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Report on Soufriere Hills (United Kingdom) — July 1996

Soufriere Hills

Bulletin of the Global Volcanism Network, vol. 21, no. 7 (July 1996)
Managing Editor: Richard Wunderman.

Soufriere Hills (United Kingdom) Dome growth continues, rockfalls and pyroclastic flows increase

Please cite this report as:

Global Volcanism Program, 1996. Report on Soufriere Hills (United Kingdom) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 21:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199607-360050

Soufriere Hills

United Kingdom

16.72°N, 62.18°W; summit elev. 915 m

All times are local (unless otherwise noted)

The following condenses daily Scientific Reports of the Montserrat Volcano Observatory (MVO) for the period 1 July-1 August. Seismic and other significant events of this month are also summarized in table 9.

Table 9. Chronology of seismicity and other major events at Soufriere Hills, Montserrat, 1 July through 1 August. The observation period is a 24-hour interval beginning at 1600 the previous day. Courtesy of MVO.

Date Volcano-tectonic Long-period Hybrid Rockfall Tremor Intensity Visual Observations
01 Jul 1996 2 11 12 57 Low --
02 Jul 1996 1 1 18 64 Low --
03 Jul 1996 -- 5 -- 59 Low --
04 Jul 1996 -- 5 25 52 Low One small ash cloud.
05 Jul 1996 -- 6 22 27 Low --
06 Jul 1996 -- 9 6 11 High --
07 Jul 1996 2 12 6 22 High --
08 Jul 1996 1 3? 3? 3? High Hurricane Bertha.
09 Jul 1996 1 1 5 9 Low --
10 Jul 1996 1 4 2 22 Low Two light ash clouds drifted W.
11 Jul 1996 2 4 2 32 Low and High Flash flooding in Upper Fort Ghaut and Tar River Valley. Few very small pyroclastic flows. Light ash fall N of Plymouth.
12 Jul 1996 4 10 11 9 Low --
13 Jul 1996 -- 9 22 14 Low-to-Moderate --
14 Jul 1996 4 22 5 18 Low-to-Moderate --
15 Jul 1996 2 13 11 14 Low-to-Moderate One small ash cloud.
16 Jul 1996 2 14 13 15 Low-to-Moderate One small ashfall in Brodericks.
17 Jul 1996 -- 8 14 24 Low Two small ash clouds drifted W.
18 Jul 1996 -- 12 17 16 Low One small ash cloud.
19 Jul 1996 4 12 11 20 Low Four small ash clouds drifted W.
20 Jul 1996 -- 4 4 21 Low-to-Moderate One small ash cloud drifted W.
21 Jul 1996 560 19 44 58 Low-to-Moderate Some small ash clouds. Few small pyroclastic flows.
22 Jul 1996 82 105 114 94 Low-to-Moderate Some small ash clouds. Seven small pyroclastic flows.
23 Jul 1996 15 24 101 150+ Low-to-Moderate Continuous ash clouds production.
24 Jul 1996 15 1 9 ~160 Low-to-Moderate One ash cloud.
25 Jul 1996 106 9 35 ~100 Low-to-Moderate One ash cloud drifted NW.
26 Jul 1996 98 5 15 102 Low-to-Moderate Some very small ash clouds.
27 Jul 1996 15 5 36 ~100 Low-to-Moderate Some small ash clouds produced light ashfall toward W.
28 Jul 1996 5 -- -- -- Low-to-Moderate Continuous ash cloud production resulted in heavy ashfall toward W. Several small pyroclastic flows.
29 Jul 1996 -- -- -- -- Low-to-Moderate Moderate-sized ash cloud caused ashfall toward WNW.
30 Jul 1996 20 6 clusters 89 Low-to-Moderate One small ash cloud. Few small pyroclastic flows.
31 Jul 1996 88 -- 178 93 High Large number of pyroclastic flows produced continuous ash clouds and heavy ashfall.
01 Aug 1996 20 36 215 117 Low Ashfall continued from the day before.

