Pyroclastic flows and surges generated by the 25 June 1997T dome collapse, Soufriere Hills Volcano, Montserrat S. C. LOUGHLIN 1 , E. S. CALDER2, A. CLARKE 3 , P. D. COLE4, R. LUCKETT 5 , M. T. MANGAN 6 , D. M. PYLE7, R. S. J. SPARKS2, B. VOIGHT3 & R. B. WATTS2 1 British Geological Survey, West Mains Road, Edinburgh, EH9 3LE, UK (e-mail:
[email protected]) 2 Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK 3 Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA 4 Centre for Volcanic Studies, University of Luton, Park Square, Luton, LU1 3JU, UK 5 British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK 6 United States Geological Survey, Menlo Park, California, USA 7 Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK
Abstract: On 25 June 1997, an unsteady, retrogressive, partial collapse of the lava dome at Soufriere Hills Volcano lasted 25 minutes and generated a major pulsatory block-and-ash flow, associated pyroclastic surges and a surge-derived pyroclastic flow that inundated an area of 4km 2 on the north and NE flanks of the volcano. Three main pulses are estimated to have involved 0.78, 2.36 and 2.36 x 106 m3 of debris and the average velocities of the fronts of the related block-and-ash flow pulses were calculated to be 1 5 m s - 1 , 16.1ms - 1 and 2 1 . 9 m s - 1 respectively. Deposits of block-and-ash flow pulses 1 and 2 partially filled the main drainage channel so that material of the third pulse spilled out of the channel at several places, inundating villages on the eastern coastal plain. Bends and constrictions in the main drainage channel, together with depositional filling of the channel, assisted detachment of pyroclastic surges from the pulsatory block-and-ash flow. The most extensive pyroclastic surge detached at an early stage from the third block-and-ash flow pulse, swept down the north flank of the volcano and then climbed 70 m in elevation before dissipating. Rapid sedimentation from this surge generated a high-concentration granular flow (surge-derived pyroclastic flow) that drained westwards into a valley not anticipated to be at high risk. Observations support the hypothesis that the interior of the Soufriere Hills Volcano lava dome was pressurized and that pyroclastic surge development became more substantial as deeper, more highly pressurized parts of the dome were incorporated into the pyroclastic flow. Surge development was at times so violent that expanded clouds detached from the block-and-ash flow within a few tens of metres of the lava dome.
The partial dome collapse on 25 June 1997 was one of the largest during the eruption and it was one of the best documented. The pyroclastic flow and surges generated by the collapse swept down the northern flanks of the volcano, killing 19 people and seriously injuring seven others. From a sociopolitical point of view, this was the most important event to occur during the eruption, and it exerted a strong influence on hazard management and decisionmaking throughout the subsequent course of the eruption. This paper describes the 25 June 1997 dome collapse, the dynamics of the consequent block-and-ash flow and pyroclastic surges, and the deposits formed. The hazards and damage caused by the flow, surges and surge-derived pyroclastic flows are emphasized. This paper is based on analysis of monitoring data, field study of deposits, observational data collected from a time-lapse video camera based at W. H. Bramble Airport (5 km to the NE of the volcano; Fig. 1), photographs and eyewitness accounts (Loughlin et al 2002). Other papers in this volume provide additional information on the 25 June deposits (Bonadonna et al 2002; Cole et al 2002; Druitt et al 2002) and the actions of the MVO scientists (Aspinall et al. 2002; Kokelaar 2002).
Construction of a lava dome in English's Crater (Fig. 1), at the summit of Soufriere Hills Volcano, began in November 1995. Pyroclastic flows generated by partial dome collapse began to develop and travel to the east in late March 1996, and first reached the sea on 12 May 1996. Dome growth continued throughout 1996 and into 1997 (Sparks et al. 1998). On 10 February 1997, pyroclastic flows overtopped the south crater wall for the first time, and during March and April 1997 flows travelled down the White River valley to the south and SW (Cole et al 1998). By the end of May 1997, the northern flanks of the volcano were threatened by dome collapse pyroclastic flows for the first time. The lava dome had reached a volume of 65 x 10 6 m 3 (Dense-Rock Equivalent; DRE) and was growing at a rate of 3 to 5m3 s-1 (Sparks et al 1998).
