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Report on Soufriere Hills (United Kingdom) — May 1996

Soufriere Hills

Bulletin of the Global Volcanism Network, vol. 21, no. 5 (May 1996)
Managing Editor: Richard Wunderman.

Soufriere Hills (United Kingdom) Dome growth and evacuation continue in May

Please cite this report as:

Global Volcanism Program, 1996. Report on Soufriere Hills (United Kingdom) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 21:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199605-360050

Soufriere Hills

United Kingdom

16.72°N, 62.18°W; summit elev. 915 m

All times are local (unless otherwise noted)

During May the dome's growth continued, accompanied by small intermittent pyroclastic flows and minor ashfalls that were mostly thought to be generated by rockfalls. Although activity during the first week of May appeared similar to the final week of April, visibility became poor after 5 May. When visible, the dome's new growth was manifested in rapid increases of summit elevation (on 19 April, 865 m; on 30 April, 896 m; on 2 May, 898 m; on 3 May, 909 m). This was followed by an apparent 2-m decrease (i.e. on 4 May, 907 m). Many rockfalls took place on the dome's NE and E flanks. Throughout early May small ash clouds repeatedly blew W depositing very small amounts of ash in the Upper Gages and Amersham areas.

Activity was characterized as slightly less elevated during the second week of May. However, visual observations on 11 May indicated that a small pyroclastic flow had traveled 300 m E of the base of old Castle Peak dome (into the Upper Tar River Valley passing just S of the path of the 3 April pyroclastic flows). Although this flow had set fire to some trees, no significant changes were observed, and small ash clouds again blew W depositing minor ash in the Upper Gages, Amersham, and Fort Barrington areas.

On 12 May the dome area discharged abnormally large ash clouds associated with at least three pyroclastic flows E of the crater down the Tar River. Relatively large ashfalls also took place in the WNW-NW sector at least as far as the coastal area (Fox's Bay). In some places the ashfall reached a maximum thickness of 3 mm. These ashfalls were reported in parts of southern and central Montserrat (including the settlements of Farrell's, Rileys, Windy Hill, Gages, Lees, St. George's Hill, Fox's Bay, Richmond Hill, Garibaldi Hill, Ile Bay, Old Towne, and Salem). Areas affected also included some settlements in the designated safe zone in the N part of the 13-km-long island (including Cork Hill, Weekes', Olveston, and Barzey's) and small amounts of ash fell in the volcano's E sector (Tar River, Long Ground, and Whites).

The 12 May episode began at about 0630 when near-continuous rockfalls took place on the dome's E flank lasting until about 0720. From 0720 to 0945 the rockfalls became intermittent and small but they still produced ash clouds. A further increase in activity produced pyroclastic flows that were seen in the Tar River Valley at around 0945, 0952, 1105 and 1153. The ones at 0945 and 1105 advanced more than 30 m over the sea; the one at 1153 stopped just short of the sea. Activity declined after about 1220 but small-to-moderate rockfalls continued intermittently.

The 12 May pyroclastic flows did not damage any structures but trees were set ablaze in the Tar River Valley area. Excellent views were obtained of the pyroclastic flows.

On 13 May, light ashfalls blew across the volcano's W and SW sectors. On 15 May small ash clouds again blew W; views then suggested that most of the rockfalls producing the ash came from the NE flank of the dome. In addition, on 15 May moderate amounts of steam escaped from the base of the dome's N side; at other times during the second week of May steam mainly escaped from the SW moat.

Rockfalls were especially abundant on 16 and 22 May. In addition, one on 19 May generated an ash plume that reportedly reached an altitude of about 1.2 km. Another on 20 May was associated with a small pyroclastic flow that traveled ~2 km NE of Chances Peak down the Upper Tar River Valley (as far as Hermitage).

Visibility was generally poor for most of the third week of May allowing only brief views into the crater to establish the dome's main areas of growth on the N and NE flanks. When visibility improved on 20 May, nine days after the previous observation on 11 May, the dome contained several smaller spines and a large broad spine at the top. The large spine rose ~20 m and leaned slightly NE. Observers saw no morphological clues for the source of the 12 May pyroclastic flows, possibly because any topographic signs may have been erased by mass wasting during the intervening week. During brief observations from a helicopter, rockfalls mainly cascaded down the dome's N and NE flanks; fewer came down the vigorously steaming SE flank. Very poor visibility returned on 21 and 22 May.

During the week ending on 29 May, visibility gradually improved allowing remote measurement of 200-250°C dome surface temperatures. Observers on 24 May saw at least three spines on top of the dome (none more than 15 m high) and vigorous steaming from both the NW moat and several areas of the dome. A mudflow that descended the Upper Tar River Valley had apparently formed due to heavy rainfall on the previous night (23-24 May). Also noted was a clear scar on the dome's lower NE flank. About a meter deep and perhaps 5- to 10-m wide, the scar provided a path for ongoing rockfalls.

Observations on 26 May indicated dome growth focused on the dome's E, NE, S, and W parts. Also during the week ending on 29 May, the absence of strong wind allowed the development of near vertical ash plumes, some of which ascended up to 2-km altitude. On 29 May observers saw several small pyroclastic flows that started near the upper dome and flowed E down the Tar River Valley, stopping no farther than the Tar River Soufriere.