Activity during 1-20 July. During the first 10 days of July activity remained at a low level, similar to the last week of June (BGVN 21:06). The most significant events were small-to-moderate size rockfalls from the growing S flank of the lava dome. The largest rockfalls produced small ash clouds that drifted with the prevailing winds, principally to the W of the volcano, toward Upper Gages, Amersham, and Plymouth.

Most of the time visibility was poor because of bad weather conditions. On 4 July a brief period of excellent viewing conditions confirmed that the dome was growing mainly in the S section of the crater. A huge slab extruded at the top SE part of the lava dome had a vertical crack down the middle; activity was concentrated around its base. Several large loose boulders were seen on the slopes of the dome. A small quantity of fresh dome material, mainly blocks, was observed in the upper reaches of Fort Ghaut. Moderate steaming and gas production were occurring from several areas.

Seismicity remained low, with volcano-tectonic events concentrated under English's Crater at depths of <2 km. Daily episodes of intermittent low-amplitude broadband tremor lasted from a few minutes to several hours. On 6 and 7 July periods of high-amplitude tremor were associated with heavy rainfall and an increase in steam venting at the summit.

On 7 July a brief period of good visibility revealed a second peak on the dome, and the accumulation of material behind Galways Wall. Intense fumarolic activity was occurring in the saddle between the two peaks. That day the elevation at the top of the dome was measured as 939 m. MVO estimated that the rate of dome growth had not changed significantly since early May.

On 10 July the seismic signals became longer and stronger. That same day a brief view of the dome showed that rockfall activity was spreading to other areas within the active SE section. More fresh material had accumulated down the S side of Castle Peak, while vigorous steaming was observed behind it. Fumaroles were active on the summit of the SE peak and in the saddle area between the two peaks inside English's Crater.

On 11 July, heavy rainfall caused flash floods in Fort Ghaut and possibly Tar River. A fine ashfall was reported in areas N of Plymouth and out to sea. Some small pyroclastic flows went into the Upper Tar River area. Helicopter inspections found that a significant amount of material had come down the N and S sides of Castle Peak and the fresh deposits were still steaming. Several erosion scars were observed on the NE flank of the dome, which was probably the source of the flows.

On 12 July the activity level decreased and it remained low throughout 20 July. However, the broadband tremor increased in amplitude, which was interpreted as a sign of increased steam emission; brief glimpses of the dome eventually revealed vigorous steaming, at times tainted with bluish vapor. That same day rockfall deposits were reported on the S and NE sides of the dome. The wet material on the NE side, around the whaleback feature, had dried out in places and two well-formed erosion chutes were present. Dome elevation was measured at 941 m.

On 17 July more new material was seen over Gages Wall and against Galways Wall. Observers on Perche's Mountain noted that most of the rockfall activity was on the SW flank of the dome.They also reported a small block-and-ash flow down the E flank of the dome around noon. On 19 July a field party working at Farrell's heard frequent rockfall activity and observed one rockfall descending the NE flank of the dome.

Activity during 21 July-1 August.On 21 July, the occurrence of 560 volcano-tectonic earthquakes marked a sharp increase in activity that lasted until August. These events originated from a shallow source beneath the crater, or just slightly NNE at <3 km. Long-period earthquakes were of moderate size whereas hybrid events were always small and occurred in a near-repetitive pattern at times so frequently to resemble continuous tremor. This type of activity had previously been associated with increased dome growth.

When weather conditions allowed, views of the dome revealed very vigorous steam emission from behind the old Castle Peak spine. On 25 July a large spine at the summit of the N peak of the dome was seen from Hermitage.

