Montserrat Volcano Observatory, Montserrat, West Indies

Special Report 03
Pyroclastic Flow Activity on 25 June 1997

MVO Staff
Montserrat Volcano Observatory
PO Box 292
West Indies
Tel: 1 664 491 5647
Fax: 1 664 491 2423

Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the Government of Montserrat.

  1. Introduction
  2. Precursive Activity in the Previous Weeks
  3. Events of 25 June 1997
  4. Post-event Activity
  5. Commentary
    Appendix 1: Time Line Summary
    Appendix 2: Emergency actions on 25 June
    List of Figures



At about 12:55 (all times within this report are local time) on Wednesday 25 June, 1997, a pyroclastic flow commenced from the lava dome in the crater of the Soufriere Hills volcano (Figure 1). In the following 25 minutes, a series of devastating flows swept the northern flanks of the volcano, down Mosquito Ghaut and followed the Paradise River almost to the sea. The flows and associated surge clouds damaged or destroyed between 100 to 150 houses, with the villages of Streatham, Dyers, Harris, Bethel, Bramble, Trants, Farms and Spanish Point being severely affected. At the time of writing 8 people are confirmed dead, and a further 11 are missing. Five people also suffered serious burns.

Special Report 03, Figure 1

Figure 1 - Small GIF - Larger GIF - Postscript

The pyroclastic flows were the largest produced during the current eruption, and the intensity of the activity exceeded that of the explosion of 17 September 1996. An estimated 4 to 5 million cubic metres of the lava dome were unloaded during the event, and the flows and surges covered an area of 4 square kilometres. An ash cloud rose to about 10 km, and ash fell over western Montserrat.

2. Precursory activity in the previous weeks


Rockfalls and rapid degradation of the north face began on 14 May after approximately two-and-half months of relative stability. The rockfalls intensified over a few days and by 19 May material had overspilled into the head of Tuitt's Ghaut. Pyroclastic flows into the northern ghauts began on 29 May with a minor flow into Tuitt's Ghaut. The size of subsequent flows into Tuitt's Ghaut increased in early June and the activity was followed by flows into Mosquito Ghaut and Gages Valley in mid-June (Figure 2). Many of the rock samples collected from the 17 June pyroclastic flow into Mosquito Ghaut were moderately vesicular and are interpreted to be juvenile material.

Special Report 03, Figure 2

Figure 2 - Small GIF - Larger GIF - Postscript

Observations in the week prior to 25 June showed that dome growth was concentrated in the summit area.

Unfortunately, the extrusion rate for June is poorly known due to low visibility, although it is likely to have been elevated because the volume of the dome as determined from a survey conducted at the end of May was higher than usual (65 million cubic metres; extrusion rate of 3.5 cubic metres per second). The dome had filled English's Crater, threatening the northern slopes of the volcano for the first time.


Hybrid earthquake swarms occurred during 13 to 27 May, with swarms comprising about 100 earthquakes per day of varied sizes. Unlike previous hybrids swarms , the activity did not consist of repetitive, identical events. Each swarm of earthquakes was followed immediately by a period of enhanced rockfall activity. When the earthquake swarms ended on 27 May, the pattern of regular periods of enhanced rockfall activity continued (Figure 3).

Special Report 03, Figure 3

Figure 3 - Small GIF - Larger GIF - Postscript

Following pyroclastic flow activity on 5 June in Tuitt's Ghaut the character of the seismicity changed slightly, with more long-period earthquakes recorded. The number of earthquakes remained quite low, not exceeding 40 per day, and the long-period earthquakes returned to normal levels after 13 June.

Hybrid seismicity restarted suddenly on the morning of 22 June. This followed a moderate pyroclastic flow in the Tar River valley, and a small swarm of volcano-tectonic earthquakes. VT earthquakes have been rare in recent months, usually occurring in single swarms.

There were 7 hybrid swarms between 22 and 25 June. The swarms gradually increased in duration and numbers of earthquakes. Within each swarm, the earthquakes were repetitive and had generally similar magnitudes, with a few larger earthquakes in each swarm. The maximum magnitudes were relatively small; much higher magnitudes had been recorded previously. The swarms on 24 and 25 June showed increasing intensity, reaching a state where repetitive events merged into continuous tremor that was difficult to distinguish from rockfall signals on the drum records.

The tendency for enhanced rockfall activity at the end of the hybrid earthquake swarms continued, although the intensity of these events generally decreased.