Terminology
Activity in May-June 1997
Collapse of the lava dome at Montserrat typically generated three components: a high-particle-concentration basal part containing abundant blocks and termed a pyroclastic flow or block-and-ash flow; a low-particle-concentration pyroclastic surge that overlies the block-and-ash flow and may detach from it and move independently; and a lofting buoyant ash plume. Rockfalls are distinguished from dome collapse pyroclastic flows by runouts < 1 km and the failure of many blocks to disintegrate completely and produce copious ash. The 25 June 1997 collapse produced an extremely unsteady, or pulsatory, block-and-ash flow. We refer to three main flow pulses, rather than three separate flows, mainly because of the continuous
Between 14 and 17 May the focus of dome growth switched from the SW to the NE side of the dome. Lava extrusion was directed to the north, and rockfalls began to spill across a broad swath from the east side to the NW side of the dome. This broadly distributed activity suggested the development of a subhorizontal shear lobe directed to the north (see 'active 17 May 1997 lobe' in Watts et al. 2002, fig. 2a). The lobe grew almost continuously through late May and into June. Swarms of hybrid earthquakes (each with a highfrequency start and long-period coda; Miller et al. 1998) accompanied this growth daily between 13 May and 27 May. Each swarm was followed immediately by enhanced rockfall activity, mainly on
nature of the seismic signal. Each pulse was capable of forming a flow front where it advanced rapidly over just-deposited or stillmoving material. Summary of precursory volcanic activity
DRUITT, T. H. & KOKELAAR, B. P. (eds) 2002. The Eruption of Soufriere Hills Volcano, Montserrat, from 1995 to 1999. Geological Society, London, Memoirs, 21, 191-209. 0435-4052/02/$15 © The Geological Society of London 2002.
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Fig. 1. Map showing the extent of the 25 June 1997 pyroclastic flow deposits. Sampling sites 1-3 are marked.
the north and NE flanks of the dome. The daily number of earthquakes during this period was typically about 100. Rockfall debris rapidly built up in the moat between the dome and the northern crater walls and, by 19 May, material began to spill over the low points of the northern crater rim into the top of Tuitt's Ghaut (Fig. 1). On 27 May, small pyroclastic flows reached 200m down Tuitt's Ghaut. On 5 June, small pyroclastic flows travelled 3.1 km down Tuitt's Ghaut, and rockfall debris began to spill into Mosquito Ghaut (Fig. 1). A relative lull in activity occurred for about a week after 6 June, during a period of poor visibility. By 14 June the level of rockfall activity increased on the northern side of the dome, and deposits extended 500 m down Mosquito Ghaut. On 16 June a significant dome collapse sent pyroclastic flows 1.5km down Mosquito Ghaut and 1.6km down the upper reaches of Fort Ghaut to the west. On 17 June pyroclastic flows travelled 4km down Mosquito Ghaut and 1.8km down Fort Ghaut. A small pyroclastic surge detached from the block-and-ash flow in Mosquito Ghaut and climbed above the channel, causing extensive tree knockdown on the steep walls of the valley. It also generated a broad singe zone that extended 400m west of Mosquito Ghaut (in the prevailing wind direction). A scar developed within the lava dome above Mosquito Ghaut as a result of the collapses on 16 and 17 June (Fig. 2a). The seismicity was relatively low between 27 May and 22 June and the daily number of earthquakes (mainly hybrid and long-period earthquakes) remained below 40. Starting on 18 May 1997 a tiltmeter installed on Chances Peak (Fig. 2) showed a regular pattern of inflation and deflation of the dome. The inflationary part of the cycle usually correlated with the hybrid earthquake swarms and the deflationary part correlated with enhanced rockfall and pyroclastic flow activity. Between 5 and 14 June the periodicity was about 12-16 hours and amplitude was 16-18 rad (Voight et al. 1998, 1999). By 16 June the tilt amplitude
