Montserrat Update 005
Date: Tue, 25 Jul 1995 16:29:18 MST
From: Global Volcanism Network
Subject: Soufriere Hills, Montserrat - GVN Report #2
To: Multiple recipients of list VOLCANO
This report from the Smithsonian Institution's Global Volcanism
Network on 25 July 1995 is preliminary and subject to change as
additional information is received.
Soufriere Hills
Montserrat, West Indies
16.72 N, 62.18 W; summit elev. 915 m
The following is based on information as of 24 July from the Seismic
Research Unit (SRU) team at the University of the West Indies and
Volcanic Alert News Releases from the Montserrat Emergency Operations
Center. The SRU maintains a seismic network on Montserrat, currently
composed of seven instruments.
On 18 July, villagers around Soufriere Hills volcano reported
unusually loud rumbling noises coming from the fumarolic areas, light
ashfall, and a strong sulfur odor. Following confirmation of these
reports, the Emergency Operations Center (EOC) in the capitol city of
Plymouth, on the coast ~4 km W of the summit, was activated and fully
operational by 1830 that night. Appropriate emergency support agencies
were notified, including the SRU in Trinidad. The EOC also set aside
two schools as refugee centers, but no evacuation was ordered.
As of the morning of 19 July, based on conversations with Montserrat
residents, SRU believed the initial eruption was a small phreatic
event with minor ashfall spread around the island by local winds. In
accord with the inferred small size of the eruption, the Synoptic
Analysis Branch of NOAA saw no evidence of a plume on satellite
imagery. Seismicity has been elevated since August 1992, and an
earthquake swarm began on 14 July. However, no additional increase in
seismicity was associated with the 18 July eruptions.
An explosion earthquake at 0924 on 19 July was centered close to the
top of Chance's Peak, the summit located on the W side of the crater
rim. A field team led by Lloyd Lynch (SRU) trekked in from the N to
make an initial inspection just after 1300. They reported minor
explosions from an area SE of Tar River Soufriere, a fumarolic area
~1.5 km NE of the summit. The explosions took place at intervals of
~20 minutes, sending ash and steam ~40 m high. Activity is centered
within the summit crater between Chance's Peak and the Tar River area.
Based on these observations, no evacuations were recommended.
William Ambeh (SRU) led another observation team on the morning of 20
July to the Paradise Estate area (~2 km N of the summit), and
additional monitoring equipment was installed in the Long Ground area
(~2.5 km NE). Reconnaissance photographs taken from a Royal Air Force
aircraft confirmed the early field reports. Later photographs taken
from a Royal Navy helicopter indicated no increased activity in the
Long Ground area.
The shallow earthquake swarm that began on 14 July ended on the 21st;
depths were 2-4 km, and the largest event was M 3.5. Volcanic
earthquakes were concentrated along the ENE and WSW areas of Lang's
Soufriere. Eruptive activity was continuing on 22 July. Early morning
ashfall was reported in Plymouth (~4 km W) and the SW-sector villages
of Gages, Parsons, and Amersham. A small steam-and-ash eruption around
0800 lasted ~10 minutes. As of 1030 on 23 July, there had been no new
volcanic activity.
At the request of Montserrat, the Government of France was sending two
French scientists (arriving on 25 July) to provide the SRU with
technical assistance and additional equipment. They will be joined on
about 26 July by five geologists from the U.S. Geological Survey's
Volcanic Crisis Assistance Team.
Soufriere Hills volcano sits on the N flank of the older South
Soufriere Hills volcano, located at the S end of Montserrat Island (13
x 8 km). The summit area consists primarily of a series of lava domes
emplaced along a ESE-trending zone. Block-and-ash flow and surge units
associated with dome growth predominate in flank deposits. The most
recent pyroclastic-flow deposits, associated with the formation of
English's Crater, have been dated at around 19,000 BP (years before
present) (Baker, 1985). Wadge and Isaacs (1988) dated a series of
eruptions at 16-24,000 BP, and note that Castle Peak dome in English's
Crater post-dates this by an unknown period of time. English's Crater
is breached to the E.
There have been no reported historical eruptions, but some undated
deposits and the cone have a young appearance. A radiocarbon date of
~320 +- 54 BP from a NE-flank pyroclastic-flow deposit is significantly
younger than other radiocarbon dates from the volcano, but could
result from the latest activity of Castle Peak. Because the sampling
site has not been relocated for confirmation, this date is considered
somewhat uncertain.
Periods of increased seismicity below Soufriere Hills volcano were
reported in 1897-98, 1933-37, and again in 1966-67. Shepherd and
others (1971) concluded that the 1966-67 seismicity was caused by a
relatively small volume of magma injected from >10 km depth into a
zone of fractured rocks below the volcano, and not from a shallow
magma body.
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.
Information Contacts: Richard Robertson, Seismic Research Unit,
University of the West Indies, St. Augustine, Trinidad; Montserrat
Emergency Operations Center, Plymouth, Montserrat.
Ed Venzke
Global Volcanism Network | Phone: 202-357-1511
Museum of Natural History, MRC 129 | Fax: 202-357-2476
Smithsonian Institution |
Washington, DC 20560 USA | Internet: mnhms017@SIVM.SI.EDU