Episodes of cyclic Vulcanian explosive activity with fountain collapse at Soufriere Hills Volcano, Montserrat

Episodes of cyclic Vulcanian explosive activity with fountain collapse at Soufriere Hills Volcano,Montserrat

T. H. DRUITT1, S. R. YOUNG2, B. BAPTIE2, C. BONADONNA3, E. S. CALDER3, A. B. CLARKE4,P. D. COLE5, C. L. HARFORD3, R. A, HERD6, R. LUCKETT2, G. RYAN7 & B. VOIGHT4

1 Laboratoire Magmas et Volcans (UMR 6524 & CNRS), Universite Blaise Pascal, 5 rue Kessler,

63038 Clermont-Ferrand, France (e-mail: [email protected])2 British Geological Survey, Murchison House, Edinburgh EH9 3LA, UK

^Department of Earth Sciences, University of Bristol, Queens Road, Bristol BS8 1RJ, UK4 Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA

6 Centre for Volcanic Studies, University of Luton, Park Square, Luton LU1 3JU, UK6 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

7 Environmental Science Department, Institute of Environmental and Natural Sciences, University of Lancaster,

Lancaster LAI 4YQ, UK

Abstract: In 1997 Soufriere Hills Volcano on Montserrat produced 88 Vulcanian explosions: 13 between 4 and 12 August and75 between 22 September and 21 October. Each episode was preceded by a large dome collapse that decompressed the conduitand led to the conditions for explosive fragmentation. The explosions, which occurred at intervals of 2.5 to 63 hours, with amean of 10 hours, were transient events, with an initial high-intensity phase lasting a few tens of seconds and a lower-intensity,waning phase lasting 1 to 3 hours. In all but one explosion, fountain collapse during the first 10-20 seconds generatedpyroclastic surges that swept out to 1-2 km before lofting, as well as high-concentration pumiceous pyroclastic flows thattravelled up to 6 km down all major drainages around the dome. Buoyant plumes ascended 3-15 km into the atmosphere, wherethey spread out as umbrella clouds. Most umbrella clouds were blown to the north or NW by high-level (8-18 km) winds,whereas the lower, waning plumes were dispersed to the west or NW by low-level (<5 km) winds. Exit velocities measured fromvideos ranged from 40 to 140ms - 1 and ballistic blocks were thrown as far as 1.7 km from the dome. Each explosion dischargedon average 3 x 105m3 of magma, about one-third forming fallout and two-thirds forming pyroclastic flows and surges, andemptied the conduit to a depth of 0.5-2 km or more. Two overlapping components were distinguished in the explosion seismicsignals: a low-frequency (c. 1 Hz) one due to the explosion itself, and a high-frequency (>2Hz) one due to fountain collapse,ballistic impact and pyroclastic flow. In many explosions a delay between the explosion onset and start of the pyroclastic flowsignal (typically 10-20 seconds) recorded the time necessary for ballistics and the collapsing fountain to hit the ground. Theexplosions in August were accompanied by cyclic patterns of seismicity and edifice deformation due to repeated pressurizationof the upper conduit. The angular, tabular forms of many fallout pumices show that they preserve vesicularities and shapesacquired upon fragmentation, and suggest that the explosions were driven by brittle fragmentation of overpressured magmaticfoam with at least 55vol% bubbles present in the upper conduit prior to each event.

Vulcanian explosions are a common feature of andesitic volcanoes(Morrissey & Mastin 2000). Examples include historic eruptions ofArenal (Costa Rica), Ngauruhoe (New Zealand), Fuego (Guata-mala) and Augustine (Alaska) volcanoes (Melson & Saenz 1973;Martin & Rose 1981; Nairn & Self 1978; Kienle & Shaw 1979).Recent examples include explosions at Lascar (Chile) in the period1986-1996 (Matthews et al 1997), Pinatubo (Philippines) in 1991(Hoblitt et al 1996) and Galeras (Columbia) in 1992 (Stix et al1997). Individual Vulcanian explosions typically discharge between102 and 106m3 of magma and comminuted accidental debris incannon-like detonations, generating buoyant plumes mostlybetween 5 and 20km high. Most of the erupted mass is dischargedon a time scale of 10 to 103 seconds. Exit velocities ranging fromabout 50 to at least 300 m s - 1 have been observed and blocks up to2m or more can be thrown ballistically up to several kilometresduring the initial vent-clearing phase (Self et al 1979; Fagents &Wilson 1993). Fallback of part of the discharging material(fountain collapse) generates pyroclastic flows. Successions ofpowerful explosions occur from some volcanoes, with intervalsranging from 102 to 107 seconds (Self et al 1979).

Vulcanian explosions are attributed to the interaction of magmawith external water or to the sudden release of highly pressurized,vesicular magma beneath a cooled or degassed cap (Self et al 1979;Fagents & Wilson 1993; Woods 1995; Stix et al 1997; Sparks 1997).Involvement of external water is invoked when there is direct fieldevidence, or when the gas content implied by models exceeds likelymaximum magmatic values of a few per cent (Self et al 1979;Fagents & Wilson 1993).

This paper concerns two episodes of Vulcanian explosions thattook place in the second half of 1997 at the lava dome of Soufriere

Hills Volcano, Montserrat. Thirteen of these occurred in Augustand 75 in September and October. A remarkable feature was therepeated and regular nature of the explosions, intervals rangingfrom 2.5 to 63 hours with a strong mode at c.10 hours. The activityin August was accompanied by cyclic patterns of edifice deforma-tion and seismic energy release (Voight et al 1998, 1999). Theexplosions generated plumes up to 15km and, in all but one,fountain collapse formed pumice-and-ash pyroclastic flows thattravelled up to 6 km from the vent. The cyclicity of the explosionspermitted accurate short-term forecasting and hazard assessmentover more than a month of intense activity. It also allowed unusu-ally detailed study by a variety of techniques. Explosions werefilmed both during the day and at night, and when possible frommultiple locations. Maximum plume heights were estimated, falloutand pyroclastic flow deposits were mapped and sampled, andmeasurements were made of the sizes and distributions of ballisticblocks. Seismicity before, during and after explosions was recordedby the Montserrat Volcano Observatory (MVO) broadband system(Neuberg et al 1998), and edifice deformation was measured by acombination of electronic distance measurement, global positioningsystem (GPS) and tiltmeter (Voight et al 1998, 1999).

We describe the 1997 explosions and their products and makeestimates of erupted masses, exit velocities, fragmentation pressuresand conduit drawdowns. In some cases only ranges and averagesof the parameters are presented, as practical considerations of timeand safety limited data acquisition for most individual events.We also describe the eruption seismic signals and recognize twocomponents: one related to the explosion itself and the other tofountain collapse, ballistic impact and pyroclastic flow transport.The data and analysis in this paper are complemented by numerical

DRUITT, T. H. & KOKELAAR, B. P. (eds) 2002. The Eruption of Soufriere Hills Volcano, Montserrat, from 1995 to 1999.Geological Society, London, Memoirs, 21, 281-306. 0435-4052/02/S15 © The Geological Society of London 2002. 281

282 T. H. DRUITT ET AL.

modelling of the explosion plumes by Clarke et al. (2002) and of theassociated conduit flow by Melnik & Sparks (2002b). Bonadonnael al. (2002a, b) describe the fallout from the explosions and developmathematical models of tephra dispersal. The pyroclastic flows andtheir deposits are described by Cole et al. (2002).

Two other periods of explosive eruption at Soufriere HillsVolcano have been described by Robertson et al. (1998) and Nortonet al. (2002). During the night of 17/18 September 1996 thereoccurred a 50-minute sub-Plinian eruption that formed a plume atleast 11.3 km high (above sea level), but no fountain-collapse pyro-clastic flows (Robertson et al. 1998). Multiple short-lived (Vulcan-ian) explosions occurred in late 1998 and in 1999, during the periodof virtually no magma extrusion (Norton et al. 2002). These rela-tively weak explosions generated plumes that typically rose toheights of 3-6 km above sea level and fountain-collapse pyroclasticflows that travelled up to 3 km from the lava dome.

We begin by summarizing the main features of the 1995-1999eruptive period and, in particular, events of the period from July toOctober 1997 during which the explosions described in this paperoccurred. Montserrat local time (four hours behind universal time)is used throughout the paper unless noted. Place names are givenon Figure 1. All plume heights are given above sea level.

The eruption of Soufriere Hills Volcano from 1995 to 1999

Soufriere Hills Volcano is an andesitic lava dome complex situatedin southern Montserrat, in the Lesser Antilles island arc. Detailedoverviews of the 1995-1999 eruptive period have been given byYoung et al. (1997, 1998), Kokelaar (2002) and Sparks & Young(2002). The eruptive vent was situated in an ancient sector-collapsescar (English's Crater) about 1 km across and open to the east(Fig. 1). The western rim of English's Crater is called Gages Walland the southern rim is called Galway's Wall (Fig. 1). The flanks ofSoufriere Hills are scarred by radial valleys (locally called ghauts),which served to channel pyroclastic flows.

Initial phreatic explosions began in July 1995. Magma reachedthe surface in November 1995, and a lava dome began to form. Thefirst dome-collapse pyroclastic flows occurred in March 1996,and flows first reached the sea down the Tar River valley in Mayof the same year. Major dome collapses occurred in July andAugust, 1996, and on 17 September 1996 a major collapse of thedome was followed by an explosive eruption (Robertson et al. 1998).Dome collapses and associated pyroclastic flows continued through-out 1997, with particularly large ones occurring on 25 June,3 August, 21 September, 4 November and 6 November (Cole et al.

Fig. 1. Map of southern Montserrat, showing the dome location inside English's Crater, the principal drainages (ghauts) around the dome, and the area ofimpact from pyroclastic surges and flows of the entire 1995-1999 phase of the eruption.

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 283

2002). The two episodes of Vulcanian explosions reported in thispaper followed the collapses of 3 August and 21 September 1997.On 26 December 1997, Galway's Wall of English's Crater failed,sending 80-90 x 106 m3 of the wall, lava dome and dome talus downthe White River valley as a debris avalanche and high-energy block-and-ash flows (Sparks et al. 2002). A pyroclastic density currentgenerated by decompression of gases trapped in the dome interiordevastated 10 km2 of southern Montserrat. Magma extrusion ceasedin March 1998, then resumed again in November 1999.

The total volume of magma discharged over the 28 months ofdome formation was 0.3 km3 dense-rock equivalent (DRE). Overallmagma flux and the sizes of gravitational collapses increased duringthe period, but with some significant fluctuations. Discharge ratesduring the first year of extrusion were mostly less than 2 m3 s - 1 , butby June 1997 had risen to more than 7-8 m3 s-1 (Sparks et al. 1998;Sparks & Young 2002).

The 1995-1999 magma is a crystal-rich andesite containingphenocrysts of plagioclase, hornblende, orthopyroxene, quartz, andFe-Ti oxides set in a groundmass of microphenocrysts, microlitesand rhyolitic glass (Murphy et al. 2000). It is believed to haveformed by heating and remobilization of a pre-existing crystal-rich(60-65 vol%) mush. The pre-eruptive liquid phase of the magmawas saturated with 4.3±0.5wt% water, as determined from glassinclusion analysis and phase equilibria studies (Barclay et al. 1998;Devine et al. 1998#). This corresponds to a water-saturated magmareservoir depth of 5 to 6 km beneath the vent, which is consistentwith seismic evidence (Aspinall et al. 1998).