Deformation monitoring at the Soufriere Hills Volcano by the MVO involves a combination of Total Station measurements (combined electronic distance measurement (EDM) and theodolite) and global positioning system (GPS). Crack dimensions in the crater walls are monitored by frequent measurements between fixed points on opposite sides of the cracks. Continuous telemetered data are collected from two tiltmeters and one extensometer installed on Chances Peak and a tiltmeter in Long Ground.

Deformation of the northern crater wall was first observed in early March 1997 by GPS surveys. The FT3 station on the crater wall (Peak C on Figure 2) had moved 15 cm to the north-west between 13 January and 3 March. Subsequently, more regular monitoring indicated continued motion to the north-west, with the total displacement reaching 21.5 cm by 12 May (Figure 4a) after which the site was considered too dangerous to visit. GPS occupations on the summit of Chances Peak (Figure 2) since July 1996 have also shown sustained motion away from the dome. Total displacement is currently 16 cm.

Special Report 03, Figure 4

Figure 4 - Small GIF - Larger GIF - Postscript

An EDM/GPS station at Farrells has shown slow movement to the north away from the dome complex (Figures 4b and 4c). Shortening of 5 cm occurred in the early stages of the eruption (up to 30 November 1995). Through 1996 two lengthening and shortening cycles developed, but since Dec 1996 movement has been sustained shortening on baselines to Windy Hill and Harris. The shortening has been at an increasing rate until the last measurement on 10 June. Long Ground underwent a slow eastwards movement of around 30 mm up to late April, after which it underwent a relatively abrupt displacement to the north-north-east by 25 mm. Displacement vectors for Farrells and points on the crater wall have been reported in the MVO Scientific Reports series (e.g., Scientific Report 64).

Crack measurements have been made on Chances Peak and Galway's Mountain since 4 December 1996 and March 25, 1997, respectively. The Chances Peak cracks showed rapid shear in early December (crack I only), mid-February and through May (Figure 4d). The total shear is currently 31 cm on Chances crack II. The Galways crack sheared by 29 cm between 3 May and 16 June. The sense of movement indicates displacement of the Galway's Wall region away from the dome complex and demonstrates the extreme stress placed on the crater walls in an area of the dome that showed no surface activity at the time.

Prior to 16 June the Chances Peak tiltmeter showed a regular pattern of inflation and subsequent deflation, directed at the dome, with a period between 12 and 16 hours and an amplitude between 18 and 25 microradians.

From early morning 16 June to late evening 17 June, the inflation-deflation cycle flattened to an amplitude between 5 and 10 microradians. At approximately 16:00 on 17 June, the inflation increased steeply, peaked at 21:00, and then rapidly deflated. This deflation preceded a collapse at 23:30 that sent pyroclastic flows 2 km down Gages Valley and 3.5 km down Mosquito Ghaut. The pre-16 June pattern returned for approximately 1.5 days after this event. On 19 June, the flattened pattern returned and persisted until the morning of 22 June.

At 05:30 on 22 June, a sharp increase in the rate of inflation occurred. Subsequent, sharp deflation at 06:30 was coincident with sustained pyroclastic flows which travelled approximately 1 km down the Tar River Valley. This event marked the beginning of a new pattern in the inflation-deflation cycles. The periodicity of the cycles shortened to 8 hours. and the amplitude increased to approximately 40 microradians. The change was accompanied by a short volcano-tectonic earthquake swarm which preceded the resumption of hybrid earthquake activity (see above). The number of hybrid earthquakes varied nearly exactly in phase with the inflation-deflation cycle, with the maximum number of hybrids occurring at the peak inflation.

Following the 25 June pyroclastic flow activity, the inflation-deflation cycle continued with the same period and amplitude which began 22 June until 5 July. Thereafter the amplitude decreased and the period increased until there was no discernible cycle after 12 July. No major pyroclastic flows occurred after 5 July (until the time of writing, 27 July).

The overall trend of the tiltmeter, prior to 25 June, showed a general inflation to the north or deflation to the south. Since 25 June, the trend shows a general deflation toward the centre of the dome.

3. Events of 25 June 1997

The hours prior to the Event:

At 03:00 a hybrid earthquake swarm began, which was similar in character to the swarms of the previous four days (Figure 5). At the maximum intensity, 4 to 5 events occurred per minute. The earthquakes were of moderate amplitude, reaching saturation on the Gages and Windy Hill drum records. The Chances Peak tiltmeter continued the previous pattern, with relative inflation of the crater area accompanying the hybrid swarm (Figure 5). The tilt levelled out at 05:20, and the volcano started to deflate at about 06:10. The swarm diminished gradually after about 06:15, with the hybrids giving way to low-level tremor at 07:05.