had flattened to only 5-10 rad and the number of hybrid earthquakes declined. At about 16:00 LT (all times are local time, LT = GMT minus 4 hours) on 17 June, inflation increased steeply, peaked at 21:00 and was followed by rapid deflation. A significant collapse at 23:00 then sent pyroclastic flows into both Mosquito Ghaut and the upper reaches of Fort Ghaut. High-amplitude tilt cycles lasted a further two days until 19 June when low-amplitude cycles returned, although the deflationary part of the cycles continued to correlate with heightened rockfall and fumarolic activity. At 05:30 on 22 June a sharp increase in the rate of inflation occurred and the subsequent sharp deflation was coincident with sustained pyroclastic flows that travelled about 1 km to the east of the dome. This was the beginning of a series of nine inflationdeflation cycles that culminated in the partial dome collapse of 25 June 1997 (Fig. 3; Voight et al. 1999). The periodicity reduced to about 8-12 hours and the amplitude increased to as much as 30 rad. Between 22 and 25 June, hybrid earthquake swarms increased in both duration and numbers of earthquakes. Each period of inflation and the associated seismic swarm was attributed to an increase in magma pressure in the upper conduit caused by the supply of gas-rich magma from depth (Voight et al. 1999). When pressure exceeded a threshold value, a plug of viscous magma was injected into the dome, conduit pressure relaxed and deflation began. Although enhanced rockfall and some pyroclastic flow activity occurred during deflation, the intensity of these did not always correlate with the intensity of the preceding earthquake swarm. However, on 24 and 25 June the hybrid earthquake swarms were intense, reaching the state whereby repetitive events merged into continuous tremor (Miller et al. 1998). Poor visibility throughout June hampered efforts to ascertain the dome volume and growth rate. During the morning of 23 June several small spines on the northern summit area of the dome were
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Fig. 2. Configuration of the lava dome (a) before and (b) after the 25 June 1997 partial dome collapse.
Fig. 3. Histogram of the number of triggered low-frequency (hybrid) earthquakes per hour compared to the radial tilt measured at Chances Peak in late June 1997. The low-amplitude tilt cycles before 22 June had no associated hybrid seismicity, whereas high-amplitude cycles that started on 22 June were accompanied by hybrid earthquake swarms during inflation. The partial dome collapse occurred at the onset of a rapid deflation (arrowed).
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S. C. L O L ' G H L I N ET AL. Fig. 4. Seismogram from seismometer at Windy Hill site (3.5km NNW of lava dome) showing precursory hybrid earthquakes merged into tremor, the onset of the pyroclastic flow and three pulses of high-intensity seismic activity corresponding to three successive dome collapses and associated pyroclastic flow pulses. Local time = GMT — 4 hours. Assuming the seismic waves travelled at about 1 k m s - 1 . the seismic signal at Windy Hill began 3-4 seconds after the collapse began: this time lag is accounted for within the error. The peaks on this seismogram probably relate to the flow fronts of each pulse passing closest to the seismic station i.e. near the junction of Mosquito Ghaut and Paradise River.
Fig. 5. (a) Map showing the probable extent of deposits formed by pyroclastic flow pulse 1 based on eyewitness evidence. (b) Map showing probable extent of deposits formed by pyroclastic flow pulse 2 based on analysis of deposits and eyewitness evidence.
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observed (from the airport) to grow by several metres over a period of six hours (Fig. 2a).
Precursory activity on 25 June 1997 At 03:00 on 25 June a hybrid earthquake swarm began, comprising four to five moderate-amplitude events per minute, and this was accompanied by tilt inflation of the summit of the volcano (Voight et al. 1998, 1999). The tilt levelled off at about 05:20 and the summit began to deflate at about 06:10 (Fig. 3). By 06:15 the hybrid earthquake swarm had also declined in intensity and by 07:05 had given way to low-level tremor. Between 06:15 and 07:15, semicontinuous pyroclastic flows travelled down Mosquito Ghaut with runouts as far as 1.5 km. There were also some rockfalls and small pyroclastic flows from the SE and east faces of the dome during this time. Inflation of the summit area then recommenced at about 09:00 and a hybrid earthquake swarm began at 10:50. The swarm intensified rapidly, reaching about six events per minute between 11.30 and 12:30. The earthquakes were all of moderate amplitude, similar to the earlier swarm. The inflation trend flattened out at 12:00 and by 12:45 the seismic activity was dominated by tremor and individual earthquakes were no longer distinguishable. A dilute steam-and-ash cloud issued from the volcano at about 12:30 and drifted to the west at an altitude of 1500m.
