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Report on Soufriere Hills (United Kingdom) — June 1996

Soufriere Hills

Bulletin of the Global Volcanism Network, vol. 21, no. 6 (June 1996)
Managing Editor: Richard Wunderman.

Soufriere Hills (United Kingdom) Dome growth continues

Please cite this report as:

Global Volcanism Program, 1996. Report on Soufriere Hills (United Kingdom) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 21:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199606-360050

Soufriere Hills

United Kingdom

16.72°N, 62.18°W; summit elev. 915 m

All times are local (unless otherwise noted)

The current eruption, which began on 18 July 1995 (BGVN 20:06), started extruding a lava dome on about 16 November 1995 (BGVN 20:11/12). What follows condenses Montserrat Volcano Observatory (MVO) Scientific Reports for the weeks ending 5 and 12 June and Daily Reports for the rest of June.

On 31 May, and 1 and 15 June, pyroclastic flows progressed several kilometers E; one to within 300 m of the sea. Associated plumes reached up to 3 km altitude. Persistent dome growth continued in June. Its talus filled the W moat, allowing rockfalls to begin escaping the crater, but none reached farther than 100 m beyond the rim. Summaries are included for visual observations, seismic observations, daily seismic event counts, and SO2 flux (tables 5, 6, 7, and 8).

Table 5. Chronology of visual observations at Soufriere Hills, Montserrat, late May through early June 1996. Poor weather conditions often prevented observations. Dated events after 12 June refer to 24-hour intervals beginning at 1600 the previous day. Courtesy of MVO.

Date Visual Observations
30 May 1996 Rockfalls concentrated on N and NE; vigorous steaming from many parts of the dome.
31 May 1996 Rockfalls mainly concentrated on N and E, but some traveled into the S moat. Two pyroclastic flows down the Tar River; the first progressed to within 300 m of the sea; the second, to well past the Tar River Soufriere. Associated ash clouds rose to 2-3 km and were blown NW. A large S-directed rockfall escaped the S crater but progressed less than another 100 m.
01 Jun 1996 Rockfalls concentrated on the dome's NE, E, and S. Small pyroclastic flow to the E (into the upper Tar River Valley); the associated plume rose to ~2 km.
02 Jun 1996 Small "whale back" extruded on the dome's E flank, just N of Castle Peak.
04 Jun 1996 Although the E dome was quiet, growth was indicated on the S and W sides. No spines were observed; instead the dome's top appeared comparatively rounded.
09 Jun 1996 Intense incandescent spots coincided with the source areas for rockfalls. New spines in the W summit area. Night incandescence and associated rockfalls; rockfalls on the dome's NE flank to its W flank.
10 Jun 1996 One of the spines seen on 9 June had fallen over. The dome overtopped the W crater rim at Gages Wall and debris began to travel into the uppermost reaches of Fort Ghaut. Dome growth indicated on the NE.
11 Jun 1996 Dome growth indicated on the NW side.
12 Jun 1996 Estimated height of the new dome's summit was 943 m.
13 Jun 1996 Having filled the moat to ~25-m depth, talus spilled out of the partly buried crater for ~100 m into the upper Gages Valley.
14 Jun 1996 During heavy rainfall, ash erupted and fell NW of the volcano (in the St George's Hill, Cork Hill, and Old Towne areas). Rockfalls were abundant; one ash emission during the afternoon was semi-continuous and lasted about an hour.
15 Jun 1996 A small (~300-m-long) pyroclastic flow S of Castle Peak; fresh deposits from two others in the upper Tar River valley (on the dome's E and NE flanks).
16 Jun 1996 Rockfalls on the dome's NE, N, SE, and W sides. On the W it was unclear how much new material escaped the crater along the upper Fort Ghaut.
17 Jun 1996 Very little new material had traveled down the upper W slopes into the upper Fort Ghaut. Talus on the dome's N side had built up to ~15 m below the rim of Farells wall. A projection along the new dome's summit measured 942 m elevation.
26 Jun 1996 Dome appeared wet and issued heavy steam from the SE flank. A stubby spine was on the NE summit.
30 Jun 1996 Dome rockfalls relatively rare but considerable new material had been deposited on the dome's E sector, in the Tar river's upper S fork. The dome's most rapid growth appeared to be on the S. Little new mass wasting on the W (down Fort Ghaut); however, steam production was concentrated in the W moat area.
04 Jul 1996 Viewed a large slab of extruded lava at the top of the SE dome. A few blocks of fresh dome lava lay in the dome's new W drainage (the upper Fort Ghaut).

Table 6. Chronology of seismically derived observations of volcanic activity at Soufriere Hills, Montserrat, 30 May through July 1996. Dated events after 12 June refer to 24-hour intervals beginning at 1600 the previous day. Courtesy of MVO.

Date Seismic Observations
30 May-03 Jun 1996 Swarm of small hybrid earthquakes of variable amplitude, occurring at the rate of 0.5-2 events/minute.
30 May 1996 Volcano-tectonic earthquake 750 m beneath the crater.
31 May-03 Jun 1996 Somewhat elevated tremor; volcano-tectonic earthquake 1.5 km beneath the crater.
01 Jun 1996 Volcano-tectonic earthquake 2 km beneath the crater.
03 Jun 1996 One ~2-hour continuous tremor episode and a second 5-hour episode running into 4 June.
05 Jun 1996 Interpreted small, local mudflow in upper Fort Ghaut.
06 Jun 1996 Sediment-laden flood towards the W (Plymouth) down Fort Ghaut.
09 Jun 1996 Areas of incandescence seen associated with rockfalls.
13 Jun 1996 Very early in the morning the Gages and Chances Peak seismic stations started to record a few small repetitive hybrid events; these slowly increased in number and were occurring at a rate of ~1 every two minutes by the end of the reporting period.
15 Jun 1996 During sustained heavy rainfall, small hybrid events appeared prior to abundant rockfalls. These hybrid events grew from ~1/minute to 5/minute and their amplitudes doubled before they decreased in number and size. Larger rockfall signals looked similar to signals from small pyroclastic flows.