Dome growth was accompanied by cyclic patterns of grounddeformation and seismicity with periodicities of 3 to 30 hours andattributed to non-linear processes of gas exsolution, crystallization,rheological stiffening and pressurization in the conduit beneath thelava dome (Voight et al. 1998, 1999). Ground deformation wasmeasured by tiltmeters installed near the rim of English's Crater.During a typical tilt cycle there was a slow inflation of the edifice(5-30urad), followed by abrupt deflation and associated gasemission, ash-venting, dome collapse or explosion. Associatedseismic swarms built up during inflation, then declined duringdeflation. Seismicity included volcanotectonic and long-periodearthquakes, rockfall, pyroclastic flow and explosion signals, andtremor. Most seismic signals forming the swarms were of hybridtype, which combined the high frequencies of volcanotectonicearthquakes with low-frequency components (Miller et al. 1998).

Two observatory stations were active during the explosive periodsof 1997. Prior to early September, the MVO was sited 6km NW ofthe dome (MVO South, Fig. 1), whereas from then onwards it waslocated in northern Montserrat (MVO North, not shown on Fig. 1).

Eruptive chronology from July to October 1997

Buildup to the August explosions

The scar left by the 4.9 x 106m3 (DRE) 25 June dome collapse(Loughlin et al. 2002) began to fill in rapidly during the last week ofJune, about 65% of the void having been filled by 1 July. Between28 June and 5 July there was intense pyroclastic flow activity.Multiple block-and-ash flows were shed up to 3.5km down Mos-quito and Fort Ghauts, up to 1.1 km down Tuitt's Ghaut, and up to0.5km down the White River valley. Many of these started with aresounding boom, and a vertical ash column ascended to an altitudeof more than 10km.

A period of intense ash emissions began on 8 July and persistedthrough 13 July. Peaks in ash emission often coincided with thepeaks of tilt cycles. Some preceded small block-and-ash flows intoMosquito Ghaut and Gages valley; small ash columns reachedheights of no more than 3km before dissipating. By 17 July thehighest point on the new dome growth nested in the 25 June scarhad reached 957m above sea level. The total volume of the domeon 17 July was estimated to be 75 x 106 m3. The level of seismic anderuptive activity in late July was generally low. The 25 June scar

had filled in, and the dome had a more or less flat summit 960mabove sea level. Activity was characterized by low-amplitudebroadband tremor associated with multiple rockfalls and smallblock-and-ash flows, particularly over Gages Wall.

The August explosions

In retrospect, tilt and hybrid cycles related to the impending explo-sive activity began on 31 July. Pale, weakly convecting plumes of ashrose almost continuously to heights of between 4.5 and 6km, andsmall block-and-ash flows travelled as far as 2 km down Gages valleyand Tuitt's Ghaut. High levels of long-period and hybrid seismicitycontinued on 1 August, peaking at one event per minute at the top ofthe tilt cycle, and a number of detonations were heard coming fromthe volcano. At least one correlated with an impulsive signalrecorded on the broadband seismometers and explosive activity wassurmised. A view of the dome on the same day revealed a smallhorseshoe-shaped depression in its west side above Gages Wall.

Activity over the next three days was characterized by furthertilt cycles and hybrid swarms every 9-12 hours. Associated ventingbecame increasingly explosive and the block-and-ash flows morevoluminous. The dome was observed at 14:00 on 3 August, when anactive face of large blocks and spines was present high above GagesWall. From 18:00 to 20:30, at the peak of the afternoon tilt cycle on3 August, a succession of block-and-ash flows travelled downGages valley. A boom heard at 16:28 at MVO South may have beena small explosion of the dome or of a gas tank ignited by the flows.Then at 18:10 a 7.0 x 106m3 (DRE) block-and-ash flow descendedthe length of Fort Ghaut to the sea, causing extensive damage inPlymouth.

The first clear explosive activity occurred on 4 August. A block-and-ash flow at 06:30 was accompanied by a loud rumbling,followed by fallout of lithic and crystal lapilli up to 5mm indiameter at MVO South. A second explosion at 16:43, following ahybrid swarm, sent a dark grey jet inclined at about 60° to thehorizontal northwards from the dome. Large blocks were observedto follow ballistic trajectories. Moments later, block-and-ash flowsswept 3.5km down Tuitt's Ghaut, 3.5km down the Tar Rivervalley to the sea, 4km down Fort Ghaut to the sea, and anunknown distance down the White River valley. The resulting ashplume rose to 4.5km, and fragments of dome rock and densepumice as large as 15 mm fell at MVO South.

Between the morning of 5 August and the morning of 12 Augustanother 11 explosions occurred. They are listed in Table 1. Eachgenerated pumice-and-ash pyroclastic flows by fountain collapseand showered the island with pumice-rich fallout. The first nineexplosions occurred regularly every 10 to 12 hours during orimmediately after hybrid earthquake swarms. The last two occurredon 11 and 12 August, and were slightly weaker than the others.

The August explosions occurred from a circular crater exca-vated in the summit of the dome. Observations of crater develop-ment were hampered by cloud cover, but the following sequence isdeduced. Rockfalls over Gages Wall at the end of July and on 1 and2 August generated a summit crater with a low lip to the west.Significant enlargement took place on 3 August during the 18:10collapse. Further enlargement of the crater took place during theexplosions of 4 August, which discharged large quantities of densedome rock as well as pumice. By midday on 5 August (after theexplosion of 04:45, but before that of 16:57; Table 1), there existed acircular, funnel-shaped crater at the top of the dome with a rimdiameter of 300 ± 20 m. The crater was seen again clearly at 07:30 on7 August, when the highest point on the northern rim lay at 940 mabove sea level, and that on the southern rim at 980m. There wasalso a low lip in the crater wall (870 m) above Gages valley, show-ing that the crater was at least 110m deep. The crater persistedthrough the explosion of the morning of 8 August, and was seenfrom MVO South at 17:00 on 9 August. When seen again at middayon 10 August, a small new lobe of lava nested within the crater wasvisible above the western crater lip. Following the final explosion on12 August, lava continued to be extruded within the crater.

284 T. H. DRUITT ET AL.

Table 1. Characteristics of the Vulcanian explosions of August, September and October, 1997

Date

4 Aug.4 Aug.5 Aug.5 Aug.6 Aug.6 Aug.7 Aug.7 Aug.7 Aug.8 Aug.8 Aug.

1 1 Aug.12 Aug.

22 Sep.22 Sep.22 Sep.23 Sep.24 Sep.24 Sep.24 Sep.25 Sep.25 Sep.25 Sep.26 Sep.26 Sep.27 Sep.27 Sep.27 Sep.28 Sep.28 Sep.28 Sep.29 Sep.29 Sep.29 Sep.29 Sep.30 Sep.30 Sep.

1 Oct.1 Oct.1 Oct.2 Oct.2 Oct.2 Oct.4 Oct.

Localtime*

06:3016:4304:4516:5704:0214:3600:3412:0521:5510:3220:5111:3810:12

00:5710:4520:4207:2300:3410:5417:1603:5411:0920:0504:2514:5600:0109:4617:1504:2810:3423:0306:2611:2316:4821:5704:4317:4405:0011:3417:4001:0512:5322:5008:33

Intervalf(hrmin)

10:1312:0212:1211:0510:349:58

11:319:50

12:3710:1962:4722:35

9:489:57

10:4117:1110:206:22

10:387:158:568:20

10:319:059:457:29

11:136:06

12:297:234:575:255:096:46

13:0111:166:346:067:25

11:489:57

33:43

Plumeheight!(km)

3.04.66.1-10.79.1

9.1-12.211.0-12.2

12.2-13.7§9.89.1-10.7

12.211.0

>9.110.7-12.29.1-10.7

10.7-12.2~7.612.2

12.212.27.6

12.2§12.2

<15.2§13.713.7

4.6-7.610.7-12.2

>12.212.2-13.79.1-13.7

<12.2<<10.7

~4.6

12.29.1

11.0

Pyroclastic

Ta W

X X

X O

X 0

X 0

X 0

X 0

X X

0 0

X

X

X

0

0

0

O

O

X

X

X

X

X

X

X

Tu

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

flows

M Ty

X X

X X

X 0

X X

X X

X X

X

X

X

X

X

X

X X

X X

x

X

0

X

X X

X

X

X

X

G

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

O

X

X

X

X

X

X

X

X

Wr

X

X

X

X

X

X

X

0

0

0

0

0

0

0

0

O

0

X

X

X

X

Date

4 Oct.5 Oct.5 Oct.5 Oct.6 Oct.6 Oct.6 Oct.7 Oct.7 Oct.8 Oct.8 Oct.9 Oct.9 Oct.

10 Oct.10 Oct.1 1 Oct.12 Oct.12 Oct.13 Oct.13 Oct.14 Oct.14 Oct.14 Oct.15 Oct.15 Oct.15 Oct.15 Oct.16 Oct.16 Oct.16 Oct.16 Oct.16 Oct.17 Oct.17 Oct.17 Oct.17 Oct.18 Oct.18 Oct.19 Oct.19 Oct.20 Oct.20 Oct.21 Oct.21 Oct.

Localtime*

18:2702:5310:4118:4102:4410:4217:5004:0616:0203:4715:1003:0312:3204:1318:4017:5707:5522:2409:3215:2401:3613:4823:1605:4708:3314:5022:2002:5106:3509:4414:2018:4804:0112:3516:0523:1806:4815:1705:1321:2705:0415:1311:3919:02

Interval!(hr:min)(km)

9:548:267:488:008:037:587:08

10:1611:5611:4511:2311:539:29

15:4114:2723:1713:5814:2911:085:52

10:1212:129:286:312:466:177:304:313:443:094:364:289:138:343:307:137:308:29

13:5616:147:37

10:0920:26

7:23

Plumeheight;

12.2-13.77.6-9.17.6-9.15.36.97.6

<12.2>9.813.7

<9.1-10.7~12.2

10.112.212.59.46.1-7.6

>4.6

3.1-7.64.6-6.1

>4.6>3.7

3.7-4.64.63.1-4.66.17.67.6-9.16.79.14.6-7.6

12.2<9.1

5.5-9.17.69.1-10.7

>9.110.7

Pyroclastic

Ta W Tu

x

xX

X X

X X

xX X

X X X

X X X

X X X

X X

X X X

X X

xxxx

O O O

O O O

X

X

O O O

xx

xX

X

X X X

X X

X

X X

flows

M Ty

x

X X

xX

O

X

X

X

x

X

0 0

O O

x

0 0

x

xx

X

G Wr

x

X X

X X

X

X X

xX X

X

X

X

xxxxX X

X X

X

O

O

X X

O O

X

X

X

X X

X X

x

* Local time is 4 hours behind universal time.fTime interval since the previous explosion.| Best estimates of maximum plume heights (above sea level) based on National Oceanic and Atmospheric Administration (NOAA) data, supplemented byMontserrat and Trinidad airport reports and by Abney-level measurements of the plume top made from the MVO. A blank denotes lack of data.§ Additional height estimates were obtained using plume-top temperatures on GOES satellite images (Bonadonna el al. 2002b) for three explosions: 12:05,7 Aug. (9.3km); 14.56, 26 Sep. (11.3km); 09:46, 27 Sep. (10.8km).f Presence (x) or absence (o) of fountain-collapse pumice-and-ash flows. Ta, Tar River valley; W, White's Ghaut; Tu. Tuitfs Ghaut; M. Mosquito Ghaut;Ty, Tyre's Ghaut; G, Gages valley; Wr, White River valley. A blank denotes lack of observations.