Dome collapse of 25 June 1997 Between 12:40 and 12:50 the tiltmeter registered the onset of a sharp deflation (Fig. 3). A strong, continuous seismic signal began at about 12:55 (Fig. 4; Baptie et al. 2002), with more intense pulses of seismic activity commencing at 12:57:15 (±20 s), 12:59:55 (±30 s) and 13:08:20 (±30 s). We interpret the strong continuous signal as a single unsteady pyroclastic flow and the three pulses as major dome avalanches that formed major pulses within the flow. Assuming the seismic waves travelled at about 1kms - 1 , the seismic signal at Windy Hill began 3-4 seconds after the collapse began; this time lag is accounted for within the error. A time-lapse video camera installed at W. H. Bramble Airport, 5 km to the NE of the volcano, recorded the fronts of the three main flow pulses as they emerged from Paradise River by Harris Lookout (Fig. 1). The arrival times of the three flow fronts at various distal locations have been estimated from the video footage. The following section describes the three block-and-ash flow pulses, based on eyewitness, video and observational data. Using data from the seismic signals and the video, we estimated mean velocities for parts of the travel path of each of the flow pulse fronts. Maps showing the extent of the deposits from pyroclastic flow pulses 1 and 2 are shown in Figure 5. The extent of deposits from the third pulse corresponds to the whole area of impact shown in Figure 1.
Pyroclastic flow pulse 1 We assumed that the initiation of the first main flow pulse was related to a rapidly emergent rockfall-type seismic signal (Miller et al 1998) at 12:57:15 (±20 s) (Fig. 4). Eyewitness accounts indicated that the first flow pulse was confined to the steep-walled channel of Mosquito Ghaut (Loughlin et al. 2002). It travelled 4.7 km and stopped in the Paradise River valley just below Bramble village (Fig. 5a). The time taken for the flow front of pyroclastic flow pulse 1 to travel the 3.9km between the source and the sharp bend at Harris village, where it was first recorded by the airport camera, was 230 ± 20s. The average velocity to this point was 16.7ms -1 (+1.6, -1.9). The flow front stopped near Bramble village about 80 seconds later, giving an average velocity for the 800m distance between Harris and Bramble village of l O m s - 1 . The average veloc-
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ity of the flow front from the dome to Bramble village, a total distance of 4.7km, was 1 5 m s - 1 (+1, -0.9). Pyroclastic flow pulse 2 Eyewitnesses observed the second pyroclastic flow pulse in the upper parts of Mosquito Ghaut as the first flow front approached Harris village (Loughlin et al. 2002). The precise onset of pyroclastic flow pulse 2 on the seismic records was not clear because there was interference between the dying signal of flow pulse 1 and the emerging, slightly higher amplitude seismic signal of flow pulse 2 (Fig. 4). The best estimate start time was 12:59:55 (±30 s). Eyewitnesses observed a pyroclastic surge moving down the northern flanks of the volcano as the block-and-ash flow travelled down Mosquito Ghaut. The surge reached the main road near Riley's Yard where it lost momentum and stopped its lateral advance (Fig. 5b). Eyewitnesses on the road near Farrell's Yard saw ash flowing west down the main road at a high angle to the original flow direction. They commented on how the ash was confined to the road, was hugging the ground, was 'boiling' and was 'moving round bends like a vehicle' (Loughlin et al. 2002). This was probably a high-concentration flow derived from the rapid sedimentation of ash near the surge front (surge-derived pyroclastic flow; Druitt et al. 2002). Pyroclastic surges detached at bends along Mosquito Ghaut and reached Mandy's gas station 2.5km north of the lava dome (Fig. 5b). Eyewitnesses heard the gas station explode as they made their escape up Windy Hill (Loughlin et al. 2002). At 13:03:10 the eastern seismic stations stopped transmitting data and we assumed this was caused by the destruction of the Bethel telephone exchange cable, either by the block-and-ash flow or a pyroclastic surge. Unfortunately, it is not clear exactly where the damage took place. The cables crossed the valley at a bridge near Bethel (Fig. 5a), but then they followed the road alongside Paradise River to the west. The bridge was 5.4km from the dome and the time between onset and cut-off was 195 ± 30 s. If the cables were destroyed at the bridge, the flow travelled this distance at an average of 26.7m s-1 (±4.8, -3.6). This is high compared to other estimates so it seems more likely that the cables were destroyed further upstream, in Paradise River valley. The block-and-ash flow continued to Trant's village (Fig. 5b), confined to Pea Ghaut the whole way. Beyond Trant's, the pulsatory advance of the block-and-ash flow was described by eyewitnesses, who saw successive small pulses burst through the stalling flow front (Fig. 6; Loughlin et al. 2002). The flow eventually reached to within 50m of the coastline, a total of 6.8 km from the dome. The flow front of pyroclastic flow pulse 2 was recorded near Bramble village after 233 seconds (a total distance of 4.7km), giving an average velocity to this point of 20.2ms - 1 (±2.9, -2.3). It continued for a further 2km and then stopped just beyond Trant's village, giving an average velocity of 11 m s - 1 for this later stage. The overall average velocity from the dome to Trant's village (a total distance of 6.7km) was therefore 16.1ms - 1 (±1.3, —1.1). The higher average velocity to Bramble village, the greater runout distance and the higher amplitude of the seismic signal (Fig. 3) all indicate that pyroclastic flow pulse 2 was significantly more energetic than pulse 1. Pyroclastic flow pulse 3 The onset of the third pyroclastic flow pulse was evident on the seismogram (Fig. 4) and was estimated to have started at 13:08:20 (±30 s). The early stages of this were not observed, so deposits and effects were used to reconstruct the event. Scouring of the walls of Mosquito Ghaut indicated that the block-and-ash flows were highly erosive and almost filled the channel during transport, thus facilitating the rapid lateral expansion and separation of the overriding pyroclastic surges. It was a surge from the third flow pulse that extended furthest to the north and
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Fig. 6. Photographs taken from the main road north of the airport of advancing pyroclastic flow pulse 2. (a) Pyroclastic flow pulse 2 moving along Farm River valley, by Trant's village, (b) An additional small pulse is shown punching through the stalling flow front (note airport to left of photo) (photos taken by R. B. Watts).
NW and formed the main surge-derived pyroclastic flows that drained westwards (Druitt et al. 2002). Erosion of the walls on both sides of the channel was extensive (but not continuous) for a distance of 2km along the flow path from the dome, particularly on the outside of bends where superelevation of the flow had occurred. The block-and-ash flow pulse continued to the NE along Mosquito Ghaut and into a steep-sided section of Paradise River with several bends (Fig. 1). Pyroclastic surges detached along the north side of the channel, scoured the outside of the bends and impacted the lower parts of Harris village (see fig. 10 in Loughlin et al. 2002). Block-and-ash flow deposits high on the outside of bends showed that superelevation occurred. Several houses on the edges of the ghaut were destroyed and the main road was buried (see fig. l0a in Loughlin et al. 2002). The superelevation of a flow as it rounds a bend reflects an approximate balance between cross-channel gradient in pressure gh/w and centrifugal foree where £ is gravity, h is elevation difference from one side of the flow to the other, w is flow width, u is flow speed and r is radius of curvature of the channel bend. The last parameter must be estimated and is somewhat problematic. On rearrangement, a simple relationship is obtained: (1) Calculations based on the highest level of the deposits on the outside of bends suggest velocities of 2 0 m s - 1 at Harris. Farther down slope, at a bend near Bramble village, eyewitnesses saw part of the block-and-ash flow spill out of the
drainage and continue for almost 1 km in a NE direction, causing extensive damage to buildings in Bethel village (Figs 7a and 8: see fig. 6 in Loughlin et al. 2002). The channel was partially filled with deposits so superelevation of the flow at the bend enabled it to escape. A surge also detached along this bend, scouring the side of the channel (Fig. 8) and barely outrunning the block-and-ash flow. The flow pulse continued northward along Pea Ghaut and part of it spilled NE onto the coastal fan at the next sharp bend to the north, near Bethel (Fig. 1 and Fig. 7b). From this point, blockand-ash flows spilled out along much of the main drainage, which had been partially filled with debris from flow pulse 2. and across the coastal fan. The distal part of the flows on the eastern coastal fan formed distinct, relatively slow-moving, ground-hugging lobes (Fig. 9a. b). These lobes had low. tapered flow fronts easily blocked by low obstacles such as garden walls. As these flows came to a halt. surges outran them by just a few metres (Fig. 11. Farm village was located on the outside of a sharp bend alongside Pea Ghaut and was partially buried by the block-and-ash flow deposits associated with pyroclastic flow pulse 3 (Fig. 10). Between Farm and Trant's the drainage channel (now Farm River) is deflected sharply towards the east by a steep hillside that cuts across the flow path at right angles. Deposits on the hillside show that superelevation occurred. Using Equation 1 the velocity of the flows here was calculated to be 8 m s - 1 . At Trant's. deposits of pyroclastic flow pulse 2 had probablyfilled Farm River valley, because flow pulse 3 spilled out and
Fig. 7. (a) Oblique aerial view SW across Bethel village with Harris Hill on the right. Block-and-ash flow pulse 3 partly escaped from the Paradise River valley at the bend (far centre of photograph) and destroyed much of Bethel village. The flow remained confined within a shallow stream valley (block-rich deposits), but an associated surge spread across a much broader area. Trees in this area were still standing but badly damaged, (b) Damaged house partially buried by block-and-ash flow deposits on the coastal fan. Flow was from left to right (photos taken by P. D. Cole).