Special Report 03, Figure 5

Figure 5 - Small GIF - Larger GIF - Postscript

Rock falls and minor pyroclastic flows commenced, fitting the established pattern. Between 06:15 and 07:15 the activity was dominated by semi-continuous pyroclastic flows travelling down Mosquito Ghaut with run outs of ~ 1 km. There were also simultaneous rockfalls and small pyroclastic flows from the south-east and east face of the dome.

Re-inflation of the dome area began at approximately 09:00. The seismic activity remained at low levels until 10:50, at which time a second hybrid swarm started. The intensity of this swarm escalated rapidly, reaching about 6 events per minute between 11:30 and 12:30. The earthquake amplitudes were uniform, and similar to those in the earlier swarm. At 12:00 the inflation trend peaked. By 12:45 the seismic record was dominated by tremor, and hybrid earthquakes were barely discernible. A dilute steam and ash cloud issued from the summit area, which was carried to the west by prevailing winds at an approximate altitude of 4,500 ft.

The Main Event:

Between 12:40 and 12:50 the tiltmeter registered the start of a sharp deflation. A strong seismic signal began at 12:55, with intensified pulses of activity at 12:57 and 13:00. At about 13:00, a dense, dark ash cloud rose vertically from the north flank of dome above Mosquito Ghaut. The cloud reached 30,000 feet in a matter of minutes. At 13:03 the eastern stations of the seismic network stopped transmitting data, because the Bethel telephone exchange (and/or the phone line across the central corridor) was destroyed by a pyroclastic flow travelling down Mosquito Ghaut. There was a third pulse of seismic activity at 13:08.

MVO staff positioned north of the airport witnessed the front of the flow coming around the bend at Pea Ghaut, just up-slope of Trant's village. At 13:15 MVO observers flying over the airport found that the initial pulse had overrun the lower part of Harris, Farm and Trant's, and had come to within 50 m of the sea. They also reported a final pulse coming down Paradise Ghaut and surges continuing to spread slowly westwards in the Spanish Point area. The final pulse of activity advanced at approximately 30 m/s across the flat land near Trants, and was captured on film by a time-lapse video recorder at the airport control tower.

Observations of the deposits and destruction of the area (Figure 6) include the following points:

  1. The main part of the flow in Mosquito Ghaut caused intense scouring to the top of the steep valley walls particularly on the outside of bends, suggesting that dense, coarse pyroclastic flows nearly filled the ghaut during transport. The scouring, however, did not extend over the lip of the ghaut. The deposits are not extensive in the upper part of Mosquito Ghaut but generally thicken towards the lower end where Mosquito meets Paradise Ghaut.
  2. Flow deposits ponded to a significant thickness, completely filling Pea Ghaut and forming a thick, broad fan emerging north-west from Paradise Ghaut just north of Bethel. Houses 200 m from the edge of the fan are completely buried.
  3. Of particular note is a separate lobe of coarse pyroclastic flow material which emanated over the lip of Paradise Ghaut immediately west of Bethel. Blocks within this coarse lobe are up to 5 m in size and caused widespread destruction to houses in Bethel village. This is the only area where a high concentration of coarse material spilled out of the main ghauts.
  4. As the pyroclastic flows emerged from the gap between peaks B and C into Mosquito Ghaut, fine-grained pyroclastic surges spread laterally onto the ridges on either side. These surges extended as far east as Paradise Estate, went northward to within 250 m of Windy Hill, inundated the entire village of Streatham, and covered to the west as far as Gun Hill. They broke and flattened trees on the ridges in the Farrell's and Paradise area. The surges did not spill into Tuitt's Ghaut to the east, but in one or two points drained into the unnamed ghaut to the west. In Streatham, trees were generally not broken or flattened and charring of trees and telegraph poles was limited to the east-south-east side. The orientation of charring, shadow zones behind a few of the houses, and the transport of a water tank in Streatham indicate surge movement in this area was to the west-north-west. Blocks above 1m in size are rare in the Farrell's area, although occasional ~0.5 m blocks are present on Farrell's road. The deposits indicate that drainage of flow material into the Dyers river occurred largely in the narrow area south of Gun Hill and west of Riley's Yard.
  5. Pyroclastic flows extended into the Belham valley travelling as far as the last of the tight bends in the valley before Cork Hill. The flow front is marked by a pile of logs oriented perpendicular to the valley axis. Vegetation destruction is limited, with most trees remaining standing even near the base of the valley. Deposits along the whole length of the Belham valley are notably fine-grained, and coarse blocks are nearly absent. In addition, two small concrete bridges were left intact at the base of the valley. This indicates that the flows which moved down the Dyers river and Belham valley were fed by relatively fine-grained pyroclastic surges which detached north-westwards from the main flow in Mosquito Ghaut.
  6. Samples of the deposit collected in the Farm River area and Spanish Point include both dense and moderately vesicular lithologies.

