Report on Soufriere Hills (United Kingdom) — April 2002
Bulletin of the Global Volcanism Network, vol. 27, no. 4 (April 2002)
Managing Editor: Richard Wunderman.
Soufriere Hills (United Kingdom) Rockfalls and pyroclastic flows originate from growing lava dome
Please cite this report as:
Global Volcanism Program, 2002. Report on Soufriere Hills (United Kingdom) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 27:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200204-360050.
16.72°N, 62.18°W; summit elev. 915 m
All times are local (unless otherwise noted)
During mid-August 2001 through February 2002 at a new lava dome continued to grow at Soufriere Hills. Small-scale dome collapses generated pyroclastic flows almost continuously, with some reaching and entering the sea on several occasions. Dense ash plumes associated with sea entry and ash venting from the summit generally drifted W and reached up to 3 km altitude. Mudflows occurred in the Belham Valley on several days during periods of torrential rainfall (BGVN 27:01). The lava dome continued to grow during February through at least mid-May 2002. Minor episodes of ash venting occurred from the summit of the dome, and at times incandescence was visible. The dome produced numerous rockfalls and small pyroclastic flows in the upper reaches of the Tar River Valley. SO2 flux rates reached up to 1,200 metric tons per day (table 40).
|Date||Rockfall||Long-period / Rockfall||Long-period||Hybrid||Volcano-tectonic||SO2 flux (metric tons/day)|
|01 Feb-08 Feb 2002||897||64||85||16||--||06 Feb: 160-380; 07 Feb: 665-790|
|08 Feb-15 Feb 2002||734||69||83||17||1||09-12 Feb: 150-420; 14 Feb: 350-650|
|15 Feb-22 Feb 2002||786||75||74||17||--||16 Feb: 600-780; 19 Feb: 90-130|
|22 Feb-01 Mar 2002||1013||124||101||5||--||--|
|01 Mar-08 Mar 2002||415||49||56||10||--||60-130|
|08 Mar-15 Mar 2002||779||67||92||6||--||40-860|
|15 Mar-22 Mar 2002||1002||108||162||3||2||395-1035|
|22 Mar-29 Mar 2002||935||80||123||3||--||1100-1200|
|12 Apr-19 Apr 2002||841||52||65||6||--||~1200|
|19 Apr-26 Apr 2002||990||66||114||31||1||~1200|
|26 Apr-03 May 2002||741||33||76||42||2||~600|
|03 May-10 May 2002||557||40||82||13||--||--|
During flights on 4, 5, and 6 February new pyroclastic-flow deposits were observed in the Tar River to the E (with some flows reaching the sea) and in the White River to the S, derived from the collapse of remnant talus material from the pre-29 July 2001 dome (BGVN 26:07). An observation flight on 14 February revealed minor rockfalls of old, inactive dome material in the upper part of the Gages region. Near-continuous rockfalls and minor pyroclastic flows occurred on the E flank. Minor rockfalls on the N flank of the active dome cascaded between the NE and central buttresses of the older inactive dome.
Activity increased beginning on the evening of 8 March. Small ash clouds (reaching ~2.1 km) arising from small collapses drifted to the W over the Plymouth and Richmond Hill area, although most of the ash fallout occurred over the sea. For a couple days during late March weak winds dispersed the ash towards the NW and N, depositing it over the main populated areas. Large spines on the dome during mid-March periodically collapsed, producing pyroclastic flows down the E flank, some of which reached the Tar River Fan. By late March minor amounts of rockfall debris from the NE flank of the dome had begun to spill into the head of Tuit's Ghaut. Ash venting appeared to have been from a pit-like depression on the summit of the dome.
Increased rockfall and pyroclastic-flow activity over the E flank of the dome coincided with periods of tremor during late April. Small, low-level ash clouds were occasionally visible on satellite imagery. Rockfalls traveled down the SE flank of the dome almost continuously. By early May rockfall talus had begun to spill over the rim of the 29 July 2001 collapse-scar in the extreme SE at the foot of Roches Mountain. Pyroclastic flows on the mornings of 1 and 2 May were the most energetic seismic events recorded for over a month. Activity increased beginning on 8 May, and rockfalls and pyroclastic flows were concentrated on the dome's NE flank.
MVO reported that weather permitting, the daytime entry zone (DTEZ) would remain open. The observatory warned that activity could increase quite suddenly, with a dangerous situation developing in the DTEZ very quickly, and that ash masks should be worn in ashy conditions. The Belham Valley was to be avoided during and after heavy rainfall due to the possibility of mudflows. Access to Plymouth, Bramble airport, and beyond was prohibited. In addition, a maritime exclusion zone around the S part of the island extends two miles beyond the coastline from Trant's Bay in the E to Garibaldi Hill on the W coast.
