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Report on Soufriere Hills (United Kingdom) — February 2001

Bulletin of the Global Volcanism Network, vol. 26, no. 2 (February 2001)
Managing Editor: Richard Wunderman.

Soufriere Hills (United Kingdom) Dome growth, rockfalls, and pyroclastic flow continue through March 2001

Please cite this report as:

Global Volcanism Program, 2001. Report on Soufriere Hills (United Kingdom). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 26:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200102-360050.

Volcano Profile |  Complete Bulletin

Soufriere Hills

United Kingdom

16.72°N, 62.18°W; summit elev. 915 m

All times are local (unless otherwise noted)

Dome growth continued from 7 October 2000 through 9 March 2001. Until the end of February 2001, the growth occurred predominantly on the E side of the volcano. However, on 25 February, the direction of the growth and the character of the seismicity changed markedly; the dome growth shifted towards the S, and the weekly number of hybrid earthquakes increased by more than an order of magnitude. During this entire period, residents were advised of the potential dangers associated with pyroclastic flows and advised to avoid the Belham valley during periods of heavy rain. Access to Plymouth, Bramble airport, and beyond was prohibited, and a maritime exclusion zone was declared around the S part of the island extending 3.3 km beyond the coastline. Since November 1999, the dome has grown at an average rate of nearly 3 m3/sec and is now at its largest size since the eruption began in 1995, with a total volume of over 120 x 106 m3 and an elevation greater than 1,000 m.

Recent MVO assessment. A summary assessment of the volcano's activity, status, and related risks covering the period from April 2000 through January 2001 was published recently by MVO. A significant finding from this assessment was: "While one prognosis is for at least a few more years of such eruptive activity, an eruption duration measured in decades has to be contemplated." Other extracts from the report are presented below.

"The period July 1995 to March 1998 was the first phase of the present eruption. A lava dome grew . . . accompanied by several hazardous phenomena.... There was then a second phase... from March 1998 to November 1999. In this phase, no significant dome growth was detected, but hazardous activity continued . . . . Dome growth resumed in November 1999... [and] represents a third phase of the eruption."

"The seismic monitoring detected relatively intense periods of rockfall and long-period (LP) activity... from August to October [2000]... and from mid-November 2000 [to January 2001].... There appears to be an underlying 14-week cycle to this type of seismic activity.... However, hybrid swarms have been rare and weak by comparison with 1997 activity. Although their generation mechanisms are still poorly understood, hybrid earthquakes may be related to fracturing of plugs that form in the conduit, and their absence suggests that conduit conditions may now be subtly different from previous stages...."

"Based on the seismic evidence, the growth of the latest... dome seems to have been different in some respects.... The only current prognostic feature in the observable seismicity is the weak 14-week cycle."

"Volcanic gases continue to be routinely monitored. The flux of sulphur dioxide over the last ten months has maintained quite high levels.... Occasional measurements of chlorine flux indicate that the flux of chlorine relative to sulphur dioxide increases... when the dome is growing."

"Two key changes have affected the morphology of the volcano since 1998... which have important implications for hazards.... Two remnants of the 1995-1998 dome remain adjacent to the northern wall of English's Crater. Over the last year, a narrow gap between the two remnants has gradually widened and deepened.... This deep gully is expected to provide a path for potential collapses on that side of the dome. Since 1997, Mosquito Ghaut has been in-filled by pyroclastic-flow and rockfall deposits and no longer exists as a pathway to channel future pyroclastic flows away to the [NE]. Thus, collapses moving down the outlet gully will run in a generally northerly direction, and these topographic changes mean that pyroclastic flows towards the north are now much more likely to be channeled into the Belham Valley...."

"The group considered the possibility of tsunami hazards that might affect other Caribbean islands. Based on modeling studies carried out by French colleagues a collapse of the current dome... into the sea is not expected to generate a tsunami sufficiently large to affect other islands. However, new data... show that there have been several very large edifice failures of the Soufriere Hills Volcano in its geological past. Another collapse of such size would cause a significant regional tsunami hazard. Thus, any precursory signs of a major edifice failure should continue to be watched for in the monitoring [program]."

"The duration of the eruption of the Soufriere Hills Volcano now exceeds about 85% of all documented dome eruptions worldwide. Most dome eruptions last only a few years, but some have durations measured in decades. Examples of the latter kind of dome eruption include the Bezymianny volcano in Kamchatka, Russia, which started to erupt in 1956 and is still active, and the Santiaguito dome in Guatemala, which started to erupt in 1923 and is also still active."