Special Report 03, Figure 6

Figure 6 - Small GIF - Larger GIF - Postscript

The elevated seismic signal persisted until 13:18, and the large deflation recorded by the tiltmeter bottomed out at 14:30. Low amplitude tremor with hybrid earthquakes continued until 15:00, at which time the seismicity dropped to background levels.

The RSAM peak for the event lasted for 30 minutes, indicating shorter but more intense activity relative to the explosion of 17 September 1996. More than 4 square kilometres were inundated by pyroclastic flows and ash surge deposits. The estimated 4 to 5 million cubic metres of material unloaded during the event left a steeply-dipping, circular scar roughly 200 m across in the north-north-west face of the dome. There was no evidence of explosive activity on 25 June. Ash fall was limited to areas west and north-west of the volcano. Maximum accumulations reached 2 mm.

4. Post-event activity

Following the end of the pyroclastic flows, the level of seismic activity remained low for several hours. However, the tiltmeter showed another inflation which started at 20:00, accompanied by a small swarm of hybrid earthquakes. In subsequent days, the inflation and deflation pattern continued, earthquake swarms became more intense and there was further pyroclastic flow activity in Mosquito Ghaut and Gages valley.

Brief views of the dome on 28 June indicated that a large part of the existing dome had been removed during the pyroclastic flow activity, and fresh dome growth was occurring within the scar at a rapid rate.

Small explosions on 27 June and into early July caused concern that the level of activity was possibly still escalating. Pyroclastic flows continued down Mosquito Ghaut and Fort Ghaut into early July, but the frequency and run-out distances of flows decreased until after 13 July when only small rockfalls occurred.

5. Commentary

The Soufriere Hills volcano was highly active for several weeks prior to the 25 June event, with dome collapse over the crater wall threatening the northern slopes of the volcano for the first time. The repetitive hybrid earthquake swarms and inflation-deflation cycles suggested that the rate of dome growth was high, and the conduit pressure was elevated. A large event was not a surprise, and the effects of the pyroclastic flow were largely as anticipated by the hazard zonation (Figure 7) and warnings issued in MVO reports throughout June (MVO daily reports).

Figure 7a Figure 7b Figure 7c
Updated February 1997 Updated 6th June 1997 Updated 24th June 1997
Small GIF - Larger GIF - Postscript Small GIF - Larger GIF - Postscript Small GIF - Larger GIF - Postscript
Moving from Zones G to A represents an increasing risk, based on an evaluation of the volcanic hazard. The status of each zone is dependent on the alert level. Potential hazards include pyroclastic flows, surges, falling rocks, mud flows and ash fall.

The surge into Dyer's Ghaut and the Belham River valley was remarkable, in that a relatively fine-grained, low volume (approximately 100,000 cubic metres) flow travelled a significant distance from the main flow path.

It is interesting that the activity continued at a high level in the days following the June 25 event. This contrasts with previous periods of enhanced activity at the Soufriere Hills volcano, in which significant dome collapse events have normally been followed by a respite in activity and a change in the eruption pattern.

Appendix 1: Time line summary

Appendix 1: Time line summary

3 March Displacement of point in north crater wall (FT3) of 15 cm since 13 Jan.

12 May Displacement of FT3 of 21.5 cm since 13 Jan.

14 May Beginning of rock fall activity on north face of dome.

19 May First rockfall spills into Tuitt's Ghaut.

29 May First pyroclastic flow activity in northern ghauts

2 June Pyroclastic flow down Tuitt's Ghaut travelling 1 km from the crater.

3 June Pyroclastic flow in Tuitt's Ghaut travelling 1.4 km.

4 June Pyroclastic flow in Tuitt's Ghaut travelling 1.8 km.

5 June Pyroclastic flow in Tuitt's Ghaut travelling 2.9 km, reaching to
250 m above where the ghaut joins Paradise River.

