Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 8, August 1995

Soufriere Hills (Montserrat)  Two additional vents open in late
       August; steam-and-ash emissions

Soufriere Hills
Montserrat, West Indies
16.72 N, 62.18 W; summit elev. 915 m
All times are local (= GMT - 4 hours)

Eruptive activity at Soufriere Hills began with a phreatic
explosion on 18 July that caused ashfall around the island
(Bulletin v. 20, no. 6). Formation of a second vent in the summit
crater on 28 July and increased steam-and-ash emissions prompted
evacuations from communities near the summit (Bulletin v. 20, no.
7). Activity was variable but generally low in early August, with
small mudflows and continued steaming. Vent 1 reactivated on 11
August, and some earthquakes were centered beneath St. George's
Hill, 3 km WNW of the summit. Relatively heavy cloud cover and bad
weather prevented observations on many days in late August.

Moderate emissions continued from both vents through 19 August
(Bulletin v. 20, no. 7). An ash-and-steam eruption from Vent 1 at
1220 on 19 August was similar in size to the two previous events
that caused ashfall in Plymouth. The eruption lasted for ~35
minutes, during which a 10-minute-long phase of more vigorous
activity deposited ~1 mm of ash in areas NW of the vent. Ashfall
along the road between Lees and Gages has caused reduced traction;
police were advising motorists to drive slower on the slippery

Another small phreatic episode at 1657 on 20 August produced minor
ashfall. Since the disappearance of continuous tremor on 19 August,
seismicity consisted of low-intensity spasmodic tremor and
occasional small volcano-tectonic (VT) earthquakes attributed to
very shallow activities under the summit area.

Formation of Vent 3 on 22 August. Seismicity generally increased in
frequency and amplitude until the largest single phreatic eruption
episode to date at 0803 on 21 August produced a column to a height
of ~2 km. Ash blown down the W flank engulfed Plymouth within eight
minutes, caused darkness for ~25 minutes, and deposited an
estimated 2 mm of ash. An analysis of the new ash showed that it
consisted only of old altered material. Other reported phenomenon
were projectiles in the Long Ground area and a density current in
Gages. Over 70 locatable VT earthquakes were recorded between noon
on 20 August through noon the next day; most of these events
occurred before the 21 August eruption and were very shallow.

Following a contingency plan, the volcano observatory was relocated
to the Vue Pointe Hotel in Old Towne (~4.5 km N of Plymouth) at
approximately 1600 on the 21st; by 1730 all of the seismic
instruments were back on-line. Seismicity was less frequent and
vigorous through noon on 22 August, with only four eruption signals
of smaller intensity and duration than the previous episodes. Over
5,000 residents were evacuated from the capitol city of Plymouth to
camps on the N part of the island. Day use was permitted, but
restricted. Government offices were also relocated.

Six phreatic eruptions occurred between noon on 22 August and noon
the next day. The largest, at 1551 on 22 August, produced ashfall
~3-3.5 km to the SW. This event was followed by three long-period
(LP) earthquakes (events associated with the movement of
pressurized fluids). When the crater area was visited on the
afternoon of 22 August it was discovered that a new vent (Vent 3)
had opened along the inside of English's Crater rim. Only 24
earthquakes were located during 21-23 August, all very shallow.
Seismicity was comparable or slightly lower during the next
twenty-four hours. There were six episodes of increased gas
venting, some of which were followed by small VT events. Fifteen
earthquakes, including two LP events, were located beneath
Soufriere Hills at depths ranging from near-surface to ~5 km.

Aerial reconnaissance on the afternoon of 24 August showed a
NNW-SSE trending line of several small explosion craters in the
summit crater, with Vent 1 at the N end and Vent 3 at the S; Vent
2 was offset to the SE. The rate of steam emission from Vent 2 was
very low, while slightly more steam was being emitted from Vent 3.
No gas emission was observed from Vent 1. Seismicity continued to
be relatively low, with seven earthquakes distributed beneath St.
George's Hill and Soufriere Hills at depths of 0-4 km. Three
episodes of increased gas venting and 30 minutes of broadband
tremor also occurred. A small mudflow originating from Gages Upper
Soufriere during heavy rains on the afternoon of 24 August flowed
through Fort Ghaut. Seismicity remained low until a swarm of VT
earthquakes that lasted from 2157 on 25 August until 0230 the next
day. Located earthquakes consisted of 22 VT events at depths of 0-5
km beneath the N flank. Five episodes of gas venting during 25-26
August had repose intervals of ~4 hours.

