows beneath turbulent pyroclastic suspensions. The 25 Juneand 26 December 1997 pyroclastic surges at Montserrat laid downthin, landscape-draping deposits, typically of two layers - a fines-depleted lower layer and an upper one richer in fines - as hasbeen described from deposits of other pyroclastic surges at MontPelee (Boudon & Lajoie 1989), Mount St Helens (Hoblitt et al1981; Fisher 1990; Druitt 1992) and Bezymianny (Belousov 1996).At Mount St Helens and Bezymianny, high-concentration flows ofsmall volume formed by remobilization of rapidly accumulated andloosely packed slope facies (Fisher et al. 1987; Hoblitt & Miller1984; Belousov 1996). Ponds of flow deposits at Mount St Helenscommonly overlie the blast surge deposit, showing that remobiliza-tion took place following initial sediment accumulation. At Mont-serrat we have not observed a surge deposit below that of its surge-derived flow, but this could be due to erosion or to the necessarilycursory nature of our field studies. It is possible that flow forma-tion at Montserrat took place both simultaneously with surgedeposition (i.e. without the material coming to rest) as well asby remobilization shortly thereafter. The presence of breakawayscars on the surface of the Belham River valley deposit shows thatparts of the flows themselves came to rest and then remobilized.We envisage a complicated combination of suspended-load fallout,advection of high-concentration flows to the valley floor, and someremobilization, in the production of the Montserrat surge-derivedpyroclastic flows.

It has been speculated that the formation of high-concentrationpyroclastic flows from turbulent suspensions is favoured by highrates of suspended-load fallout (Druitt 1992, 1998). Rapidsedimentation of granular materials can, under some conditions,generate excess pore pressures in the accumulating sediment layer,reducing the intergranular friction and allowing the sedimentedlayer to flow away (Miller 1990; Freundt 1999). The averagesediment accumulation rate in the middle reaches of Dry Ghautwas probably a few millimetres per second. Higher values werelikely in the upper ghaut and along the valley axis due to densitystratification effects (Valentine 1987). Bursik et al. (1998) estimatedan average bulk density of c. 1 .5kgm - 3 for the Mount St Helenspyroclastic surge 12km from source, which also generated high-concentration pyroclastic flows of local extent, and this corre-sponds to an accumulation rate of a few millimetres per second(Equation 3). We tentatively infer that sediment accumulation ratesof a few millimetres per second may be required for the formationof pyroclastic flows beneath turbulent pyroclastic suspensions.Values of this order are predicted by theoretical models of ignim-brite emplacement, which is known to involve widespread formationof highly concentrated granular flows from turbulent suspensions(Bursik & Woods 1996; Druitt 1998; Freundt 1999).

The deposits of surge-derived pyroclastic flows are massive ornormally graded and are characteristically finer grained than othertypes of pyroclastic flow deposits. They consist principally of ashwith subordinate lapilli, blocks comprising only a small percentageof the deposit. The relatively fine grain size of these flows isattributed to their origin by sedimentation from turbulent suspen-sions in which the transport of abundant coarse blocks was notpossible. The fine grain size may be a distinguishing feature ofsurge-derived pyroclastic flows at other volcanoes, and may aidrecognition of their deposits in ancient successions. The depositsare, however, relatively thin and, owing to their deposition alongvalley axes, readily eroded, giving them poor preservation poten-tial. The surge-derived flow deposits in the Belham River valley andDry Ghaut had both been largely removed by erosion a year aftertheir formation.

The recognition of surge-derived pyroclastic flows has impor-tant implications for volcanic hazards assessments. The ability ofpyroclastic surges to generate highly mobile, high-concentrationpyroclastic flows poses an important, but hitherto underestimated,threat around lava domes. A particularly important feature of these

flows is their ability to develop in drainages different from thoseaffected by the main block-and-ash flows and associated pyroclasticsurges. They therefore have the potential to impact upon areas notpreviously anticipated in hazards analyses.

The Department for International Development is thanked for financialsupport in monitoring Soufriere Hills Volcano. E.S.C. acknowledges aNERC studentship, L.J.R. a studentship from the University of Luton,and R.S.J.S. a NERC Professorship. Helicopter pilots J. McMahon andA. Grouchy skillfully transported the authors in the field. M. Branney,A. Freundt and P. Kokelaar kindly reviewed the manuscript. This paperis published with the permission of the Director of the British Geolog-ical Survey.

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