Following the initiation of Vulcanian activity in early August,major changes were made to the volcanic hazards map of theisland, resulting in an enlargement of the evacuated zone andnorthward displacement of inhabitants under threat from this newstyle of activity (Kokelaar 2002).

Renewed dome growth after the August explosions

The new dome lobe in the summit crater developed a large spine,the elevation of which was 950m by 13 August. By 14 August thecrater was nearly filled in, and by 19 August the entire dome had

reached its pre-3-August volume, growth being concentrated in anarea about 150m wide above Gages valley. Initially during thisperiod the seismic activity continued to be characterized by intensehybrid earthquake swarms occurring at approximately 8-hourintervals. The intense hybrid activity that characterized the Augustexplosive period continued until 19 August, with block-and-ashflows occurring mainly in the upper Gages valley from the growingdome, immediately after peaks in earthquake activity.

Throughout late August and early September, rockfall andpyroclastic flow signals dominated the seismic records, with activityconfined to the western and northern flanks of the dome. A smalldome collapse (about 1 x 106m3) took place on 30 August. A slow

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 285

increase in block-and-ash flow activity occurred through mid-September, with material being shed across Farrell's Plain and intoTuitt's Ghaut. Rapid dome growth was occurring at this time, withthe unstable active face located above the northern wall of thecrater. Earthquake activity remained at a low level, with occasionalswarms of hybrid earthquakes, some of which included the largestindividual events recorded by the broadband network up to thattime. Heightened long-period earthquake activity was also notable,with events often occurring immediately before pyroclastic flowgeneration. At least one long-period event, on 16 September,correlated with an audible detonation from the dome.

By 28 August, the dome volume was more than 80 x 106 m3 andthe total volume of magma erupted to form both lava andpyroclastic deposits was 160 x 106m3 DRE.

Dome collapse of 21 September

Hybrid earthquake swarms recommenced 24 hours prior to thedome collapse of 03:54 on 21 September and continued after thecollapse at the same level. The swarms were neither long nor intensecompared with previous ones; individual events were not large andneither were there visual signs of imminent collapse.

The onset of increased seismic amplitude related to this collapseis timed at 03:54, although the first 8 minutes of activity was not at ahigh level, probably registering small dome collapses with pyro-clastic flows in the upper and middle parts of Tuitt's Ghaut. A sharpincrease in signal amplitude at 04:02 marked the start of sustainedhigh-amplitude signals on all stations, typical of those generatedduring major dome collapse. Minor pulsing occurred throughoutthis phase, which lasted until 04:17, a total of 15 minutes. Withinthis phase, the highest amplitude signals were recorded between04:11:1 and 04:13:7; the signal amplitude at this time was markedlyhigher than at any other time, peaking at 04:12:1. The signal haddropped to background level by 04:24, giving a total duration forthe dome collapse of 30 minutes.

During the collapse, which involved 11.0 x 106m3 (DRE) of thedome, block-and-ash flows moved down Tuitt's Ghaut and White'sGhaut to the ocean, spreading out over the area of Spanish Point(Cole et al. 2002). Associated pyroclastic surges covered interfluvesbetween the ghauts, causing the burning of Tuitt's and parts ofother villages not overrun by the flows. The ash plume associatedwith the block-and-ash flows reached an altitude of 9-12 km,causing ash fall over much of Montserrat.

The post-collapse dome had a deep scallop-shaped scar onits northern flank, extending back about 300m. There was a

prominent opening and chute above the head of Tuitt's Ghaut,eroded by pyroclastic flows as dome collapse proceeded.

The September and October explosions

The first explosion of this episode occurred about 20 hours later at00:55 on 22 September and the last at 19:02 on 21 October. A totalof 75 explosions occurred over a 30-day period (Table 1). Intervalsvaried between 2.5 and 33.5 hours, with an average of 9.5 hours.No systematic variation of plume height with time occurred from22 September until 11 October, when there occurred a series of atleast ten relatively weak explosions with plume heights of 7 km orless over a period of five days (Table 1). Gaps in the plume-heightrecord at this time mean, however, that it cannot be excluded thatlarger explosions also occurred. After 16 October, plume heightsincreased again until the end of the explosive episode on 21 October.

The first explosion produced a crater at the southern edge of the21 September collapse scar. The shape and size of this craterchanged only gradually thereafter, becoming larger in diameter andprobably deeper. The volume of the dome at this point was68 x 106m3. An estimate of the volume of the crater excavatedduring the first few explosions was 2 x 106m3 (Fig. 2). Successiveexplosions deposited a tephra rampart on the northern side of thecrater, effectively completing the near-circular crater wall across the21 September collapse scar. Minor reaming of the crater wallsoccurred throughout the explosion sequence, but good views intothe crater were scarce so that accurate estimates of the volumeincreases were impossible to obtain.

Within a day of the cessation of explosions on 21 October, alava lobe was seen growing within the crater. Within a few days,lava had filled the crater and overspilled the tephra rampart, and by3 November had largely filled the entire 21 September collapse scar.Observations also suggest that a small lobe started to grow in thebase of the explosion crater during the longest break betweenexplosions (2 to 4 October; Table 1).

The Vulcanian explosions

The characteristics of the explosions are listed in Table 1. Of the 88explosions, 37 occurred during hours of darkness and about 30under poor weather conditions. About 20 explosions were wellobserved and documented, five of which were at night. The follow-ing descriptions are based on field observations and on ongoing

Fig. 2. The crater formed early during theexplosion sequence of September andOctober 1997. The photo is taken from theNE. The crater has a rim diameter of about300 m and opens into Tuitt's Ghaut.

286 T. H. DRUITT ET AL.

Fig. 3. Sequence of events during a typical Vulcanian explosion in 1997.

analysis of video footage. To a first approximation, the explosionsand their products were similar and they are described together.The main features of a typical explosion are shown schematically inFigure 3. Sequences of photographs of three typical explosions areshown in Figures 4, 5 and 6.

General description of the explosions

Visual precursors to the explosions were rare, although on severaloccasions increases in fumarolic activity around the dome werenoted in the preceding seconds to minutes. Each explosion con-sisted of two fairly well defined phases: (1) an initial, high-intensityphase lasting about 10 minutes and including the main explosionand peak magma discharge rates (a few tens of seconds), foun-tain collapse, formation of a buoyant eruption plume, and theascent of the plume to its neutral buoyancy level in the atmosphere;(2) a drawn-out, much lower-intensity waning phase lasting a fewtens of minutes (typically 1 to 3 hours) and characterized by rela-tively weak, pulsatory venting of gas and ash. Fountain collapsewas limited to the first 10-20 seconds of each explosion.

Each explosion began with the rapid rise of numerous dark-greyfinger jets of ash and debris (Fig. 3a). Condensation of atmosphericmoisture ahead of the jets, indicative of shock waves, was observed

in some explosions (Clarke el al. 2002). There then followed a loudand steady roaring, like an aircraft, punctuated by further detona-tions as subsequent jets were discharged. The initial jets wererelatively weak, but subsequent ones became progressively morevigorous with time over the first few seconds of the explosion. As thejets rose, they decelerated rapidly. Decimetre- to metre-sized ballisticblocks detached from the leading edges of many jets and werethrown outwards in high, curving arcs as far as 1.7km from thedome. Impact of the blocks with the ground kicked up clouds of ashvisible from a distance (Fig. 6). The finger jets, having reached theirmaximum elevation, then collapsed back towards the ground.Simultaneously, part of the material that had ingested enough airbegan to rise as one or more buoyant plumes, which then mergedinto a single large plume. Viewed from a distance, the collapsingfountain took the form of a hemispherical cap, through which thebuoyant plume then pierced (Fig. 3b). The fountain-collapse heightwas typically between 300 and 650 m above the crater rim, although,in some explosions, later jets remained momentum-driven up tomore than 1000 m. Impact of the collapsing material with the groundgenerated highly expanded pyroclastic surges, which swept outfrom the volcano with frontal velocities of 30-60 m s - 1 (Figs 3c, 4, 5and 6). On some occasions, the surges moved almost as quicklyhorizontally as the central plume was ascending vertically, so thatthe explosion cloud appeared to expand equally in all directions.The surges decelerated rapidly 1-2 km from the dome, then lifted

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 287

Fig. 4. Photographs of the explosion of12:05 on 7 August 1997. The plume fromthis explosion ultimately rose to about13 km. Times after the onset of theseismic signal: (a) 27 s, (b) 36 s, (c) 42 s,(d) 48s, (e) 58s, (f) 76s. The collapsingfountain is visible to the left in (a) and tothe right in (b) and (c). Fountain collapsegenerated pyroclastic surges and flows inall the major drainages around thevolcano. The three individual plumesshown in Fig. 15 and discussed in the textare numbered 1 to 3; g, Gages Mountain;sg, St George's Hill. For scale, the top ofthe plume in (e) is about 3 km above sealevel and 2 km above the lava dome.Photographs taken from NW of thevolcano by K. West.

off the ground to form buoyant ash plumes (Fig. 3d). Shortly there-after, highly concentrated pumice-and-ash pyroclastic flows wereobserved advancing at about 10ms - 1 down one or more valleysaround the dome as thin (0.5-1 m), granular avalanches with asso-ciated billowing clouds of elutriated ash. It is surmised that the pyro-clastic flows formed by rapid fallout of debris from the collapsingfountain and initial pyroclastic surges. In most of the explosions,pyroclastic flows occurred in all major ghauts around the dome.Some explosions in September and October were angled to thenorth, probably because the horseshoe-shaped dome summit crateropened in that direction (Fig. 2). Runout distances varied betweenexplosions, but were typically 3-6 km, with the flows taking up toa few hundred seconds to reach their distal limits. Only one explo-

sion (06:35 on 16 October) did not generate pyroclastic flows, andanother generated only very small ones (05:47 on 15 October).

Fountain collapse was clearly visible at night. First, a brightlyincandescent cloud was seen rising over the dome. Moments later, aring of coarse, incandescent debris fell back from height along steep,outwardly inclined trajectories onto the slopes surrounding thedome. Fountain collapse was short-lived, no more than 10 to20 seconds. Incandescence in the initial pyroclastic surges dis-appeared rapidly over a few seconds as the surges entrained airand cooled.

The central explosion plumes had fast-rising bulbous heads andnarrow central stems (Figs 4, 5 and 6) and reached heights of3-15 km, with an average of about 10 km (Figs 7 and 8). The height

288 T. H. DRUITT ET AL.

Fig. 4. (continued)

estimates (probably ±10%) were supplied by the US NationalOceanic and Atmospheric Administration (NOAA) Satellite Anal-ysis Branch in Washington DC, based on ash-cloud movements tiedto radiosonde wind data from Puerto Rico and Guadaloupe. Addi-tional ground-based height estimates made on Montserrat using anAbney level were in broad agreement with the NOAA heights. Forthree explosions, height estimates were obtained using plume-toptemperatures from GOES satellite images (Bonadonna et al. 2002b;Table 1). Upon attaining their maximum altitude, which typicallytook about 10 minutes, the plume heads spread out to form umbrel-la clouds (Fig. 9), which then detached from their stems and werecarried to the north or NW by high-level (8-18 km) winds. Satelliteimages of one such cloud (14:56 on 26 September) are presented by

Bonadonna et al (2002/?). Some plumes were richer in steam andthus paler in colour than others and tended not to rise as high. Theweaker plumes also had more poorly developed umbrellas, andwere more liable to be blown off-centre by the wind. Ash cloudsgenerated by the lofting of the pyroclastic surges and by elutriationfrom pyroclastic flows were gradually drawn up and incorporatedinto the central plume by inward-moving currents of air (Fig. 3d).

