Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 7, July 1995
Soufriere Hills (Montserrat)  Steam and ash emissions from two
       vents in the summit crater

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

Soufriere Hills volcano (figures 1 & 2) began erupting on 18 July
from a fissure vent (Vent 1) within the summit crater (Bulletin v.
20, no. 6). The initial small phreatic eruption spread minor ash
around the island. The next day the airport on Montserrat issued a
Notice to Airmen (NOTAM) after a reconnaissance flight at 0745
reported flying through volcanic ash. Seismicity and minor phreatic
explosions continued in the following days (Bulletin v. 20, no. 6).

Another NOTAM on 26 July renewed the warning to aircraft and
reported sporadic ash emissions. A second vent formed on 28 July
(figure 2), and a third on 20 August. Gas samples taken in late
July from fumaroles at Tar River and Galway's were unchanged from
1989. Samples of ash showed no juvenile components through at least
30 July. Seismicity in late July remained at about the same level
as previously. A distinct odor of hydrogen sulfide (H2S) was
detected in Plymouth, the capitol of Montserrat, during late July,
but sulfur dioxide (SO2) was not detected until 30 July.

Monitoring efforts. Monitoring of the eruptive activity and
scientific advice to the Government of Montserrat are being
provided by an international team of volcanologists. The first
response to the crisis was provided by the Seismic Research Unit
(SRU) at the University of the West Indies in Trinidad, which had
maintained a seismic network on the island. Their team was later
supplemented, at the request of Montserrat, by scientists from the
U.S. Geological Survey (USGS), the Guadeloupe Volcano Observatory,
and the United Kingdom. French scientists arrived on 25 July to
sample gases. The USGS arrived on 26-27 July with additional
seismometers, tiltmeters, and a correlation spectrometer (COSPEC).

The USGS team set up a seismic data analysis system to
automatically locate earthquakes in near real-time, and made
improvements to the existing seismic network. By 30 July,
volcanologists were monitoring 10 channels of component signals
from 8 seismic stations; another station was added soon after. New
telemetered tiltmeters at Spring Estate (2.5 km SW of the vents),
Amersham (3.7 km WSW), and near Long Ground (2 km NE), were
operational by 2 August.

Formation of Vent 2 on 28 July. A volcano-tectonic (VT) earthquake
swarm that began at 0854 on 28 July lasted for >2 hours;
instruments detected ~50 events of M >1. Coincident with this
seismicity, a new vent opened SW of Castle Peak (Vent 2),
along-strike with the fissure vent that had been intermittently
active since 18 July. Vigorous jetting from the new vent at 1814 on
29 July, associated with ~22 minutes of tremor and earthquakes,
occurred during heavy rainfall and was accompanied by a small
mudflow. Following this episode, Vent 2 was estimated to be 1-2 m
in diameter and was jetting steam and a small amount of fine ash
~100 m high with a loud roaring sound. During an overflight on 30
July Vent 1 was producing only wisps of steam, while Vent 2
continued to jet a large amount of steam and fine ash.

Because of increased steam emissions, on 28 July local authorities
ordered an evacuation of the Long Ground area (figure 2). The
evacuees returned to their homes the next morning. Two episodes of
increased seismicity on 29-30 July caused no observable changes at
the vent area. Small (mostly M <1) VT earthquakes continued through
31 July at a relatively steady but low rate. This background
activity was punctuated once or twice a day by continuous
seismicity lasting several minutes to 2 hours. Some of these
episodes may have been associated with vigorous venting of steam
and ash. Others were volcanic tremor with coincident earthquakes.
One low-amplitude tremor episode on 31 July lasted for several

Low-level activity in early August. During 1-3 August there were
fewer high-frequency, VT earthquakes, plus some long-period (LP)
earthquakes. Preliminary locations for the LP events were at depths
of 5-6 km, slightly deeper than the VT events. Emergent
"cigar-shaped" signals, that probably correspond to vigorous steam
venting, occurred a few times each day. Heavy rainfall on 3 August
triggered a small, non-destructive mudflow during the night in a
stream valley that runs through Plymouth. Normal infiltration of
rain water may have been reduced by the relatively impermeable
layer of fine ash that had accumulated on the upper slopes of the

Following 12 hours of unusually low seismicity, vigorous steam and
ash emission began at 0852 on 4 August. This phreatic eruption
lasted ~10 minutes, producing a dark, ash-laden column visible from
most of the island. Seismicity associated with the eruption
included several LP events. An aerial inspection revealed that the
eruption had enlarged Vent 1 to ~10 m across and 10 m deep.