Table 7. Daily counts of seismic events at Soufriere Hills, Montserrat, 30 May-1 July 1996. The amount of tremor is described qualitatively (high-low) or using analysis from the Daily Reports (particularly after 12 June). Dated events after 12 June refer to 24-hour intervals beginning at 1600 the previous day. Courtesy of MVO.

Date Volcano-tectonic Long-period Hybrid Rockfall Amount of Tremor
30 May 1996 1 5 0 17 Low to intermediate (3 hours of continuous tremor)
31 May 1996 1 14 96 97 Intermediate to high
01 Jun 1996 1 0 307 116 Intermediate to high
02 Jun 1996 0 0 132 83 Intermediate to high
03 Jun 1996 0 1 19 32 Intermediate to high
04 Jun 1996 0 5 18 51 Low to intermediate
05 Jun 1996 0 17 8 57 Low to intermediate
06 Jun 1996 0 13 4 49 Low to intermediate
07 Jun 1996 0 0 1 13 Low to intermediate
08 Jun 1996 0 0 1 51 Low to intermediate
09 Jun 1996 0 3 1 54 Low to intermediate
10 Jun 1996 1 15 2 54 Low to intermediate
11 Jun 1996 0 12 5 87 Low to intermediate
12 Jun 1996 0 2 1 59 Intermediate
13 Jun 1996 1 2 (tab 4) 39 5.5 hours
14 Jun 1996 -- 12 25 34 --
15 Jun 1996 -- -- (tab 4) 198 --
16 Jun 1996 -- 15 21 149 1 hour
17 Jun 1996 -- 16 2 49 12 hours
18 Jun 1996 1 13 0 81 --
19 Jun 1996 -- 7 8 92 --
20 Jun 1996 -- 1 7 63 --
21 Jun 1996 -- 3 5 53 6.6 hours
22 Jun 1996 -- 5 0 28 Low
23 Jun 1996 -- 1 4 60 7 hours
24 Jun 1996 -- 29 3 62 Very low
25 Jun 1996 -- 7 2 37 Low
26 Jun 1996 -- 4 6 37 --
27 Jun 1996 -- 6 7 59 --
28 Jun 1996 2 14 11 66 --
29 Jun 1996 1 4 4 55 --
30 Jun 1996 -- -- ~55 -- --
01 Jul 1996 2 11 12 57 4 hours

Table 8. COSPEC measurements of SO2 flux at Soufriere Hills. Courtesy of MVO.

Date Number of measurements SO2 flux (metric tons/day)
30 May 1996 2 224
31 May 1996 2 88
01 Jun 1996 3 52
02 Jun 1996 6 193
03 Jun 1996 3 192
04 Jun 1996 4 240
05 Jun 1996 7 194
07 Jun 1996 3 84
08 Jun 1996 5 269
09 Jun 1996 4 126
10 Jun 1996 5 59
12 Jun 1996 5 168
15 Jun 1996 -- 135
17 Jun 1996 6 125
19 Jun 1996 -- 170
24 Jun 1996 -- 76
28 Jun 1996 -- 160

During the week ending 5 June gas measurements using a Fourier Transform Infrared Spectrometer showed substantial errors (50-100%), but did establish that at ground level on the lower slopes of the volcano (excluding Upper Amersham), the ambient concentrations of HCl and SO2 in the air were well below 100 ppb. The SO2:HCl ratio was generally well below 1.0. The only exception to this, on 24 May, was when the measurement errors were large. The SO2:HCl ratios could be used to make an argument about status of the magma chamber. Assuming that this ground-level SO2:HCl ratio was the same as in the plume, then the low ratios measured would indicate a moderately degassed magma chamber.

During 6-12 June, ash generation was generally low and few ash clouds emerged from the crater area; seismically detected rockfalls decreased with respect to the previous week and although no swarm of hybrid earthquakes occurred as in the previous week, the abundance of long-period earthquakes did appear similar to the previous few weeks.

EDM deformation measurements often detected an overall shortening rate along survey lines of ~1 mm/day during the interval from the beginning of December through the end of April. During 1 May-4 June there was a shortening rate of 2.4 and 2.1 mm/day in the volcano's N region (White's and Long Ground respectively); however, the shortening rate subsequently returned to ~1 mm/day.

On 12 June it was noted that during the previous 7-day interval, a ~12 mm/day shortening rate occurred on the N line (Long Ground to Castle Peak). In contrast, during this interval the W and N triangles continued to show no changes in line length above the error of the method.

On 13 June the E triangle was remeasured; its previous measurement was on 11 June: the Long Ground to Castle Peak line shortened by 9 mm and the Whites to Castle Peak line shortened by 6 mm. On 15 June it was reported that the W triangle's line lengths had recently shortened by ~1 mm/day. On 19 June it was found that NE sector (White's and Long Ground to Castle Peak) line lengths shortened by 2.5 cm over 5 days; this result extended a 3-week trend of 3-5 mm/day shortening here. Despite these larger than typical deformations in the NE sector during June, during the same month it was reported that the tiltmeters at Long Ground had remained stable for the past 10 months.

Scientists also noted that by June considerable new dome lava and talus had piled against the crater's W wall. Still, the June EDM surveys failed to show corresponding movement in this portion of the older edifice.

Geological Summary. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.mvo.ms/).