After several minutes, each explosion settled into a phase ofwaning, relatively low-intensity discharge, generating a low, bent-over plume transported mainly to the west or NW on low-level tradewinds (below about 5 km). This decoupling of the high umbrella(north or NE) and low, waning plume (west or NW) was charac-teristic of many of the explosions. The waning plume then

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 289

Fig. 4. (continued)

decreased slowly in height over a period of 1-3 hours. Ventingduring the waning phase was often pulsatory on a time-scale of1-2 minutes.

Lightning occurred during many of the explosions, and wasparticularly evident at night. It appeared in the eruption columna few seconds after the initial explosion and continued for up to10 minutes. Cloud-to-cloud strikes accompanied by thunderclapswere most common.

Despite the presence of condensed steam in some of the plumes,it is not believed that external water played any significant role in theexplosions. The groundwater system of Soufriere Hills Volcano hadessentially dried out by mid-1997 after 20 months of lava extrusion,and there was no evidence of a significant source of groundwater.

The explosion of 12:05 on 7 August, 1997

This explosion was studied in particular detail from video footage.The events and their timing are listed in Table 2, which is exploitedlater to compare with calculations. The explosion was filmed by atime-lapse video camera mounted at MVO South (frame interval1.4 s), two ordinary video cameras at MVO South and at Fleming,and an ordinary video camera aboard the MVO helicopter. Stillphotographs were taken roughly every 2 s from MVO South until45 s into the explosion. Another set of images taken by K. West isshown in Figure 4. Correlation of all image sets permitted detailedreconstruction of the explosion. The timer on the Fleming videocamera had been previously correlated with GPS time to ±0.5s

Fig. 5. Explosion at 14.02 on 6 August 1997, showing the typical development of the buoyant plume, which rose to between 9 and 12 km above sea level. Partialfountain collapse during the initial stages of the explosion sent pyroclastic flows and surges down valleys to the north (left) and west (right). either side of GagesMountain (g). Photographs taken from MVO South by T. H. Druitt.

Fig. 6. Explosion at 15:13 on 20 October 1997. Fountain collapse generated pyroclastic surges and flows visible to the west (right) and north (left) of Gages Mountain (g). Ash was thrown up by the ground impact ofballistic blocks (b). The buoyant plume ultimately rose to about 10km. Note the buildings for scale in the foreground. Photographs taken from the NW by P. Cole.

292 T. H. DRUITT ET AL.

Fig. 7. Histograms of plume height andexplosion repeat interval for the 1997Vulcanian explosions.

Fig. 8. Variations of (a) plumeheight and (b) explosion repeatinterval with time during theepisode from 22 September to21 October 1997. The repeatinterval is that which precededthe explosion concerned,which is why none is reportedfor the first explosion. Thehorizontal axis shows theexplosion number during thisperiod (see Table 1). Anabsence of bars indicates alack of data. In mid-Octoberthere occurred a series ofrelatively weak explosionswith short repeat intervals.

allowing correlation of images with the broadband seismic signalmeasured by the Galway's Estate seismometer (Fig. 10).

Time zero in Table 2 is taken as the onset of the seismic signal (asreceived at the Galway's Estate seismometer) at 12:04:44. Emer-

gence of the first jet above the crater rim, as estimated by back-extrapolation of height-time curves (see below), followed 1 s later.Given that the velocity of long-period seismic waves at Montserratis 1800-1900ms-1 (Neuberg et al. 1998). the travel time for the

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 293

Fig. 9. (a, b) Development of the umbrella cloud during a typical explosion (09:46 on 27 September 1997). The plume height in (b) is about 10.8 km above sealevel. Photograph taken from a boat NE of Montserrat by B. Poyer. The boat was retreating from Montserrat, so that (a) was taken from closer than (b).

seismic signal from the dome to the seismometer was about 1.5 s.About 2.5 s therefore separated the onset of the seismic signal andemergence of the first jet above the crater rim. Given the verticalexit velocity of this jet (80 ± 10ms - 1 , see below), this suggests adepth of about 200m for the summit crater at the time of theexplosion, which is compatible with the observed crater diameter(300 ± 20 m) and a typical angle of rock stability.

The explosion generated at least three separate plumes (num-bered 1 to 3) visible from MVO South and Fleming that merged,after about 100s, into a single, large plume (Fig. 4). There mayhave been other plumes not visible from these observation points.Plume 1 emerged first over the southern or southwestern flank of thedome, rapidly followed by plume 2 to the north (Fig. 4a). Eachdecelerated as it rose, part collapsing back to the ground, and partascending buoyantly. The fallback height of the two plumes wasestimated visually as a few hundred metres above the crater rim andthe collapsing material first hit the ground behind Gages Mountainabout 18s into the explosion (Table 2). The resulting pyroclasticsurge was first visible behind Gages Mountain at 22.8 s. As this surgetravelled out from the dome, a thin veil of ash was thrown up allover Gages Mountain, either due to seismic shaking or to a blast ofpreceding air. The surge then ramped over the north face of GagesMountain before decelerating abruptly, ceasing forward motion,and lifting buoyantly off the ground at about 45 s (Fig. 4b, c and d).Collapse over the northern flanks lagged a few seconds behind thatover Gages. The northern fallback curtain was first observed at19.1 s and it hit the ground three seconds later (Fig. 4a). The result-ing surge was first visible at 27.8 s advancing at about 45 m s-1 downthe headwaters of Mosquito Ghaut.

Large blocks thrown northwards ahead of plume 2 followedballistic trajectories. The first blocks were seen to hit the ground at21.6s, throwing up ash. Impacts then migrated northwards awayfrom the dome, reaching the maximum range of 1.6 km about 5 slater (26.9 s). At 27.9 s, as plumes 1 and 2 became buoyant, plume 3broke out at high speed at the top of the column (Fig. 4b). It rapidlydecelerated and began to rise buoyantly, before separating into two(Fig. 4c and d). Plume 3 remained momentum-driven up to 1200mabove the crater rim.

After a few tens of seconds, thin pumice-and-ash pyroclasticflows were observed travelling slowly down Tuitt's Ghaut, MosquitoGhaut and the Tar River valley, reaching their maximum limits of3-6 km about 200 s after the onset of the explosion. The centralplume rose to between 12.2 and 13.7km according to NOAA data,forming a large umbrella. The waning phase of the explosion lastedan hour.

Explosion products

Fallout tephra from the explosions had three sources (Bonadonnaet al. 2002b). (1) Pumiceous (and minor lithic) blocks, lapilli andash from the main central plumes and umbrella clouds. Pumiceclasts as large as 10cm in mean diameter fell on St George's Hill,6.5cm on the South Soufriere Hills, 6.5cm on northern Plymouth,4.5cm on Windy Hill and at Cork Hill, 4cm at MVO South and2 cm in northern Montserrat. (2) Ash from plumes generated by thelofting of pyroclastic surges and by elutriation from pyroclasticflows (termed co-pyroclastic-flow plumes). This was mostly drawn

294 T. H. DRUITT ET AL.

Table 2. The 12:05 explosion of 7 August 1997

Time (s) Event

0 Start of explosion seismic signal (phase 1)*1 Emergence of explosion jet 1 at 95 ± 10ms - 1

7 Emergence of explosion jet 2 at 95 ± 10 m s - 1

17 Emergence of explosion jet 3 at > 130ms - 1

17.4 Fallout visible behind Gages Mountain from MVO South18 Start of seismic signal from fountain collapse and pyroclastic

flows (phase 2)19.1 Fallout curtain descending over the north flank21.6 First ballistics hit Farrell's Plain, 1.2km north of the vent22.0 First ballistics hit Paradise Plain, 1.2km north of the vent22.2 Collapsing fountain hits the north flank22.8 Pyroclastic surge visible behind Gages Mountain26.9 Ballistics reach maximum range on Paradise Plain, 1.6km

north of the vent27.8 Pyroclastic surge in Mosquito Ghaut, 1.7km from source,

travelling at c. 4 5 m s - 1

27.9 Jet 3 arrives at the top of the plume34.3 Pyroclastic surge passes Gages soufriere on the west flank45 Pyroclastic surge ramps over Gage's Mountain and lofts58 Pyroclastic surge reaches maximum runout on the Farrell's

Plain and begins to loft70 Drop in intensity of the phase 2 seismic signal108 Pyroclastic flows reach the foot of St George's Hill on the

west flank167 Pyroclastic flow reaches the Paradise River, 3.5km from

source, at 10ms - 1

187 Pyroclastic flow level with Harris, 3.4km from source, at9ms-1

202 Pyroclastic flow reaches sea on Tar River delta, 3.3 km fromsource, at 13-25ms-1

300 End of pyroclastic flow seismic signal; continuing tremor (phase 3)c. 3600 End of the explosive eruption

*The seismic signal was measured at the Galway's Estate station (Fig. 10).The time for seismic waves to reach this station from the dome was about1.5s, so emergence of jet 1 occurred about 2.5s after the onset of theexplosion seismic activity.

well defined levees and snouts rich in relatively low-density pumiceboulders. Temperature measurements made approximately 2 hoursafter flow emplacement ranged from 180 to 220°C. This is consis-tent with the night video footage of the explosions, which showsthat the discharging material lost heat very rapidly during foun-tain collapse, presumably by entrainment of air, as also seen innumerical models of the explosions (Clarke et al. 2002).

Vesicularities of clasts (>3cm) from the explosion depositswere calculated from density measurements made by the water-immersion technique. Vesicularities of pumice clasts from bothfallout and pyroclastic flows (from all but the first three explosionsin August) ranged from 55 to 75vol% (26 clasts). In contrast,ballistic blocks discharged during the initial vent-clearing phase ofeach explosion were mostly dense, with Vesicularities much lowerthan 55%. Dense clasts also occurred in the pyroclastic flows, butonly up to 5% by volume (Cole et al. 2002; Clarke et al 2002). Theseare interpreted as fragments of crater rubble, degassed magmaplugging the conduit prior to each explosion, or material picked upfrom the ground. No systematic differences in pumice vesicularitywere observed between different explosions, with the exception ofthe first three of the August series, in which the Vesicularities of fall-out clasts ranged from 0 to 75 vol%. The ejection of importantquantities of dense andesite as fallout could be due to reaming outof the August summit crater by these early explosions. It is notknown if a similar process took place in September.

A notable feature of the fallout pumices was the high abun-dance of tabular clasts with planar or curviplanar surfaces and sharpedges (Fig. 12). Many pumices were lenticular in cross-section, withone flat face and a convex face on the other side. Between 20 and50% of fallout pumices a few centimetres in diameter had tabularshapes. Many had been slightly deformed and rounded by minorpost-fragmentation inflation, showing that their angular shape wasnot due to ground impact. Neither were they associated on theground with other pieces of the same block, as expected if impactbreakage had occurred. The shapes of the clasts are attributed tobrittle fragmentation of an already vesicular magmatic foam withat least 55 vol% bubbles. In contrast, pumice blocks in pyroclasticflows from the explosions were typically rounded and subequantdue to abrasion during fountain collapse and transport.

up and mixed into the central plumes; however, where observedseparately, fallout from this source was often sporadic due togeneration from surges and flows in different sectors around thevolcano. (3) Ash from the low, waning plume.