Concern was heightened after the phreatic eruption on 4 August and
the increasing seismicity. As a precautionary measure, the elderly
and infirm from the villages of Long Ground, Bramble Village,
Bethel, Farms, and Trants were resettled on the N part of the
island on 6 August. Aged and infirm in areas from Harris to Gages
and N and S of the immediate area around Fort Ghaut in Plymouth,
were also relocated to the N. Able-bodied residents of Long Ground
were advised to move to shelters at night. Further restrictions may
have been enforced during the next week.

Vent 2 was full of water on 5 August, apparently ground water
forced from the volcano, and muddy water flowed from it through the
Hot River drainage. On 6 August a large steam plume with minor ash
rose from the vent area. By 7 August Vent 2 had grown ~20 m to the
NW, in the direction of the 18 July fissure, the muddy water was
gone, and jetting of steam and varying amounts of fine ash
continued. The abscence of water emissions from the vent area after
7 August suggested that the volcano may have been "drying out,"
possibly due to increased heatflow.

Eighteen locatable earthquakes (M <1) during 4-5 August were
centered <=5 km beneath the vent area or slightly NE of it. There
were roughly equal numbers of VT and LP earthquakes. Approximately
200 events during a 24-hour period on 5-6 August were mostly VT
events with a few LP events; some were felt in Plymouth. On 7
August, spasmodic tremor and small VT earthquakes occurred
throughout the night and into the next day. The seismicity in early
August was believed to be related to "boiling" of the hydrothermal
system. However, the seismic energy release was dominated by
low-amplitude tremor generated by degassing and steam eruptions at
Vent 2.

Ground tilt recorded at Long Ground appeared to reverse on 5 August
from steady deflation (down toward the vent area) since the station
was installed, to apparent inflation (up toward the vent area).
Vigorous venting on 6 August caused several microradians of tilt at
two tiltmeters. Tiltmeters recorded small tilt events through 7
August, some of which seemed to correlate with periods of strong
seismicity. However, there was no consistent pattern, suggesting
that any deformation was relatively minor. A small tilt event
occurred coincident with the 6-7 August seismic swarm.

Earthquakes declined on 9 August, but tremor caused by steam
venting continued. At about 0715-0745 there was a relatively large
steam venting episode. Vigorous steaming continued from Vent 2 on
10 August, but tremor intensity decreased and the number of small
VT earthquakes increased to 1-2/hour. Most of the earthquakes were
centered 2-5 km beneath the vent area. Seismicity was slightly
lower during 10-11 August, with seven VT earthquakes, two
individual low-frequency events, and three of four periods of
continuous tremor lasting ~2 hours. On 10 August tilt appeared
stable as measured by titlmeters and along a short leveling line
near Broderick's Estate, ~3 km SW of the vent area, that was last
measured ~10 years ago.

Reactivation of Vent 1. Vent 1 reactivated on 11 August and seemed
to be emitting steam on the 12th. Steady steam emissions continued
from both vents through 13 August. Seismicity was slightly lower on
11-12 August, with five small VT earthquakes and two periods of
continuous tremor (~2 hours total). Increased gas venting around
1621 on 12 August triggered a swarm of VT events that continued
into the next afternoon. The swarm consisted of >134 earthquakes,
of which 38 were felt, the largest at around 0221 on 13 August (M
~3.5). Epicenters clustered 2-6 km beneath St. George's Hill (3 km
WNW of the summit).