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Fig. 8. Oblique aerial view south across Bethel village with Pea Ghaut to the right showing block-and-ash flow deposits where flows escaped Paradise River valley. Part of block-and-ash flow pulse 3 spilled out of Pea Ghaut at the bend near the confluence with Tuitt's Ghaut (arrow 1 see also Fig. 7) and part at the next bend near Bethel (arrow 2). These flows escaped the drainage channel as a result of superelevation at bends. Note erosion of the outside of the bend in Pea Ghaut. Fine-grained surge deposits surround Bethel church and the house in the lower left foreground, where a family survived indoors (photo taken by S. C. Loughlin. BGS NERC).
inundated the remaining part of Trant's village (Fig. 11). Eyewitnesses observed several secondary pulses associated with flow pulse 3 (e.g. Fig. 1 11c). Pyroclastic flow pulse 3 was not observed on the video until 265 seconds after its onset, when the flow front was clearly visible approaching Trant's village. The poor visiblity up to this point was caused by the lingering ash cloud from flow pulse 2. It is estimated that the flow front had travelled 5.8 ± 0.1 km to that point and therefore that the average velocity was about 21.9ms - 1 (+3.2, -2.6). Assuming the body of the block-and-ash flow had a flow front velocity of 2 0 m s - 1 near Harris village (see above), its velocity on the upper volcano flank may have been much greater. The flow front decelerated in the final stages before it came to a halt beyond Trant's village, 6.7km from the dome. Knowing the velocity, the peak flux of flow pulse 3 can be calculated. If we assume (based on evidence of erosion and surge detachment), that at peak flux the block-and-ash flow filled Mosquito Ghaut from source to bend 'X' (Fig. 12) and the minimum velocity of block-and-ash flow pulse 3 was within the range 20-30 m s - 1 , then the peak flux would have been 6-8 x 1 0 4 m 3 s - 1 . Dynamics and effects of surges Trees on the north flank of the volcano were flattened by surges that detached from the block-and-ash flow in Mosquito Ghaut. There were three bends from which significant surge detachment
occurred along Mosquito Ghaut (shown in Fig. 12). Surge transport indicators on the eastern side of Mosquito Ghaut imply a northerly motion and the implication is that a surge detached from a gentle bend about 0.7 km from the dome (Fig. 12). It appears that this north-directed surge drained back into Mosquito Ghaut near its confluence with Paradise River (Fig. 1). The second bend is along the flow path about 1.1 km from the crater rim. where the direction of drainage changed from NW to north. The outside of the bend was scoured and there was no evidence that these eroded slopes were created by undercutting and collapse. It is assumed that the surge continued on a northwesterly trajectory across the north flank of the volcano. This is confirmed by scouring of trees on the eastern margin of Tyre's Ghaut to the west (Fig. 1); trees were scoured on their SE sides, suggesting that they were impacted by NW-directed surges. The major bend in Mosquito Ghaut ('X' in Figs 12, 13 and 14) is about 1.38km from the crater rim along the flow path. At this bend, the drainage swings from NNW to east and the outside of the bend was strongly scoured (Fig. 12). The surge that detached here impacted Streatham and Windy Hill, where transport direction was indicated by scouring and charring of standing trees and telegraph poles on the upcurrent side, bent reinforcement bars and by ash shadow zones behind some houses (see flow direction arrows on Fig. 1). A corrugated iron sheet was wrapped around a tree at the foot of Windy Hill (Fig. 15a). Most indicators suggested a flow direction towards the north and NW but in the headwaters of Dver's River, flow direction was to the west.