Except within a couple of kilometres from the lava dome, thetotal fallout from individual explosions seldom exceeded just ascattering of clasts or a layer of ash no more than a few millimetresthick. Fallout distributions were complicated by mixing of materialfrom the three sources and by variable wind directions, windintensities and plume heights. Western Montserrat was affected byfallout from all three sources, whereas fallout from the umbrellasdominated in the north.

A map of the areas impacted by the fountain-collapse pyro-clastic flows and surges is given in Figure 10. Pyroclastic flowstravelled down the Tar River and White River valleys as far as thesea, and down Fort Ghaut to within a few hundred metres of theshoreline. Pyroclastic flows discharged to the north by the Augustexplosions travelled down Tuitt's and Mosquito Ghauts, then on asfar as the village of Farm (6 km). Those in September and Octoberdid the same, but also entered White's Ghaut and spread out overthe surface of the 21 September dome-collapse block-and-ash flowdeposit (Cole et al. 2002), locally reaching the sea. Some pyroclasticflows in September and October were channelled preferentiallydown Tuitt's Ghaut because the explosions were angled to thenorth. Many explosions sent flows down Tyre's Ghaut, then intoDyer's River valley.

The deposits from the pyroclastic flows are described by Coleet al. (2002). They consisted of numerous anastomosing lobes withmultiple breakouts (Fig. 11). The distal ends of individual lobeswere typically 10-50m wide and 0.5-1 m thick (Fig. 11c,d), with

Cyclic patterns of edifice deformation, seismicityand explosion

Cycles of edifice deformation and hybrid seismicity were closelyassociated with the August explosive activity (Voight et al. 1998,1999). A typical cycle consisted of slow inflation of the domefollowed by rapid deflation and explosion, giving a saw-toothpattern on tiltmeter records (Fig. 13a). Seismic swarms began up toseveral hours before each explosion and culminated near the tiltpeak or during the deflation phase at between one and four eventsper minute (Fig. 13b, c). Deformation records ceased during theexplosion of 16:57 on 5 August, when the only operating tiltmeter(Fig. 10) was destroyed, but the cycles continued to be evident fromthe seismic record. All but one of the August explosions occurredshortly before or shortly after the peak in seismicity. The closecorrelation between seismicity and eruption onset permittedaccurate prediction of the explosions during this period. Repeatintervals between the August explosions ranged from 10 to 14hours, except for the last two of the series, which were preceded byintervals of 63 and 23 hours respectively (Table 1).

In contrast, there was much less precursory seismicity beforeeach of the September and October explosions. Precursor seismicswarms occurred on 14 occasions, although only a few includedperiods of intense seismicity such as those preceding most of theAugust explosions. When a swarm did occur, it tended to continuefor a short time after the explosion. On no occasion did theprecursor seismicity develop in such a way as to enable accurateforecasting of explosions as in August. There was also no tiltmeterfunctioning at the time, so there are no tilt data from this period.

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 295

Fig. 10. Map of the pyroclastic flow and pyroclastic surge deposits from the 1997 explosions. The locations of the seismometers and tiltmeter operational at thetime are also shown. The tiltmeter was destroyed at 16:57 on 5 August and was not replaced until after the explosions had finished.

Explosion forecasting in September and October relied on theobserved periodicity of the explosions themselves, which rangedfrom 2.5 to 33.5 hours, with an average of 9.5 hours (Figs 8 and 9).

Seismic signals from the explosions

Each explosion generated a seismic signal captured by the MVObroadband system. A typical example (16:57 explosion on 5 August1997) is shown in Figure 14a, with a blow-up of the first 1.4 min inFigure 14b. Each signal began abruptly and remained at a highlevel for several minutes, then decayed over a period of a few tens ofminutes to 3 hours. Selected signal parameters are listed in Table 3.

There were three discrete phases to each signal: (1) a long-periodpart, typically of 10-20 s duration; (2) a higher amplitude pyro-clastic flow signal lasting a few minutes; and (3) harmonic tremorlasting 1-3 hours (average 80 minutes) during the long period ofwaning discharge that terminated each explosion. Phases 1 and 3had similar frequency spectra, with the main energy between 0.6 and1.7 Hz (Fig. 14c). Phase 2 contained much more energy distributedover a range of higher frequencies, principally 2 to 20 Hz.

Filtering of the raw signals enabled us to separate the signals intolow-frequency and high-frequency components (Fig. 14a and b).

We applied a time-domain recursive Butterworth filter between 0.5and 1 Hz and a high-pass filter at 2 Hz. The low-frequency compo-nent was present throughout all three phases of the seismic sig-nal, whereas the high-frequency component occurred only duringphase 2 when it was superimposed on the low-frequency one.

The low-frequency component is interpreted as the vibrationalresponse of the magmatic conduit to the explosion itself (Neuberg& O'Gorman 2002). During most explosions, it remained at ahigh level for about 45-70 s before falling to a much lower level,which agrees with visual observations for the duration of peakdischarge. The low-frequency component fluctuated in intensitywith time, as visible on Figure 14b for the explosion at 16:57 on5 August. This is typical of the amplitude modulation of a long-period seismic signal (Neuberg & O'Gorman 2002), and is notthought to be due to eruption unsteadiness. Video analysis ofthe 12:05 explosion of 7 August revealed no obvious correlationbetween eruptive intensity and the amplitude of the low-frequencyseismic component.

The high-frequency component of phase 2 was due to a combi-nation of ballistic impact, fountain collapse and pyroclastic flow,and had a frequency spectrum typical of pyroclastic flow signals, forexample at Montserrat (Miller et al. 1998) and Mount Unzen, Japan(Uhira et al. 1994). Its onset coincided with the first impact of the

296 T. H. DRUITT ET AL.

Fig. 11. Pyroclastic flow deposits from the 1997 Vulcanian explosion., (a) Buff-coloured pumice-and-ash pyroclastic flow deposits from the August explosiveperiod overlying grey block-and-ash flow deposits of the 25 June 1997 dome collapse. View looking NNE down Tuit ts Ghaut and Pea Ghaut. Paradise Rivervalley joins Tuitt's Ghaut from the left. (b) Buff-coloured pumice-and-ash pyroclastic flow deposits from August 1997 in White River valley. The domelies hidden in cloud to the right. Road for scale. (c) Pumice-rich snout of a pyroclastic flow lobe from the September/October 1997 explosions, near SpanishPoint. The lobe is about 1 m high and about 12m across. It overlies block-and-ash flow deposits of the 21 September 1997 dome collapse. (d) Pyroclasticflow lobes from the September/October 1997 explosions overlying block-and-ash flow deposits of the 21 September 1997 dome collapse. Houses of thecommunity of Spanish Point on the left. The lobes have well defined levees and snouts rich in coarse pumice boulders.

collapsing fountain and ballistic blocks with the ground. This isconfirmed by analysis of the 12:05 explosion of 7 August, in whichphase 1 lasted 18s, and the end of phase 1 correspondedapproximately with the collapsing fountain hitting the flank of thevolcano behind Gages Mountain (Table 2). Ballistics were firstobserved to hit the north flank of the volcano 21.6 s into thisexplosion and the collapsing fountain touched down 0.6 s later. The10-20 s duration of seismic phase 1 is therefore the transit time forfountain collapse, i.e. the time for the momentum-dominated

eruption jets and ballistic blocks to reach their maximum height,then fall to the ground. The duration of the high-frequency signal(i.e. of phase 2) corresponded with the time necessary for thepyroclastic flows to reach their distal limits (200-500 s).

The abrupt drop in intensity of the phase 2 signal about 55 safter the explosion onset (arrow, Fig. 14a) is believed to be dueto pyroclastic flows nearest the seismometer concerned ceasingmovement, while those in other valleys further away were still inmotion, resulting in a drop in apparent seismic energy production.

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 297

Fig. 12. Fallout pumices from the Augustexplosions. The fragments are tabularwith angular edges and curviplanarsurfaces, and were formed by brittlefragmentation of a pressurized magmaticfoam resident in the conduit prior to eachexplosion. The scale is in centimetres.

Fig. 13. (a) Tiltmeter and (b, c) seismicdata for the period 30 July to 14 August1997. The tiltmeter (Fig. 10) wasdestroyed by the explosion at 16:57 on5 August. Cycles of slow inflation of thedome, followed by rapid deflation, areevident from the tilt data. Seismic dataare from the St George's Hillseismometer (Fig. 10) and show cyclicvariations in total seismic amplitude(RSAM) and number of triggers per10 minutes that are in phase with the tiltcycles. Explosions are marked by theletter E and the dome collapse of 18:10on 3 August by the letter C. The eventsmarked P were the two relatively weakinitial explosions of 4 August (Table 1).

Eruptive volumes

We now estimate the volumes of magma discharged during theexplosions and the partitioning of material between fallout tephraand pyroclastic flows. The fallout includes that from (1) the main,central plume and umbrella, (2) the co-pyroclastic-flow plumes, and(3) the low, waning plume. We use the following mean densitiesbased on field and laboratory measurements: airfall ash 1100 kgm - 3 ,pumice-and-ash flow deposit 1350 kgm - 3 , and dense juvenileandesite2600kgm -3 .

The uncompacted volume of fallout tephra from the months ofAugust, September and October has been estimated by Bonadonnaet al (2002b) at 22 x106m3. All but 106m3 of this (21 x 106m3 or8.9 x 106m3 DRE) is attributable to the 88 Vulcanian explosions.The estimate is based on extrapolating isopach data to infinityusing an exponential decay model. It is thought to be a minimumestimate, at least 10% too low, because study of GOES satelliteimages shows that considerable quantities of very fine (2-20 um)ash in fact travel further than predicted by the exponential model(Bonadonna et al. 2002b). Raising the figure by 10% and dividing

298 T. H. DRUITT ET AL.

by 88 gives an average of at least 1.1 x 105m3 DRE of fallouttephra per explosion.

Another estimate of fallout volumes can be obtained fromplume height data (Table 1). The maximum ascent heights of the

explosion plumes ranged from 3 to 15km. Since the duration ofpeak discharge (c. 50 s) was an order of magnitude shorter than thetime for the plumes to reach their maximum altitude (c. 500 s), theplumes can be treated as discrete thermals of a given initial mass.This agrees with visual observations that the plumes developed thebulbous heads characteristic of thermals (Figs 4, 5 and 6).

The ascent height of a volcanic thermal in the atmosphere canbe expressed approximately by:

H = 1 . 8 9 ( Mc[T- To])0.25 (1)

where M is the mass of solids, T is magma temperature (about1100K; Barclay et al. 1998), T0 is the atmospheric temperature atvent level (about 300 K), c is the specific heat of the solids (typically1 1 0 0 J k g - 1 K - 1 ) , and is the fraction of particles contributing tothe thermal mass of the plume (Morton et al. 1956; Woods &Kienle 1994). This relationship has been shown to be appropriatefor fine-grained ash clouds with a major part of their ascent belowthe tropopause (Woods & Kienle 1994). On the timescale (t) ofplume ascent in the atmosphere, particles of radius r can attainthermal equilibrium with the gas phase only if:

r < (2)

(Woods 1995) where K is the thermal diffusivity of the particles(c. 10-6 m2 s - 1 ) . The Montserrat plumes took on the order of 500sto reach their maximum heights, so only particles smaller than 1 cmor so attained thermal equilibrium with the gas during plumeascent. These typically constitute about 80% of particles dischargedduring explosive eruptions (e.g. Druitt 1992; Woods & Bursik1991), so 0 is taken as 0.8.