The rockfall activity, mainly on the NE flank of the dome, increased daily. Periods of near-continuous rockfalls were reported after 27 July. Most of the rockfalls were channeled down the NE-flank gully; none reached as far as the Tar River Soufriere. Small pyroclastic flows from the E and NE parts of the dome occurred daily into the Tar River Valley until they filled the entire valley area. Most of the local vegetation was set on fire by these flows. Associated ash clouds caused light to moderate ashfalls on 27, 28, and 29 July. One eyewitness reported on an electronic forum that during the ashfall of 28 July visibility in Plymouth was reduced to the less than one-half of a city block.

The dominating event on 31 July was a sequence of pyroclastic flows in the Tar River valley. It started at 1150 hours with a series of small- to moderate-sized rockfalls, which gradually led to the pyroclastic flow. Four flows occurred within a period of four minutes, with the last three eventually reaching the sea. A helicopter inspection confirmed that the pyroclastic flows were confined to the Tar River Valley. Light steam emission was observed from the area where the pyroclastic flows entered the sea and from the Tar River Valley.

The ash cloud generated by the pyroclastic flows attained a height of 6.4 km above sea level, according to Bramble Airport Control Tower. The ash cloud produced significant ashfalls in most areas of central Montserrat (Lees, Gages, St. George's Hill, Cork hill, Garibaldi Hill and Fox's Bay) and a far N as Woodlands. Lighter ashfalls were reported in Amersham and Plymouth. An eyewitness posted to an electronic forum that during the 31 July ashfall there was ". . . total darkness, the electricity had gone off . . . then it started to rain. The windows . . . facing the mountain became almost solid black. All the rest were covered with some mud . . . . When I got up to [my pickup the] windows, top, and hood were covered with ~1.5 inches [~3.8 cm] of mud."

That same day five episodes of high-amplitude, low-frequency, harmonic tremor were recorded at intervals of ~4 hours. After each period the signal decayed first into smaller hybrids and then to background noise. These signals could be related to movement of magma at shallow depth as the process of dome growth continued.

GPS, EDM, and COSPEC measurements. The poor weather conditions in July prevented most of these measurements. COSPEC data collected during an all-day experiment on 30 June showed no systematic variation in the SO2 production; on 10 July ~88 tons/day were measured.

A GPS survey carried out on the E side of the volcano on 7 July showed that no significant movement had taken place there since 18 June. Data collected on 10 July from Tar River, Harry's, O'Hara's, and Dagenham showed changes <5 mm in all lines since the survey started on 11 April.

EDM measurements showed an increase in the shortening rate from a few millimeters/day at the beginning of the month up to 1.5 cm/day toward the end of the month for the lines of the E triangle (Whites-Castle Peak-Long Ground) (table 10). Lengthening (1 and 2.3 cm) was measured on 30 July, when the dome elevation was found to be 923 m.

Table 10. EDM data from Soufriere Hills, Montserrat, 1 July through 1 August 1996. Courtesy of MVO.

Date Shortening (-) or lengthening (+)/day(s) Line or triangle
01 Jul 1996 -0 Amersham-Dagenham-Chances
12 Jul 1996 -few mm Whites-Chances Peak-Long Ground
18 Jul 1996 -1.5 cm/2 days Whites-Chances Peak
18 Jul 1996 -1.5 cm/2 days Long Ground-Chances Peak
24 Jul 1996 -8 cm/4 days Whites-Chances Peak
24 Jul 1996 -8 cm/4 days Long Ground-Chances Peak
26 Jul 1996 -3 cm/2 days Whites-Chances Peak
26 Jul 1996 -3 cm/2 days Long Ground-Chances Peak
29 Jul 1996 -3 cm/3 days Whites-Chances Peak
29 Jul 1996 -1.3 cm/3 days Long Ground-Chances Peak
29 Jul 1996 -17.5 cm/14 days Tar River-Chances Peak
30 Jul 1996 +1 cm/day Whites-Chances Peak
30 Jul 1996 +2.3 cm/day Long Ground-Chances Peak
01 Aug 1996 -4 cm/day Whites-Chances Peak

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/).