Montserrat Volcano Observatory, Montserrat, West Indies

Scientific Report 67
21 June 1997

This report covers the 14-day period to 24:00 (local time) on 21 June.


Activity has remained high over the reporting period with rockfalls occurring from a 180o-wide sector of the dome. A slight shift in activity towards the west resulted in rockfalls and pyroclastic flows spilling over the northern crater rim into Mosquito Ghaut and Upper Fort Ghaut for the first time. A significant collapse of the dome on the 17 June resulted in pyroclastic flows travelling 4 km down Mosquito Ghaut and 2 km down Fort Ghaut.

Visual Observations

Observations of the dome have been severely hampered by low cloud. A clear view of the dome from Harris and Windy Hill on 8 June showed the north and north-east side of the dome to be active with rockfalls and small pyroclastic flows. Activity also continued on the east and south-east-east face of the dome with small slow moving pyroclastic flows with run-outs of < 1 km observed between 10 and 14 June. A slight increase in rockfall material was noted at the head of Mosquito Ghaut on 14 June and a helicopter inspection flight on 14 June revealed lobate pyroclastic flow deposits extending ~500 m down Mosquito Ghaut. A small chute had been cut into the crater rim and vegetation had been stripped from the peak B and C region immediately north of the crater rim. During the same flight fresh rock fall material was observed ~200 m down upper Fort Ghaut. Observations of the dome revealed a steep vertical face on the northern side, a blocky eastern flank and a collection of stubby spines on the summit. By 16 June pyroclastic flow activity down Mosquito Ghaut had cut a shallow chute extending upwards and to the west of the steep vertical face into the north-north-west side of the dome. Significant pyroclastic flow activity from the north-west face of the dome at 15:50 sent simultaneous pyroclastic flows over both Gages wall down upper Fort Ghaut and also down Mosquito Ghaut. Observations of these flows indicated near continuous pyroclastic flow formation with high velocity pulses down Mosquito Ghaut. An inspection flight immediately following this activity showed flows had reached 1.5 km down Fort Ghaut ~50 m before Gages Lower Soufriere. A dome collapse event began at ~23:00 on 17 June. A field team at the airport reported incandescence from the summit region with rockfalls occurring over the whole of the eastern face. Observers at the MVO reported large ash clouds being generated over the Windy Hill area from pyroclastic flows in Mosquito Ghaut. A particularly large ash cloud pulse was observed. A dawn inspection flight on the morning of 18 June revealed pyroclastic flow deposits to 4 km down Mosquito Ghaut and further flow deposits in Fort Ghaut extending to ~2 km from the Gages crater wall. Partial views of the dome above Mosquito Ghaut revealed a spoon-shaped collapse scar excavated into the northern flank of the dome. Examination of the deposits in Mosquito Ghaut indicate that the flows had a minor surge component except in the first 1 km where extensive tree knockdown occurred on the east side of the valley. A large singe zone extending 400 m west of Mosquito Ghaut had formed in the fields of Farrells estate. All vegetation was burnt and fine ash was deposited in the fields. This was probably a result of a co-ignimbrite cloud ramping over the lip of the ghaut as the flow hit a large bend in the valley. Temperature patches were fixed on trees at Farrells estate, Farrells target and at Harris. Further down Mosquito Ghaut absence of tree knockdown except at bends in the valley and a singe zone of only a few metres indicates that surges were poorly developed. The deposits were sampled on 19 June at the end of the flow below Harris and at Farrells. The flows were still highly fluidised and hot, and there were still several logs burning in the ash.

By 21 June, the continued rockfall activity had almost completely filled the chute above Mosquito Ghaut.


Seismically the whole two week period was dominated by rockfall and pyroclastic flow signals, and these corresponded to observed activity on the dome from the east face across to the north-west face. These events continued to be largely concentrated in pulses which occurred approximately every 20 hours. In the interval between the pulses rockfall activity continued at a much reduced level. A low level of hybrid and long period activity was observed throughout the period, with events of both types frequently triggering rockfalls. Events of these types were not concentrated within the pulses of high rockfall activity, but spread throughout the day.

Table 1: Earthquake types

These earthquake counts are of events that triggered the broadband network's event recording system between 0000 and 2400 each day (local time).