Equation 1 yields individual plume masses ranging over threeorders of magnitude, equivalent to 0.01-17.5 x 105m3 DRE ofmagma, reflecting the wide range of plume heights. The heightsused were the mean values of the ranges given in Table 1. They werecorrected by first subtracting the height of the dome during theexplosions (c. 1000m), since the buoyant thermals formed abovethe dome, not at sea level. The average DRE volume calculated inthis way was 3.8 x 105m3, which is over three times that estimatedfrom the field measurements given above (1.1 x 105m3).

One reason for the discrepancy may be the parameter 0, whichis poorly constrained. Another reason may lie in the plume heightestimates. The calculated volumes are a strong function of plumeheight due to the one-quarter-power dependence in Equation 1.Independent estimates of plume heights for three explosions madefrom GOES images (Bonadonna et al. 2002b) yield lower valuesthan those provided by NOAA (Table 1), the differences rangingfrom 10 to 30%. If we reduce all the plume height estimates by 20%and reapply equation 1 while retaining 0.8, we obtain a range

Fig. 14. (a) Seismic signal from the 16:57 explosion of 5 August1997, recorded on the seismograph at Galway's Estate(Fig. 10). The unfiltered signal shows phase 1, phase 2 and thefirst minute of phase 3. The abrupt drop in intensity of thephase 2 signal (arrow) may be due to pyroclastic flows nearestthe seismometer coming to rest. The low-frequency (0.5-1 Hz)filtered component is attributed largely to fragmentation andconduit flow. The high-frequency (>2 Hz) component is due tofountain collapse, ballistic impact, and pyroclastic flow.(b) Enlargement of the first 1.4 minutes of the same signal.Phase 1 is interpreted as the time for the ballistic blocks andcollapsing fountain to first hit the ground. Pulsing of the0.5-1 Hz component is attributed to resonance of seismicwaves in the conduit. (c) Frequency spectra for the differentphases of the explosion signal, as well as for the backgroundseismicity. Two spectra are shown for phase 2: one prior to theintensity drop (arrow in (a)) and one after it. The low-frequency component of the explosion signal is presentthroughout the explosion, whereas the high-frequencycomponent occurs only in phase 2.

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 299

Table 3. Characteristics of the explosion seismic signals

Date

4 Aug.4 Aug.5 Aug.5 Aug.6 Aug.6 Aug.7 Aug.7 Aug.7 Aug.8 Aug.8 Aug.

1 1 Aug.12 Aug.

22 Sep.22 Sep.22 Sep.23 Sep.24 Sep.24 Sep.24 Sep.25 Sep.25 Sep.25 Sep.26 Sep.26 Sep.27 Sep.27 Sep.27 Sep.28 Sep.28 Sep.28 Sep.29 Sep.29 Sep.29 Sep.29 Sep.30 Sep.30 Sep.

1 Oct.1 Oct.1 Oct.2 Oct.2 Oct.2 Oct.4 Oct.

Time(local)

06:3016:4304:4516:5704:0214:3600:3412:0521:5510:3220:5111:3810:12

00:5710:4520:4207:2300:3410:5417:1603:5411:0920:0504:2514:5600:0109:4617:1504:2810:3423:0306:2611:2316:4821:5704:4317:4405:0011:3417:4001:0512:5322:5008:33

Durationphase 1(s)

10

1819171918

1210.6108

109.66

14.58.5

14141717172022.210.417.512.923.5187

34164

14187

1715

Max.verticalvelocityexplosion(nms - 1)*

43960

1012706556089160504108495034500

319505612524113169322596520763

89255324017256277721352262372383622420814339

13986813179

1332801014240622295542146820005

1898801950

5442413294794429 55130904

Durationphase 2(s)

360

300

330270300240

300

480360270

300

330240300300300240300270360270240300270240270270270210240300240450

Maxverticalvelocitypy flows(nms-')t

12323515535210730040525425146463919651

103013429646551535585504835743045939329407846517927

11598630784296958299054007216107599035951

32284384984794412119

161781

Date

4 Oct.5 Oct.5 Oct.5 Oct.6 Oct.6 Oct.6 Oct.7 Oct.7 Oct.8 Oct.8 Oct.9 Oct.9 Oct.

10 Oct.10 Oct.1 1 Oct.12 Oct.12 Oct.13 Oct.13 Oct.14 Oct.14 Oct.14 Oct.15 Oct.15 Oct.15 Oct.15 Oct.16 Oct.16 Oct.16 Oct.16 Oct.16 Oct.17 Oct.17 Oct.17 Oct.17 Oct.18 Oct.18 Oct.19 Oct.19 Oct.20 Oct.20 Oct.21 Oct.21 Oct.

Time(local)

18:2702:5310:4118:4102:4410:4217:5004:0616:0203:4715:1003:0312:3204:1318:4017:5707:5522:2409:3215:2401:3613:4823:1605:4708:3314:5022:2002:5106:3509:4414:2018:4804:0112:3516:0523:1806:4815:1705:1321:2705:0415:1311:3919:02

Durationphase 1(s)

19134

1421145

242215132016171514

391813

10343119

44126

26

1526

8102255171821

Max.verticalvelocityexplosion(nms-1)*

525042141

56443178941720288929210

96213173423166334058213034217223 9153979815306

283992082814414

1653848463

3252186747061

32847393572251735000

3387027381149237655

4655430973155886633620120

Durationphase 2(s)

270240210300240270300

300300

300300300300

270210240300240

240

0

300240360450480270270480240480

Max.verticalvelocitypy flows(nms-1)t

28641409141959456612393813766455690

492573682854754749803835681507410357424325726

15813300444873125354

25185340414591415536

011275151173979052444

52215326204551242377

1 1 1 0284407423411784783376522438

* Maximum vertical component of the velocity spectrum for the explosion component of the seismogram. Values for August were measured from theseismometer at Galway's Estate. Those for September and October were measured on the Windy Hill seismometer. The two data sets are therefore not directlycomparable.t Maximum vertical component of the velocity spectrum for the pyroclastic flow component of the seismogram, measured at Windy Hill.

of DRE volumes from 0.01 to 6.6 x 105m3, with an average of1.4 x 105 m3, which is more consistent with the field estimate.

The total volume of pumice-and-ash pyroclastic flow depositsgenerated during the two episodes of Vulcanian explosions was32 x 106m3, equivalent to an average of 1.9 x 105m3 DRE perexplosion. This was calculated from volume surveys of the mainghauts carried out throughout the Soufriere Hills eruption. Themethod involved surveying of the ground surface by helicopterusing rangefinder binoculars and GPS (Sparks et al. 1998). Only forone explosion (15:17 on 18 October) was a detailed survey carriedout of the pyroclastic flow deposits from a single event in enoughdetail to calculate a volume. The resulting map is given in Coleet al. (2002). This explosion was average in magnitude (NOAA-estimated plume height 9.1 km), and the calculated pyroclastic flow

volume (2.3 x 105 m3 DRE) agrees broadly with the overall averagegiven above.

The variation of pyroclastic flow volume with explosion magni-tude cannot be determined. There are indications from the data inTable 1 that flows from larger explosions (as indicated by higherplumes) entered more ghauts around the dome, and thus wereperhaps more voluminous, although this is not possible to quantifyowing to the incompleteness of the observations. On the other hand,there is no obvious correlation between pyroclastic flow runout andplume height. For example, flows from the relatively small explo-sions at 11:34 on 1 October (plume height 4.6 km) and at 17:57 on 11October (plume height 6.9 km) had runouts down Tuitt's Ghaut of4.5 and 5.5 km respectively, which is comparable to, or greater than,those from explosions with higher plumes. Neither is there any

300 T. H. DRUITT ET AL.

correlation between plume height and the amplitude or duration ofthe high-frequency (pyroclastic flow) component of the explosionseismic signals that might suggest systematic variation of pyroclasticflow volume with explosion magnitude.

We conclude that the average Vulcanian explosion duringAugust, September and October 1997 discharged a total of about3.0 x 105m3 DRE of magma - 1.1 x 105m3 as fallout and 1.9 x105m3 as pyroclastic flows - although there was a large variationfrom the smallest to the largest explosions. Since the volume of ashin the co-pyroclastic-flow plumes was c. 10% of the total fallout(Bonaonna et al. 2002b), the partitioning of magma during anaverage explosion is estimated to have been as follows: centralplume, umbrella cloud and waning plume 1.0 x 106m3, pyroclasticflows 1.9 x 106m3, and co-pyroclastic-flow ash plumes 0.1 x 106m3.About two-thirds of the material ejected during an average explo-sion underwent fountain collapse to form pyroclastic flows.

Exit velocities during the explosions

Exit velocities during the explosions (the velocity at which thematerial left the dome summit crater) were estimated from analysisof video footage, distributions of ballistic blocks and, more crudely,from the observed fountain-collapse heights and durations of phase1 of the explosion seismic signals.

Observed fountain-collapse height and duration of seismicphase 1

A crude estimate of exit velocity (u) at the start of each explosioncan be made from the estimated fountain collapse heights(h = 300-650 m above the summit crater rim) during the initial10-20s of the explosions. Numerical modelling has shown thatfountain-collapse height can be approximated by h u2/2g(Dobran et al. 1993; Clarke et al. 2002), which implies a verticalexit velocity at crater-rim level of 80-115m s - 1 . For such velocities,the transit time (t) of the collapsing fountain from initiation toground impact would be approximately t 2u/g, or 16-23 s, whichis consistent with the observed durations of phase 1 of the explosionseismic signals (10-20 s).

Analysis of video footage

More accurate estimates of vertical ascent velocities were made forthe three main plumes of the explosion at 12:05 on 7 August, usingthe northern crater rim (940m) as reference level. Video footagefrom two sites (MVO South and Fleming) was analysed in order toconstruct height-time curves and thus to estimate ascent velocities.The height of each of the three plumes was measured as a functionof time and corrected for perspective effects using the equations ofSparks & Wilson (1982). Distortions were small because the plumeswere filmed from distances of several kilometres and scalingcorrections were approximately linear. The vertical and horizontalfields of view for the two cameras were determined by filming aknown landscape using the same settings as during the explosion.Tracings of the three plumes are shown in Figure 15, and theheight-time data are shown in Figure 16a. Plume ascent velocitieswere calculated using best-fit polynomials to smooth the height-time curves, then differentiating the polynomials (Fig. 16b). For allthree plumes the velocity first decreased, reached a minimum in therange 15 to 30ms - 1 , then increased again. The velocity minimum isinterpreted as the transition from momentum-driven to buoyancy-driven behaviour and occurred at heights of 450-650 m (plumes 1and 2) and 1200m (plume 3) above the crater rim.

Exit velocities for the three plumes were estimated by back-extrapolation of the velocity curves to crater-rim level. Plume 2left the crater 7 s into the explosion with a vertical velocity of80 ± 10ms - 1 ; but, since it was initially inclined at about 60° tothe horizontal, the absolute exit velocity was about 95 ± 1 0 m s - 1 .

Fig. 15. Tracings of plumes 1 to 3 of the explosion of 12:05 on 7 August(Fig. 4). The data were measured from video footage taken from Flemingand corrected for perspective effects using the equations of Sparks & Wilson(1982). Numbers are the time in seconds after the onset of the explosion andrefer to the overlying plume front. The tracing intervals are either 2 or 3s.The star marks the vent location.