7-14 June Rockfall and pyroclastic flow activity concentrated in Tuitt's

15 June Significant changes in Mosquito Ghaut. Pyroclastic flow material
advanced 500 m down the ghaut. Debris from a small rockfall in Gage's Valley.

16 June Pyroclastic flows in Gage's Valley travelling 1.6 km from the crater 
rim. Smaller pyroclastic flows occurred in Mosquito and Tuitt's Ghaut.

17 June Significant pyroclastic flow activity in Mosquito Ghaut to a
distance of 4 km. Lesser activity in Gage's Valley with pyroclastic
flows travelling 200m further than previously.

22 June Moderate pyroclastic flow in Tar River followed by a short
volcano-tectonic earthquake swarm and resumption of hybrid earthquake
swarm activity

24 June Small pyroclastic flows down Mosquito Ghaut for the first time
since 17 June, travelling 1 km from the crater rim. Focus of dome growth
confirmed at top of Mosquito Ghaut.

25 June
	03:00 Start of hybrid earthquake swarm
	06:00 to 08:00 Deflation accompanied by small pyroclastic flows in Mosquito Ghaut
	10:50 Start of hybrid earthquake swarm
	12:00 Crater inflation peaked
	12:45 Volcanic tremor, steam and ash production
	12:55 Start of pyroclastic flow activity
	12:57 First pulse in seismic activity
	13:00 Second pulse in seismic activity
	c. 13:00 First pyroclastic flow observed in Mosquito Ghaut from MVO
	13:02:12 First flow seen from airport around Harris Hill (didn't get much further than Farms)
	13:03 Seismic signals from eastern stations lost
	13:07:40 Second pulse seen from airport (probably reached furthest run-out distance)
	13:08 Third pulse in seismic activity
	13:13:49 Third pulse seen from airport (distinct pulse of co-ignimbrite ash through cloud of second pulse)
	13:20 End of seismic activity

Appendix 2: Emergency actions on 25 June

07:00 to 08:00 Heightened concerns about possible pyroclastic flow
activity. MVO observer at airport alerted. Police HQ contacted, advised
to keep checkpoint to Plymouth closed for the time being (it later
transpired that essential services were already working in Plymouth,
without the knowledge of the duty scientist). MVO observers deployed at
Windy Hill to observe small pyroclastic flows in Mosquito Ghaut.

08:20 Police Commissioner advised that checkpoints into Plymouth can
open, and essential services allowed access.

12:30 Police Commissioner visits MVO, briefed by Chief Scientist (CS).
Check points into Plymouth closed.

12:45 Deputy Chief Scientist (DCS) arrives at MVO. CS / DCS discuss
situation. Duty scientist at airport put on alert.

12:45 to 12:55 Essential services in Plymouth advised to evacuate. Port
Authority, Monlec, Montserrat Mills contacted successfully but direct
contact with Texaco fuel tanker not made (the tanker later cut the fuel
lines to the port and successfully withdrew). Plymouth siren sounded.
Live update of situation broadcast on Radio Montserrat. Field team at
Whites advised to speed up operation. Field team requests permission to
enter central corridor area, refused by CS and advised to withdraw to

12:55 Start of pyroclastic flow activity

c. 12:58 Phased evacuation of airport recommended by MVO.

c. 13:00 Immediate evacuation of airport initiated. Staff clear of
airport in less than 5 minutes, plane on ground takes off. HE the
Governor at airport, accompanies MVO scientists to safe viewing position
north of the airport.

c. 13:00 Acting Governor, Chief Minister, Minister for Education and
Health, Minister for Agriculture, Trade and Environment arrive at MVO
for briefing.

c. 13:00 Two MVO staff members take off from Vue Pointe in helicopter.

c. 13:10 All MVO staff accounted for and confirmed safe.

c. 13:15 Acting Governor, etc. leave MVO for Emergency Operations Centre
(EOC), accompanied by DCS and Dr Baxter.

13:16 Helicopter reconnaissance over the airport picks up survivors in
the Trants area.

13:20 Ash advisory issued by MVO. Due to phone line problems, fax
reached only 2 of 5 intended recipients.

14:00 Helicopter searches for survivors in Spanish Point area.

14:45 Survivors sighted in Harris, evacuation initiated by MVO

15:06 Search and Rescue team approaching from west by road report hot
ash at Dyers.

15:14 Hot ash reported by Police in Bellham Valley near Cork Hill

16:00 CS briefs authorities at EOC of current situation

The figures are available in a small image format (usually 600x400 GIF), a large image format (usually 1200x800 GIF), and Postscript

Montserrat Volcano Observatory