Date		VT	Hybrid	LP	Dome RF	LPRF*	HYRF*
08 June 97	0	14	14	107	10	8
09 June 97	0	11	26	148	17	6
10 June 97	0	14	22	136	15	8
11 June 97	1	22	13	130	6	12
12 June 97	0	16	27	129	12	11
13 June 97	0	11	14	109	10	2
14 June 97	0	11	4	109	0	7
15 June 97	0	8	2	158	1	5
16 June 97	6	2	4	173	3	2
17 June 97	0	5	6	187	6	4
18 June 97	0	8	8	216	4	4
19 June 97	0	0	5	176	3	0
20 June 97	0	3	7	174	4	3
21 June 97	0	9	10	159	7	9

* LPRF: LP earthquake followed by rockfall signal. HYRF: Hybrid earthquake followed by rockfall signal. The LPs, hybrids and rockfalls in these signals are also counted in their respective columns.

Ground Deformation

A shortened version of the GPS network EASTNET, including Harris, Windy Hill and Farrells, was occupied on 10 June which highlighted the movement of the Farrells site to the north-north-west. Farrells has moved almost 5 cm to the north since 14 December 1996 and, recently, has done so at an accelerating rate. A GPS occupation of Chances Peak triangulated with Harris Lookout (M18) and Windy Hill was carried out on 16 June and the general westward trend since the previous occupation on 4 June had continued. The Chances and Galway's cracks were measured on 16 June. Little change had occurred on the Chances crack since it was last measured on 8 June. However, there was 29 cm of sinistral shear on the Galway's crack since 3 May. Overall Chances and Galway's cracks indicate southerly movement of the block containing Galway's wall away from the dome.

Farrells site is now too dangerous to visit and so a fixed reflector has been installed there. It has been shot several times from Windy Hill but does not yet show any consistent trend.

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 end of the reporting period.

Dome Volume Measurements

Poor visibility hindered dome volume measurements, but some static photos were obtained on June 16 during a brief, partly clear window. Combined GPS/binocular surveys were carried out on the deposits in Tuitts Ghaut (9 June) and Mosquito Ghaut (20 June). The volume of deposits in Tuitts Ghaut was estimated to be approximately 420,000 cubic metres (340,000 cubic metres dense rock equivalent), and in Mosquito Ghaut the deposits were 810,000 cubic metres (650,000 cubic metres dense rock equivalent).

Environmental Monitoring

Rain water samples were collected from sites around the volcano on 10 June. The results are shown in Table 2. These values are nearly all within WHO standards for drinking water.

Table 2 Rainwater geochemistry 1 to 10 June 97

Location		pH	Cond.	TDS	Flrd	Chlrd	Slpht
				mS/cm	g/l	mg/l	mg/l	mg/l
Weekes 			6.52 	0.064	0.032	0.25	8	nd
Police HQ 		7.19	0.372	0.186	>1.50	*	*
Dyers 			7.72	0.412	0.206	>1.50	9	70
MVO 			7.30	0.060	0.030	0.60	5	nd
Molyneaux 		6.40	0.145	0.072	0.25	23.2	9

nd: not detected
*: no reagent available

Ash was collected from sites around the volcano on 9 June and 17-18 June. On 9 June, small accretionary lapilli were common at the Plymouth sites. The ash was widely distributed from Brodericks to Dyers with the thickest ash fall (2.5 mm) at Upper Amersham. A similar amount of ash was collected on 17-18 June although there was a significant coarse grained component (up to 5 mm) in samples close to the volcano.

Diffusion tubes measuring the averaged background SO2 since 1 June were collected on 18 June and have been sent to the UK for analysis. Further tubes were put out at Upper Amersham, Lower Amersham, Weekes, Police HQ and Whites Yard.

The miniCOSPEC is being used to measure SO2 flux from the volcano. Daily averages during the reporting period are shown in Table 3. These levels are high compared with the general background level for the eruption to date and reflect the high level of activity.

Table 3 Daily average SO2 flux using miniCOSPEC (tonnes/day)

Date	t/d
10 June	842 
11 June	839 
12 June	363 
14 June	442 
15 June	634 
16 June	409 
17 June	450 
19 June	618 
20 June	1171
21 June	921 

Staff Changes
Wilkie Balgobin (Seismic Research Unit)

Lloyd Lynch (Seismic Research Unit)

Dr Willy Aspinall is the current chief scientist.

Montserrat Volcano Observatory