The data for plume 1 do not allow accurate extrapolation, but thevelocity and emergence angle were about the same as for plume 2.Plume 3 left the vent vertically 17 s into the explosion at a velocityof at least 130ms - 1 .

Clarke et al. (2002) have analysed the same video footage of the12:05 explosion on 7 August using a similar method, but withoutdistinguishing the three individual plumes recognized here (i.e. bytracing the progress of the front of the entire plume). Their data areconsisted with an initial exit velocity of 1 1 0 ± 9 m s - 1 and avelocity-minimum height of about 450m above the crater rim. Theyalso analysed footage of the 14:36 explosion on 6 August, for whichthe exit velocity and velocity-minimum height were estimated to be1 3 0 ± 7 m s - 1 and 650m respectively.

A more detailed analysis of video footage of three explosions inOctober 1997 (17:50 on 6 October, 16:02 on 7 October and 12:32 on9 October) has been carried out by Formenti & Druitt (in prep.).The analysis involved plotting height-time curves for individualfinger jets and extrapolating velocities back to crater-rim level usinga fluid dynamic model for a jet. The calculated exit velocities rangefrom 40 to 140m s-1 and in all three explosions increased with timeas fragmentation progressed to deeper levels in the conduit. Thusthe slowest jets emerged at the start of the explosion and the fast-est ones about 10s later. Higher exit velocities may have occurredsubsequently, but any such jets were hidden by the billowing cloudsof ash.

Analysis of ballistic trajectories

The locations and sizes of ballistic blocks also served to constrainexit velocities. Ballistic crater fields were mapped by helicopterfollowing the August explosions using onboard GPS, and blockdiameters were estimated to within ±10 cm (Fig. 17). No accuratemeasurements were made in September and October due to safetyconsiderations, but observations indicate that they were similar. TheAugust explosions threw blocks out to 1.7 km from the crater centre.The distribution was approximately symmetrical around the dome.Blocks with the largest ranges had diameters of 0.7m (to the north)

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 301

Fig. 16. (a) Heights and (b) velocities as afunction of time for the three plumes of the12:05 explosion on 7 August (Fig. 15). Thelines simply connect the data points.

and 1.2m (to the south). Blocks smaller than 0.4m are not shownin Figure 17, but became increasingly abundant nearer the dome,reflecting the stronger air drag experienced by small blocks (Bower& Woods 1996).

Two models (Self et al 1980; Waitt et al 1995) have been used toestimate launch velocities for the ballistic blocks. In the Self et al.(1980) model the maximum range for a block several decimetres insize is achieved for an optimum launch angle of about 35°. Launchangles as low as 30° are observed on video footage, so the optimumangle was assumed in the calculations. Selected launch velocities andtravel times calculated using the model are shown on Figure 17.These take into account the elevation difference between the craterrim and landing site. Blocks 0.7m large on Farrell's Plain requirelaunch speeds of about 160ms - 1 to reach their range of 1.6km,

540m below the crater rim. The 1.2m block launched over Gal-way's Wall requires a velocity of 135ms - 1 . The highest velocities(250 ms - 1 ) are required by 0.4m blocks that landed 1.7km from thevent. However, it was unclear in the field whether these were frag-ments of larger blocks that had broken on impact, so the result isambiguous. Launch velocities up to 160m s-1 are therefore requiredby the Self et al. (1980) model to explain the ballistic data, which ishigher than plume exit velocities measured from video footage (upto 140ms -1). Calculated flight times for the ballistics, which rangefrom about 17 to 27 s (Fig. 17), are, however, broadly consistent withobservations of the 12:05 explosion of 7 August, in which the firstballistic impacts on Farrell's Plain were observed at 21.6 s (Table 2).

The same ballistic data have been modelled by Clarke et al.(2002) using the numerical scheme of Waitt et al. (1995). This

302 T. H. DRUITT ET AL.

Fig. 17. Sizes and ranges of ballistic blocksfrom the 13 explosions in August 1997. Thenumbers are the launch velocities and flighttimes calculated using the model of Self elal. (1980), taking into account air drag andthe elevation differences between the impactsites and the crater rim. The calculationsassume an optimum eruption angle of 352 tothe horizontal. Locations shown by squaresare south of the dome, dots between TarRiver valley and White's Ghaut, upward-pointing triangles between White's Ghautand Tuitt's Ghaut, and downward-pointingtriangles between Tuitt's Ghaut andMosquito Ghaut.

requires lower initial velocities for the same 35° launch angle asused above (< 125ms - 1 as opposed to < 160ms - 1) , since the dragcoefficients assumed in this model result in lower form drag than inthe model of Self et al. (1980). The result is in better agreement withthe exit velocities observed.

Fig. 18. Summary of a single explosivecycle in 1997: (a) immediately prior toexplosion onset; (b) during the explosion;(c) during the interval betweenexplosions. See the text for discussion.

Discussion

Explosion cyclicity

Activity of Soufriere Hills Volcano in 1997 involved a regime of cyclicexplosive behaviour, with an average interval of about 10 hours.Repetitive Vulcanian explosions have been reported from othervolcanoes. For example, six explosions occurred regularly at MountNgauruhoe, New Zealand, on 19 February 1975 at intervals ofbetween 0.5 and 1 hour (Nairn & Self 1978). Over the period 12 to 14

June 1991, a sequence of four explosions occurred at MountPinatubo, Philippines, at intervals ranging from 10 to 28 hours,culminating in the climatic eruption (Hoblitt et al 1996). At VolcanGaleras, Colombia, six explosions took place at intervals of between8 and 181 days in 1992-1993 (Stix et al. 1997). Twenty-three explo-sions occurred at Tokachi-dake, Hokkaido, between 16 December1988 and 5 March 1989, an average of three to four days apart(Katsui et al. 1990).

Repetitive explosive behaviour at Montserrat is attributed to thecyclic build-up and release of magmatic pressure beneath a rheo-logically stiffened plug of degassed magma at shallow levels in theconduit below the dome (Voight et al. 1998, 1999). A type of stick-slip effect has been invoked to explain cyclic conduit pressurizationat Montserrat, resulting in cyclic deformation of the dome andsurrounding terrain (Voight et al. 1999; Denlinger & Hoblitt 1999;Wylie et al. 1999). Hybrid earthquakes are attributed to hydro-fracturing and associated gas flow in rock or crystal-rich magma at

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 303

Table 4. Estimates of physical parameters for the 1997 explosions

Parameter

Plume height (km)Ascent duration of plume (s)Total magma volume discharged (m3 DRE)Duration of peak discharge (s)Exit velocity (ms - 1)Fountain-collapse height (m)Mass fraction entering collapse fountainInitial velocity of pyroclastic surges (ms - 1 )Runout of pyroclastic flows (km)Runout duration of pyroclastic flows (s)Typical velocity of pyroclastic flows (ms - 1 )Fragmentation pressure (MPa)Velocity of fragmentation wave (ms - 1 )Conduit withdrawal depth (km)Duration of waning phase of explosion (min)Explosion interval (min)Magma ascent velocity between explosions (ms - 1 )

Value*

c. 10(3 to 15)c.5003x 105

45-7040-140300-650c.2/330-603-6c.500105-1510-500.5 to >2c. 60 (20-190)c. 600 (100-3 700)>0.02

* Average values, with ranges in brackets.

the peak of each pressurization cycle (Neuberg et al. 1998; Voightet al. 1999). Synchronized tilt cycles and hybrid swarms duringthe August explosive episode provided accurate indicators of thepressurization state of the system, enabling MVO volcanologists toanticipate many of the explosions successfully and to reduce thethreat to the population. They also facilitated study of the explosionsand their products. Subsequently, in September and October, whenthere was no tiltmeter and hybrid swarms were weak or absent, thestrong periodicity of the explosions themselves played this role.

Explosion mechanisms

Figure 18 summarizes schematically the events throughout onecycle of 1997 explosive activity at Soufriere Hills Volcano. Our bestestimates of the physical parameters are given in Table 4. Explo-sive eruption commenced when the conduit overpressure exceededthe strength of the cap of degassed crystal-rich magma. Duringthe initial few seconds, crater rubble and fragments of disrupted,degassed plug were thrown out, forming ballistic showers. A frag-mentation wave then descended the conduit into the region ofpressurized magma, resulting in a rapid escalation of exit velocitiesfrom about 40 to 140ms - 1 . Eruption was highly unsteady, peakdischarge lasting just a few tens of seconds with the highest intensityover the first 10-20 s.

Each explosion discharged on average about 3 x 105 m3 DRE ofmagma. Since the conduit diameter during the 1995-1999 phase ofthe eruption is estimated at 25 to 30m from spine dimensions andthe widths of early vents (Watts et al. 2002), and is not believed tohave varied greatly with time (Voight et al. 1999), the conduitdrawdown during an average explosion was about 500 m below thecrater floor (which itself was about 800m above sea level). This is aDRE drawdown; the actual average drawdown of vesicular magmawould have been greater, but not more than 1 km. The largestexplosions must have emptied the conduit to depths of 2km ormore. Since peak discharge lasted a few tens of seconds, the velocityof the fragmentation wave down the conduit is constrained to havebeen of the order of l0-50ms-1, although the initial value couldhave been greater. High-intensity eruption probably ceased oncethe wave reached a level in which the magma pressure was notsufficiently large to drive fragmentation. Ash then continued to bedischarged for 1-3 hours, but at a greatly reduced rate.

Numerical models of the plume dynamics (Clarke et al. 2002)and conduit flow (Melnik & Sparks 2002b) reproduce several keyfeatures of the explosions, including their highly transient nature,peak exit velocities in the range 80-140 ms - 1 , and dischargedurations and conduit drawdowns comparable to those observed.The models are based on the rapid decompression and expansion of

Fig. 19. Explosion magnitude (as measured by plume height) as a functionof the time interval between explosions: (a) interval preceding the explosion,and (b) interval following the explosion. See the text for discussion.

gas-rich, pressurized magma beneath a degassed plug, thus support-ing this interpretation of the eruption dynamics.

The eruption columns were partially unstable in all but oneexplosion. On average about two-thirds of the erupted materialcollapsed back to form pyroclastic surges and flows, while the otherthird, including probably a large proportion of smaller particles,was carried up into the plume. However, these proportions mayhave varied considerably between individual explosions. Fountaincollapse occurred in the first 10-20s of each explosion from a fewhundred metres above the crater rim. Vertical velocity profiles inthe plumes reveal velocity minima corresponding to the transi-tion from momentum-driven to buoyancy-driven behaviour. This isanalogous to the superbuoyant regime of sustained eruption col-umns, which is intermediate between fully stable (convective) andfully unstable (collapsed) regimes (Bursik & Woods 1991) and isseen in the explosion simulation of Clarke et al. (2002).