"There is currently a lack of clarity about the legal responsibilities and obligations of the MVO... [and other official organizations] in relation to providing warnings of detected ash injections to civil aviation authorities. The group recommends that HMG [Her Majesty's Government] investigate as a matter of urgency where responsibility lies and what [organization] should issue such warnings."

Activity since 6 October 2000. Until the end of 2000 the summit was dominated by a broad lava spine inclined at a steep angle towards the E. On 25 October the spine had a peak elevation of 1,030 m and by 13 November had grown to 1,077 m, the greatest height measured throughout the eruption. On 5 December the top of the spine was at 1,060 m, while the flat top of the main dome was between 1,020 and 1,030 m. By the end of December the spine had grown back to 1,071 m. Two large near-vertical spines were observed on 4 February 2001, but both had collapsed by the following week. A large stubby spine visible in the S part of the summit area on 22 February rose to 1,068 m.

Rockfalls took place throughout the period. Until 25 February 2001, these traveled predominantly down the E or NE side of the dome, and occurred typically in numbers of hundreds per week (table 36). Some of these glowed and occasionally produced small ash clouds, but none reached altitudes greater than 3,000 m. These rockfalls contributed to an accumulating talus fan in the upper reaches of the Tar River valley. The talus began to bury the remnant buttress of older dome material on the NE flank that formed during the 1995-98 growth phase. Rockfalls that occurred after 25 February traveled predominantly S of the dome, mainly in the upper reaches of the White River valley. (N.B. The White River is sometimes confused with White's Ghaut, which lies to the N of the dome.)

Table 36. Seismic and gas data from the Soufriere Hills during 20 October 2000 to 9 March 2001; not every week had reported data. Courtesy of MVO.

Week Rockfall Hybrid Volcano-tectonic Long-period Range of Average Daily SO2 (tons/day)
20 Oct-27 Oct 2000 214 9 4 35 235-2252
27 Oct-03 Nov 2000 146 20 3 19 --
10 Nov-17 Nov 2000 207 33 7 144 --
24 Nov-01 Dec 2000 491 13 -- 69 1020 (28 Nov)
01 Dec-08 Dec 2000 547 15 1 72 --
15 Dec-22 Dec 2000 423 12 1 74 400 (20 Dec)
22 Dec-29 Dec 2000 708 10 2 53 745-1100
12 Jan-19 Jan 2001 943 -- -- 54 345 (18 Jan)
19 Jan-26 Jan 2001 417 1 -- 55 330-350
26 Jan-02 Feb 2001 313 8 21 45 105-360
02 Feb-09 Feb 2001 409 5 1 40 180-500
09 Feb-16 Feb 2001 500 2 1 15 80-670
16 Feb-23 Feb 2001 486 18 6 53 210-720
23 Feb-02 Mar 2001 729 388 3 58 180-1400
02 Mar-09 Mar 2001 629 280 4 45 100-1230

Pyroclastic flows were also produced throughout the period. A small one on 15 November 2000 traveled N from the summit, entered the upper reaches of Tyre's Ghaut, and reached ~1 km away from the dome. On 17 November pyroclastic-flow deposits were noted in the upper reaches of Tuitt's Ghaut and White's Ghaut on the volcano's NE side; this was the first new dome material to have traveled down the notch between the N and NE lobes from the 1995-98 dome. By 8 December 2000 the notch between the central and NE buttresses of the 1995-98 dome was 60 m wide. Another small flow occurred down the White River valley on 1 February 2001 and traveled about 1 km from the dome. On 8 February 2001 new pyroclastic-flow deposits had formed in the upper portion of Tuitt's Ghaut up to ~300 m from the dome. By mid-February new pyroclastic-flow deposits had also formed down the Tar River on the E flank, and, by 23 February, had reached as far as the old coastline. New deposits were also seen by 23 February in the S White River valley just 50 m short of the coastline.

On 25 February 2001 a pyroclastic flow spread over the N and central parts of the White River fan. A hybrid earthquake swarm occurred after this collapse (table 36). Subsequently, small pyroclastic flows traveled into the upper portion of the White River valley and were accompanied by banded tremor and weak hybrid earthquakes. By 9 March steady dome growth appeared to have resumed.

Seismicity and COSPEC measurements of SO2 are presented in table 36. The SO2 data are in the range of average daily values (in metric tons/day) measured during the report week and include the range of data obtained from both helicopter and static mounted sensors.

Geologic Background. 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), Mongo Hill, Montserrat, West Indies (URL: http://www.mvomrat.com/).