Formation of Vent 4 on 27 August. On 27 August there was one
episode of broadband tremor that lasted ~20 minutes and a small
mudflow in Fort Ghaut that began around 0820. From 26 to 28 August,
fourteen small VT earthquakes were located beneath Soufriere Hills
and St. Georges Hill at depths of <5 km. During this same period
there were twelve episodes of increased gas venting. One episode at
1443 on 26 August ejected ash that could be seen from the Vue
Pointe Hotel; other emissions caused light ashfall in the Tar River
area. Observations on the morning of 28 August confirmed the
presence of a fourth vent that had probably opened the day before.
Located on the NNE flank of Castle Peak dome, it was vigorously
emitting steam and ash through mid-day on 28 August; emissions from
the other vents were low. Eight VT earthquakes were located beneath
Soufriere Hills at depths of 0-4 km. Five episodes of increased gas
venting occurred.

Vent 4 was still emitting mainly steam at a reduced rate on 29-30
August. Another nine episodes of increased gas venting were
detected, and five small shallow earthquakes were located beneath
and to the N of the Soufriere Hills during 29-30 August. Unusually
good visibility allowed Castle Peak dome to be inspected at around
0900 on 30 August. Steam emissions from all vents were low and
there was no ash. The main vent system (a linear chain of vents
extending from Vent 1 on the NW margin of the dome SE to the S
margin of the dome) had enlarged since 24 August. Mud or muddy
water was locally present in the bottom of the main vent system.
Several pools of standing water were located atop Castle Peak, and
the moat pond on the NW side of the dome still existed. A recent
mudflow from the W side of the dome southward down the Tar River
had buried Vent 2, on the S side of the dome.

In terms of earthquake activity, the 24-hour period beginning at
1400 on 30 August was probably the most active since the 21 August
phreatic eruption. Thirty-four shallow earthquakes were located WNW
of Soufriere Hills. A few earthquakes were also located beneath
Windy Hill (3.5 km NNW) and in the area between Windy Hill and
Soufriere Hills. Seismicity decreased the next day, when only ten
shallow earthquakes were located WNW of Soufriere Hills; two were
also located beneath Windy Hill. In addition, four LP earthquakes
occurred at shallow depths beneath the NW edge of Soufriere Hills.
During these two days, thirteen episodes of increased gas venting
were detected, but steam and ash emissions from all vents remained

Deformation and SO2 measurements. A review of the Brodrick's
dry-tilt data completed on 23 August indicated that some
deformation of the volcano may have occurred between January and 9
August, confirming that magma may be at a shallow depth (as
suggested by the earthquake data). Tiltmeter readings in late
August were generally within background noise levels; no tilt
related to volcanism was observed. EDM reflectors were deployed on
30 August in Gages Upper Soufriere and on Castle Peak dome.

COSPEC gas measurements taken on the afternoon of 20 August
indicated that the rate of SO2 emission was just above the
detection level, ~50 metric tons/day (t/d). Additional measurements
taken during favorable conditions on the next afternoon and morning
of 22 August did not detect any SO2. This lack of SO2 was thought
to be a result either of the system running out of gases or a
sealing off of the fluid access path to the surface. A COSPEC
flight on the afternoon of 23 August detected a slight trace of SO2
(~40 t/d) while a flight the next morning showed none. The flux
rate on the morning of 26 August was ~50 t/d, and on 28 August was
~85 t/d. Further COSPEC measurements on 29, 30, and 31 August
showed no detectable SO2.

Information Contacts: Volcanic Disaster Assistance Program, USGS
Cascade Volcano Observatory, 4200 MacArthur Blvd., Vancouver, WA
98661 USA; Seismic Research Unit, University of the West Indies,
St. Augustine, Trinidad; Emergency Operations Center, Government
Information Unit, Chief Minister's Office, P.O. Box 292, Plymouth,