Seismicity during May is summarized in table 3. Intense hybrid seismicity took place on 2-3 May; otherwise seismic activity for late April through May was dominated by near-continuous broadband tremor, in some cases lasting up to several days. Tremor duration remained qualitative because it was saved on analog recorders; the gains and filters on these recorders were periodically changed in order to look at other types of seismicity, leaving no consistent record for quantitative analysis. In addition to tremor, rockfall signals were also common.

Table 3. Seismic data from Soufriere Hills, May 1996. Courtesy of MVO.

Date Volcano-tectonic Long-period Hybrid Rockfall Amount of tremor
02 May 1996 0 32 52 46 Intermediate
03 May 1996 1 2 345 50 Intermediate to high
04 May 1996 0 5 11 27 Intermediate
05 May 1996 0 11 1 67 Intermediate to high
06 May 1996 0 2 6 55 Intermediate
07 May 1996 0 7 5 50 Low
08 May 1996 0 21 5 64 Low
09 May 1996 0 21 0 73 Low
10 May 1996 1 16 0 97 Low
11 May 1996 1 4 0 62 Low
12 May 1996 0 6 0 109 Low
13 May 1996 0 15 0 127 None
14 May 1996 0 18 0 147 None
15 May 1996 2 50 67 103 None
16 May 1996 0 2 12 80 Low to intermediate
17 May 1996 0 4 8 33 Low to intermediate
18 May 1996 1 12 2 25 Low
19 May 1996 1 9 13 34 Low to intermediate
20 May 1996 0 7 8 43 Intermediate
21 May 1996 0 4 0 32 Intermediate to high
22 May 1996 0 7 0 60 Intermediate to high
23 May 1996 0 12 0 64 Intermediate to high
24 May 1996 0 19 0 50 Low
25 May 1996 0 17 1 104 Low
26 May 1996 0 12 8 114 Intermediate
27 May 1996 1 13 5 85 Intermediate
28 May 1996 1 13 4 86 Intermediate to high
29 May 1996 0 12 3 83 Low to intermediate
30 May 1996 1 5 0 17 Low to intermediate
31 May 1996 1 14 96 97 Intermediate to high

Some of the deformation measurements made during May were taken on the E and S triangles on 26 May. The line lengths on the southern triangle had shortened by 8 to 9 mm since 21 April, while the eastern triangle had shortened by ~1 cm since 20 May. These data obtained by the EDM technique were consistent with recent GPS measurements conducted by the Alan Smith and colleagues from the University of Puerto Rico.

The bulk of the SO2 flux measurements were made with a car-mounted COSPEC driven under the plume (between Cork Hill and St. Patrick's) at ~20 km/hr (table 4). Wind speeds were measured with a hand-held annemometer before and after each day's runs at Windy Hill (3.4 km N of Chances Peak), the windiest spot accessible by road. Typical SO2 fluxes were in the range of 25-205 metric tons/day (t/d). An exception was the 13 May measurement of 357 t/d.

Table 4. Correlation spectrometer (COSPEC) SO2 flux measurements at Soufriere Hills, 28 April-22 May 1996. Courtesy of MVO.

Date Number of measurements Mean (t/d) Sigma
28 Apr 1996 4 26 5
29 Apr 1996 3 86 10
01 May 1996 5 97 29
02 May 1996 3 177 29
03 May 1996 5 89 11
04 May 1996 5 76 17
05 May 1996 3 54 10
09 May 1996 4 138 11
10 May 1996 5 123 46
11 May 1996 4 96 30
13 May 1996 3 357 119
17 May 1996 5 130 29
18 May 1996 5 129 39
19 May 1996 5 203 54
20 May 1996 4 164 31
21 May 1996 5 205 56
22 May 1996 -- 130 --

Resettlement. Since 3 April shelters have housed 1,381 residents. About another 3,000 people rented or shared accommodations in the homes of friends and relatives. The W. H. Bramble airport remained open. Pre-fabricated buildings were erected and church and school buildings were converted to temporary shelters; in addition, the government prepared an ancillary hospital and a power station in the safe area; it made road repairs, upgraded fuel storage, relocated livestock on farms, and established programs for sport, culture, counselling, and guidance.

As of 24 April no plan for mass off-island evacuation for the island's 10,000 inhabitants had been implemented; instead the British and CARICOM governments favored voluntary evacuation. Some residents could remain on Montserrat at the N end of the island, in the area considered comparatively safe by Wadge and Isaacs (1988) and by scientists at MVO. Participants who go to the U.K. could be eligible for employment, income support, housing, and the enrollment of children in British schools for two years.

Reference. Wadge, G., and Isaacs, M.C., 1988, Mapping the volcanic hazards from Soufriere Hills Volcano, Montserrat, West Indies using an image processor: Journal of the Geological Society of London, v. 145, no. 4, p. 541-551.

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/); Alan L. Smith, Univ. Puerto Rico, Dept. of Geology, Mayaguez, PR 00680 USA.