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Fig. 9. (a) Oblique aerial photograph showing several lobes from block-and-ash flow pulse 3 advancing through Spanish Point, (b) The front of a flow lobe (central lobe in (a)) following a shallow ditch at Spanish Point (photos taken by P. D. Cole).
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Fig. 10. Aerial view of the block-and-ash flow and surge deposits on the coastal plain. Pea Ghaut ran along the foot of the hills to the right but was filled with block-and-ash flow deposits. Part of Farm village was not buried and can be seen at the right margin of the flow deposits. At the very distal limit of the surge deposits, thin lobes extended at high angles to the flow direction. These were minor surge-derived pyroclastic flow deposits, probably formed by rapid suspended load deposition as the surge ran out of momentum and stalled (photo taken by G. E. Norton, BGS r NERC).
Surges detached at bends partly as a result of centrifugal forces when the block-and-ash flow changed direction; the continued motion of the surge in a straight line would be driven by momentum. However, these particular bends also corresponded with a constriction of the channel, a reduction in cross-sectional area and a decrease in slope from about 30° to 17° (Fig. 14). Deposits from flow pulses 1 and 2 would have accumulated in the shallow channel thus reducing its depth, and this, combined with the natural constriction, would have caused flow pulse 3 to bank up, thicken and fill the channel. The surge cloud would then have been unconfined and easily detached from the block-and-ash flow. Momentum carried the surge that detached at bend 'X' up Windy Hill to an elevation of 70m above the main road. Its velocity, in the vicinity of Farrell's Yard and Riley's Yard (Fig. 1), can be estimated at 3 7 m s - 1 by converting kinetic to potential energy: (2)
The calculation neglects frictional effects, but assumes that the base of the surge cloud rose 70m; the possible influence of surge cloud thickness on upslope sedimentation is not considered. On the upper flanks of the volcano the surges felled many trees (Fig. 12), but north of the main road the damaged trees remained standing (see fig. 9 in Loughlin et al. 2002). The ability of a surge to topple trees depends on the dynamic pressure (Valentine 1998): (3)
At Farrell's Yard and Riley's Yard (Fig. 1), leaves and some branches were stripped off trees but few trees were blown down. According to Valentine (1998), such light to moderate damage
requires dynamic overpressures of about 1000 Pa. Assuming this was the case, and assuming a velocity of 3 7 m s - 1 , the maximum surge density was therefore approximately 1 . 6 k g m - 3 . Valentine (1998) suggested a 90% tree blowdown, as observed on the proximal flanks of Mosquito Ghaut, requires dynamic overpressures of 2000 to 2400 Pa. Assuming the density of the surge was higher near source (a minimum of 1 . 6 k g m - 3 ) , the maximum proximal velocity required to produce the damage was 50-55 m s - 1 . Larger assumed average densities would require lesser surge velocities to achieve the same dynamic pressure. In reality, the vertical distribution of cloud densities is non-linear as discussed by Clarke & Voight (2000). Despite such complexities, it is clear that a rapid reduction in dynamic overpressure of surges occurred between the proximal region and Farrell's Yard. This was caused by the rapid decrease in velocity as a result of slope reduction, and a decrease in density as a result of rapid suspended-load fallout. A 6m diameter water tank was transported about 250m to the WNW, across the lower slopes of Windy Hill, and came to rest on top of an overturned car (Fig. 15a), implying that the dynamic pressure was still significant as the surge moved uphill. In Streatham, bitumen roof tiles were stripped off and some windows were cracked and broken, but structural damage to concrete block walls was minimal. Fires ignited inside concrete buildings with broken windows, but at the margins of the surge these fires did not necessarily spread throughout the house (see fig. 11 in Loughlin et al. 2002). It was hot enough to ignite wooden structures in the Streatham area including doors, rafters and huts. Most damage in Streatham and Windy Hill was caused by heat. The temperature of deposits in Dyer's River valley five days after the event was 410 C (Druitt et al. 2002), which is therefore the minimum temperature of the sediments in the surge. Eyewitnesses on Windy Hill described
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Fig. 11. Photographs from the main road above the airport showing the advance of pyroclastic flow pulse 3. (a) Pyroclastic flow pulse 3 emerged from the dissipating ash of flow 2 and spilled northwards out of Farm River, (b) The block-and-ash flow inundated Trant's village and spread across surrounding fields, (c) A further minor pulse escaped Trant's River and is clearly visible here at the break of slope (photos taken by R. B. Watts). how paint blistered on houses. Grass was singed in a sear zone 5 m were particularly prevalent in the Harris-Bethel area where slope was reduced and the flow slowed down. Hydrothermally altered blocks in the block-and-ash
flow deposits were probably entrained from the substrate (see Cole et al. 2002), but may have included blocks of dome rock altered by fumarolic activity. Slightly vesicular andesite blocks were present, particularly in the thin margins of the coastal block-and-ash flow deposits. Low-density debris, such as tree trunks and domestic gas canisters, were also abundant at flow margins. Grain-size analyses of three typical block-and-ash flow deposit samples show the poor sorting ( > 2.5) and the coarse nature of these deposits (median diameter < 1 ), compared to the other deposits (Fig. 16). Further accounts of block-and-ash flow deposits, including 25 June deposits, are given by Cole et al. (2002).