Seismicity and SO2 flux. Since 4 February SO2 measurements were carried out using a remote, telemetered Differential Optical Absorption Spectrometer (DOAS) that scans through the plume, yielding over 600 measurements of SO2 emission rates per day. The highest SO2 fluxes were measured after pyroclastic flows. SO2 emission rates decreased dramatically during early March (table 40).
A swarm of hybrid earthquakes on 22 April was followed by increased numbers of long-period events and a surge in the number of rockfalls over the next four days. Banded tremor also followed the swarm. Weak periods of tremor occurred approximately every 20 hours during 26 April-3 May, and each lasted a few hours. Fluctuations in SO2 emission rates in late April appeared to reflect variations in the intensity of rockfall activity.
Morphology of the lava dome. During early February the lava dome continued to grow primarily on the E and NE sides, and by late February growth was focused on the E side. The summit of the dome was blocky and massive, in contrast to the spines of previous weeks. On 19 February the dome was crowned by a large spine inclined steeply up towards the SE. The spine changed in size and shape, as it periodically collapsed or disintegrated and grew again as fresh material was extruded. On 26 February the spine had a height of 90 m above the general level of the summit area. At this stage the top of the spine had an elevation of 1,080 m, the highest point measured during the eruption to date.
Observations in early March revealed that the summit of the dome had a generally spiny appearance and on several occasions was crowned by a large spine directed upwards at a high angle towards the E. During mid-March the summit of the dome was dominated by fast-growing large spines (50-70 m high). Theodolite measurements of the dome taken on 20 March yielded a dome height of 1,039 m.
During mid-April, dome growth shifted to the SE area of the dome complex, although small rockfalls occurred in other areas. The summit area had evolved from a large striated lobe to a series of small spines. By late April the lobe on the SE portion of the dome had reached 1,041 m elevation and the NE lobe, which had been highly active during the previous two weeks, stagnated at a height of 1,020 m elevation. Lava dome growth continued on the E side of the dome complex during early May.
The closest GPS station to the dome showed sustained outward movement of ~0.5 cm per month. During periods of dome building, slow subsidence took place at the closest sites at Hermitage, Whites, and Harris. Since January, the EDM reflector on the N flank showed a 5-cm movement away from the lava dome.
Hazard assessment. On 11 March 2002 the Montserrat Volcano Observatory (MVO) issued the following preliminary statement concerning the history and hazard assessment of the current eruption: "The Soufrière Hills Volcano continues its second phase of sustained dome growth, which began in November 1999. Since September 2001, the dome has grown at an average rate of about 2 m3/s (or 400,000 metric tons per day). The summit region of the dome has now reached an altitude of ~990 m, having filled most of the depression formed by the large dome collapse of 29 July 2001. The dome has mainly grown towards the E, although there was a period during late November and early December 2001 when growth was directed W.
"During [September 2001 to March 2002] there have been fluctuations in activity as recorded in seismicity and gas emissions. Pyroclastic flows and almost continuous rockfalls have occurred, mostly directed down the Tar River Valley. For prolonged periods in the last six months, there have been cyclical patterns of enhanced seismicity lasting for a few hours to about a day, during which rockfall and pyroclastic-flow activity has been more intense.
"Continued growth of the dome over this period has meant that hazard levels close to the volcano have increased slightly compared with . . . September 2001. Risk levels will fluctuate as the configuration of the dome changes. In an extreme scenario, a switch in the direction of growth to the N or NW could result in more hazardous conditions along the margins of the Exclusion Zone. Consequently, increased levels of risk might develop in the populated areas bordering the Belham River. Across the remainder of the island, however, it is considered that the general level of risk to the population from volcanic activity is unchanged.
"The main hazards remain pyroclastic flows, explosions, falls of ash and small stones, and volcanic mudflows. The increasing knowledge of the volcano acquired by the experienced observatory staff allows patterns of eruption behavior to be recognized and some forms of activity to be anticipated. During a large dome collapse or explosion, heavy ashfall and the fall of small rock fragments can be expected in the populated areas if the wind is in an unfavorable direction. However, a detailed study of the hazard due to fall of rock fragments has recently been completed, and this indicates that outside the Exclusion Zone significant falls of rock fragments large enough to cause serious injury are unlikely.
"At the moment there is no sign of the volcanic activity diminishing. It is most likely that the eruption will continue for a number of years, although the volcano may be evolving into a persistently active state with the eruption continuing for even longer periods, either continuously or intermittently."
General References. Baker, P.E., 1985, Volcanic hazards on St. Kitts and Montserrat, West Indies: Journal of the Geological Society, London, v. 142, p. 279-295.
Shepherd, J.B, Tomblin, J.F., and Woo, D.A., 1971, Volcano-seismic crisis in Montserrat, West Indies, 1966-67: Bulletin of Volcanology, v. 35, p. 143-163.
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, London, v. 145, p. 541-551.
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.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).