A mudflow from Vent 2 along the Hot River early on 12 August
blocked the road for ~1 km between Tar River and Perche's Estates.
Steady steam output from Vent 2 continued throughout 14 August.
Steam emissions from Vent 1 were intermittent and occasionally
changed composition. On 14-15 August there were three VT
earthquakes, three B-type events, two low-frequency events, and
five degassing episodes followed by tremor.

After three days of consistently lower activity (as of 11 August),
daytime occupancy was recommended for able-bodied residents of the
evacuated villages. However, the sick and bedridden were to remain
at shelters. The reactivation of Vent 1 caused additional
evacuations during 12-14 August from the previously mentioned
villages, but able-bodied residents returned again on 15 August.

The seismographs recorded sustained low-frequency tremor from noon
on 16 August through noon the next day. Degassing from Vent 2
continued at nominal to vigorous rates with occasional increases in
acoustic intensity and changes in color of the output. Six of the
13 locatable VT earthquakes on 16-17 August were beneath Soufriere
Hills; the others, including one felt strongly in Plymouth at 0143,
were scattered within 4 km NE to NW of St. George's Hill. Vent 2
exhibited loud roars and intense venting coincident with heavy
rainfall. The morning of 18 August was very overcast and inspection
of the vents was not possible, but tremor continued, and there were
six locatable VT earthquakes. During 18-19 August there was
continuous low-frequency tremor and moderate emissions from Vent 2.
Vent 1 was obscured for most of the period, but venting noise was
generally low. Several low-amplitude extremely short-duration VT
earthquakes on the Gages seismograph were buried within the
background tremor signal, locatable events were generally 2-3 km
beneath Soufriere Hills.

Tiltmeter observations remained within background noise level
between 7 and 19 August. COSPEC measurements of SO2 flux taken from
a helicopter after 30 July averaged 300 metric tons/day (t/d) until
the morning of 6 August (table 1). After reaching a high of ~1,200
t/d on 6 August, values decreased and stabilized below an average
of 200 t/d through 18 August.

An ash explosion on the morning of 20 August formed a third vent in
the summit crater and prompted evacuations of up to 5,000 people.
The observatory location was moved as a result, delaying the daily
reports. Because this occurred near our deadline, details will be
provided next month.

Information Contacts: Volcanic Disaster Assistance Program, USGS
Cascade Volcano Observatory, 4200 MacArthur Blvd., Vancouver, WA
(URL:; Seismic Research Unit,
University of the West Indies, St. Augustine, Trinidad; Montserrat
Emergency Operations Center, Plymouth, Montserrat.

Figure 1. Map of Montserrat showing selected towns and features.

Figure 2. Shaded topographic map of Soufriere Hills volcano and the
city of Plymouth. The summit is located on the SW crater rim at
Chances Peak. Modified from the "Tourist Map of Montserrat" and
reprinted with the permission of Lands & Survey Department,
Plymouth, Montserrat.

Figure 3. Photograph showing Vent 2 within the summit crater of
Soufriere Hills volcano, 28 July 1995. View is to the WSW; the S
wall of English's Crater is left of the vent. Castle Peak, to the
right of the vent, is the youngest dome of the volcano. Courtesy of
Mitch Lewis Enterprises and Tom Casadevall, USGS.

Table 1. Summary of SO2 measurements at Soufriere Hills determined
by COSPEC, 30 July-18 August 1995. Courtesy of the USGS.

Date      COSPEC SO2 (t/d)   Comments
30 Jul-   200-600            First flight on 30 July.
  3 Aug   (average 300)      Determined that the only plausible
                             source is a degassing magma body.
4 Aug     ~550               Measurements spanned an episode of
                             vigorous venting.
6 Aug     ~1,200             Morning measurement.
6 Aug     ~250-300           Later in the day.
7-9 Aug   ~200
10 Aug    ~245 +- 25
11 Aug    ~190 +- 30
12 Aug    ~                  Wind not adequate for measurement
14 Aug    ~150 +- 20
16 Aug    ~163 +- 25
17 Aug    ~111 +- 35
18 Aug    ~180 +- 30