Once each explosion was over, magma rose in the conduit byviscous flow at a couple of centimetres per second or more (1-2 km in10 hours). This exceeds the critical ascent speed of about l^cms - 1

for amphibole breakdown and explains the presence of hornblendephenocrysts lacking breakdown rims in the explosion pumices(Devine et al. 1998b). In at least some cases the conduit was tot-ally refilled prior to the next explosion and a small dome appeared inthe crater. Repressurization of the conduit then occurred untilconditions were right for the next explosion, although the exactmechanism is not well understood. Figure 19 shows that a weakcorrelation exists between plume height and the intervals betweenexplosions for both the August and September-October episodes.Positive correlations exist between (1) plume height and the intervalprior to a given explosion (Fig. 19a) and (2) plume height andthe interval following a given explosion (Fig. 19b). Correlation 1would suggest that large explosions result from long precedingintervals, perhaps allowing the build-up of larger magma pressuresin the conduit. Correlation 2 is consistent with a scenario in whichlarge explosions drain the conduit to deeper levels, so that longer

304 T. H. DRUITT ET AL.

intervals are then required to refill the conduit prior to the nextexplosion. The data do not distinguish between these two mechan-isms, although correlation 2 appears visually to be slightly betterthan correlation 1.

Conduit pressurization and explosive fragmentation

Pressurization of the magmatic conduit at Montserrat is attributedto non-linear vertical pressure gradients caused by large viscosityvariations that accompany exsolution of water from magma (Sparks1997; Massol & Jaupart 1999). Magma viscosity is a strong functionof water content, particularly at low pressures (Hess & Dingwell1996). The estimated viscosity of non-degassed magma at Mont-serrat is about 106 Pas and that of completely degassed magmaabout 1014Pas (Voight et al. 1999). This is believed to have gener-ated large magma overpressures (magma pressure minus lithostaticpressure) at shallow levels in the conduit. Another effect is the devel-opment of high gas pore pressures in the ascending magma due to(1) viscous resistance to vesicle expansion, which increases as theliquid exsolves gas (Massol & Jaupart 1999), and (2) growth ofmicrolites in the degassed, undercooled liquid, which forces furthergas into vesicles (Stix et al 1997; Sparks 1997).

Tilt amplitudes and far-field deformation measurements atMontserrat are consistent with maximum magma overpressures ofabout ten to a few tens of megapascals a few hundred metres belowthe base of the dome (Shepherd et al 1998; Voight et al. 1999). Theconduit flow modelling of Melnik & Sparks (2002a) predicts steeppressure gradients and overpressures up to l0MPa in the upperconduit.

The angular, platy shapes of many of the 1997 fallout pumiceswith 55-75 vol% vesicles are consistent with brittle fragmentationof a pressurized magmatic foam present in the upper conduit priorto each explosion. Brittle fragmentation of magma requires steeppressure gradients and fast decompression rates in order to drivethe magma through the glass transition limit (Dingwell 1996). Thishas been observed experimentally by Alidibirov & Dingwell (1996),who showed that pressure differentials across the fragmentationinterface of a few megapascals can be sufficient to drive brittlefailure, generating platy fragments with shapes very much like thoseat Montserrat. Recent experimental work has shown that the tensilestrength of crystal-rich magma like that at Montserrat may be ofthe order of 20MPa or more (Martel et al 2001). As the frag-mentation wave descended the conduit during each explosion, thepressurized foam broke up into tabular fragments that were thenaccelerated to the surface. Magma fragments erupting from the ventapparently had sufficiently high viscosities due to gas exsolution tosuppress post-fragmentation expansion, enabling pumices to retainvesicularities and angular shapes close to those acquired at frag-mentation (the viscosity quench effect; Thomas et al 1994). Pumiceincorporated into pyroclastic flows were subsequently rounded byabrasion during transport.

The presence of magmatic foam with at least 55% bubbles inthe upper conduit can be used to provide an independent estimateof magma pressure prior to each explosion. The bulk water contentof the magma prior to ascent was about 1.6±0.3wt%, based onthe water content of glass inclusions (4.3±0.5wt%; Barclay et al1998; Devine et al 1998a) and the estimated crystal content in themagma reservoir at 5-6km depth (60-65vol%; Murphy et al2000). In the Appendix we show that the total confining pressuresrequired for magmatic foam with 1.6 ±0.3 wt% water to have vesi-cularities of 55-75% are 5-15MPa. One key feature of theexplosions is that the products are dominantly pumiceous, withdense clasts making up no more than 5% of those erupted (Clarkeet al 2002). Given that an average explosion emptied the conduit toabout 500 m (DRE) depth, the plug of degassed magma present inthe conduit prior to each explosion can have been no more thanabout 25m thick, corresponding to an overburden of less than1 MPa. Given that pressures of 5-15 MPa are required in the mag-matic foam just below this cap, this suggests that the foam musthave been very significantly overpressured (by at least a few mega-

pascals) relative to the overlying plug and surrounding conduitwalls. The presence of pressurized, gas-charged magma at very highlevels in the conduit immediately prior to each explosion is consis-tent with the observation that exit velocities in excess of 100 m s-1 ormore were achieved only a few seconds after each explosion began(Clarke et al 2002; Melnik & Sparks 2002b).

Initiation of episodes of explosive activity on Montserrat

Each of the two episodes of explosive activity in 1997 was triggeredby a major dome collapse (3 August and 21 September), as was theexplosive eruption on 17 September 1996 (Robertson et al 1998). Ineach case, sudden removal of part of the dome led to the conditionsfor explosive fragmentation. This was not immediate, the delaysbeing 2.5 hours (17 September 1996), 10 hours (3 August 1997) and20 hours (22 September 1997), showing perhaps that time was neces-sary for the build-up of sufficient conduit pressure for this to occur.Many dome collapses occurred during the 1995-1999 period, butonly three are known to have triggered major vertical explosions. Weexclude here the relatively weak explosions of late 1998 and 1999,which may have been triggered by slow pressure build-up in theslowly crystallizing lava dome and conduit during the period ofvirtually no magma extrusion (Norton et al 2002). One importantfactor was probably that the 17 September 1996 and 21 September1997 collapses resulted in two of the largest height reductions of theactive dome-growth area during the 1995-1999 period (130m and230m respectively), causing large decompressions of the conduit (atleast 3.5 and 6 MPa). The height reduction from the 3 August 1997collapse was not observed clearly, but it is inferred to have been atleast 110m (3 MPa) from the form of the crater observed four dayslater. Sudden decompression of at least 3 MPa therefore appearsnecessary to trigger explosive fragmentation at Montserrat. Conduitflow beneath lava domes involves complex feedback effects andsudden decompressions can force systems from effusive to explosivebehaviour (Jaupart & Allegre 1991; Woods & Koyaguchi 1994). Anadditional effect in 1997 may have been the high magma dischargerate. The time-averaged magma discharge rate increased throughoutthe 1995-1999 period, and by August 1997 had reached 7-8 m3 s-1

(Sparks et al 1998; Sparks & Young 2002). High discharge ratesfavour explosive fragmentation by limiting the time available formagma degassing during ascent (Jaupart & Allegre 1991; Melnik &Sparks 2002a). High magma flux during August, September andOctober of 1997 may have helped to prime the conduit for explosiveactivity once a suitably large dome collapse occurred. Strangely, thelargest dome collapse of the 1995-1999 period (26 December 1997;Sparks et al 2002) decompressed the conduit by at least 8 MPa.giving rise to a violent lateral blast and pyroclastic density current,but triggered no vertical explosion from the conduit and producedlittle pumice. This highlights the complexity of the system and theexistence of important effects not considered here.

Conclusions

Two episodes of cyclic explosive activity occurred at Soufriere HillsVolcano in 1997. Thirteen explosions took place in August andanother 75 in September and October. The activity had a majorimpact on southern Montserrat and triggered northward enlarge-ment of the evacuation zone in mid-August.

Like the explosive eruption of 17 September 1996. both episodesin 1997 were preceded by major dome collapses that decompressedthe conduit by 3 MPa or more. Delays of 3 to 20 hours then followedbefore explosive activity commenced. Large gravitational collapsesare a prerequisite for vertical explosive eruption at Montserrat.

The explosions were highly unsteady, with the most intensephase lasting only a few tens of seconds. Peak discharge was accom-panied by ballistic showers, exit velocities up to 140 m s - l , and (in allbut one event) fountain collapse from a few hundred metres abovethe crater rim over the first 10-20s of each explosion. Pyroclasticflows travelled up to 6km down all major drainages around the

EPISODES OF CYCLIC EXPLOSIVE ACTIVITY 305

dome and entered the sea on the south and east coasts. Buoyanteruption plumes with large, bulbous heads rose to 3-15 km in theatmosphere, then spread out as umbrella clouds. After 10 minutes orso, each explosion settled into a waning phase that typically lastedan hour and generated a low, bent-over ash plume. Fallout andpyroclastic flows/surges from the explosions accounted on averagefor one-third and two-thirds of the magma discharged, respectively.The explosions emptied the conduit to a depth of 0.5-2 km, perhapsmore in some cases.

Filtering of explosion seismic signals permitted distinction of alow-frequency (c. 1 Hz) component due to the explosion itself anda high-frequency (>2 Hz) component due to ballistic impact, foun-tain collapse and pyroclastic flow. Relative timing of the onsets ofthe two components provided information on the flight durationsof ballistic blocks and on the transit time for fountain collapse,from inception to first ground impact.

Explosions in August were accompanied by cyclic patterns ofseismicity and edifice deformation. Repeated slow inflation, followedby rapid deflation, of the volcano recorded cycles of build-up, thenrelease, of pressure beneath the dome. The explosions were driven byrapid decompression and brittle fragmentation of overpressuredmagmatic foam in the upper conduit and occurred at intervals of 2.5to 63 hours, with a mean of 10 hours. Synchronized tilt cycles andhybrid earthquake swarms during the August explosions providedaccurate indicators of the pressurization state of the system, enablingvolcanologists to anticipate many of the explosions and reduce thethreat to the population. In September and October, when there wasno tiltmeter and hybrid swarms were weak or absent, the strongperiodicity of the explosions themselves played this role.

We thank the staff of the M VO for their very important contributions in thestudy of the 1997 explosions. D. Lea, M. Sagot and D. Williams kindlyprovided us with video footage of the explosions and allowed us to studyit. D. Williams helped us in the analysis of video footage. B. Poyer kindlyprovided the photographs in Figure 7. Careful reviews by T. Koyaguchi,L. Wilson and P. Kokelaar are gratefully acknowledged.

Appendix

Estimation of fragmentation pressures during the explosions

We estimate the pressure necessary for the magma with 60-65vol% crystals (Murphy et al. 2000) and a bulk water content of1.6±0.3wt% to have 55-75 vol% vesicularity. Consider a unitvolume of crystal-bearing magmatic foam immediately prior tofragmentation. The volume fraction of bubbles is X and the volumefraction of crystals in the liquid phase is F. The masses of gas,liquid, and crystals are given by:

Mg = pgX

M1 = A(1-A-)(1-F) (Al)Mc = Pc(l - X)F

where M is mass, p is density and the subscripts g, 1 and c stand forgas, liquid and crystals respectively. Given the solubility law forwater in magmatic liquid, n = P1/2, where n is a mass fraction anda is approximately 4.1 x 10-6 Pa1/2 for rhyolite (the composition ofinterstitial glass in the pumices), we can write the mass balanceequation for water in the foam

Mg + M1aP1/2 = N(Mg + M1 + Mc) (A2)

where N is the bulk mass fraction of water in the magma. Thedensity of the gas is given by:

(A3)

where T is temperature (about 860°C or 1133 K; Barclay et al. 1998)and r is the gas constant (462 J kg - 1 K-1 for water). Given a bulkwater content N, we can use Equation A2 to estimate the vesi-cularity X of the foam as a function of pressure P. For a bulk watercontent of 1.6±0.3wt%, bubble contents of 55-75 vol% requirepressures in the range 5-15MPa.

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