Pyroclastic surge deposits Extensive pyroclastic surge deposits (Fig. 1) defined a broad fanshape emanating northwards from the dome. Fine-grained surge deposits were also observed in the upper part of Tyre's Ghaut, a few hundred metres west of Mosquito Ghaut. These presumably were deposited by a surge cloud that expanded rapidly almost from source and surmounted an intervening ridge. Site 1 (Figs 1 and 17) shows two units of broadly equal thickness each comprising massive, fines-rich ash and lapilli. It is believed that each unit represents the surge deposit from flow pulses 2 and 3. A very thin layer of ash at the base may have been deposited from the poorly developed surge and/or lofting ash clouds associated with flow pulse 1. Alternatively, the ash layer may have been deposited from earlier pyroclastic flows, for example the flow on 17 June. The foot of Windy Hill (site 2; Figs 1 and 17) was affected only by a surge associated with block-and-ash flow pulse 3 and two layers were identified. The deposits varied in thickness, filling minor
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On the coastal plain, distinctive fine-grained deposits occurred at the distal margins of the block-and-ash flow lobes. They were typically up to 0.5m thick, thinning gradually towards the margins, and they lacked blocks or other coarse debris. They did not extend farther than 100 m beyond the flow deposits. It could be argued that these distal deposits were simply from fine-grained block-and-ash flows, the inevitable result of slow transport of fine-grained material, but the well sorted nature of the deposits suggests that they were surge-related.
Surge-derived pyroclastic flow deposits
Fig. 16. Median grain size (Md ) versus sorting ( =( 84- 16)/2) for the 25 June 1997 pyroclastic deposits. Tie lines link samples from the same location. Filled squares = block-and-ash flow deposits. Circles = fines-poor basal layer of the surge deposits. Open squares = fines-rich upper layer of the surge deposits. Triangles = surge-derived pyroclastic flow deposits.
depressions (e.g. furrows and hollows tens of centimetres deep) in the pre-existing topography. Sections here and at site 3 (Figs 1 and 17) typically showed a lower fines-poor layer of friable medium to coarse ash and lapilli that thickened into depressions and thinned onto highs. In ploughed fields this lower layer thinned and pinched out onto the ridges between furrows oriented approximately perpendicular to the flow direction. This layer could locally be subdivided further into a lower, finer-grained part rich in sheared soil clasts and vegetation fragments, and an upper, coarser part, rich in small fragments of dome rock (rare clasts up to 10cm). The upper layer was a brown, fines-rich ash layer of almost constant thickness that mantled the topography. Gas-escape structures were common within the upper layer (pipes of fines-poor ash and lapilli) and were usually rooted on fragments of carbonized organic debris or coarse clasts. Plumes of smoke and gases could be seen rising from small vents in the surge deposits in the days after the 25 June event, and were probably caused by burning vegetation. The deposits were typical of pyroclastic surge deposits formed following other dome collapses during this eruption (Cole et al. 2002). The upper fines-rich layer usually mantled the ground and normal grading of coarse clasts occurred locally (e.g. at site 3; Figs 1 and 17). Fines-poor samples had a median diameter (Md ) of -0.5 to 3 whereas fines-rich surge deposits had a Md of 2.5 to 4 (Fig. 16). Blocks
