Smithsonian Institution
Bulletin of the Global Volcanism Network, v. 20, no. 11/12, November-December 1995
Soufriere Hills (Montserrat)  Dome building, minor ash eruptions

Soufriere Hills
Montserrat, West Indies
16.72 N, 62.18 W; summit elev. 915 m
All times are local (=3D GMT - 4 hours)

Although there was relative quiet during October (Bulletin v. 20,
no. 10), during the first 10 days of November three large phreatic
eruptions occurred. Each of these eruptions blanketed Plymouth, 4.5
km W of the active vent, with ~2 mm of ash (table 1). Dome growth
within the crater started on 16 November, the estimated date when
juvenile material first reached the surface, and continued through
at least December. Estimates of the dome's rate of growth from 16
November to 6 December were on the order of 0.5 m^3/sec.

Small rockfalls from the flanks of the new, locally incandescent
dome were witnessed on several occasions. During early December,
debris from a larger rock avalanche was seen in the moat of
English's Crater. As of early January, neither local avalanches nor
material liberated during the failure of spines escaped the crater
area. The limited mobility of the rock avalanches suggested they
were not propelled by gas explosions with great overpressures.
Although floods and dilute mudflows were distinguished seismically,
no significant debris avalanches or pyroclastic flows occurred.

Heavy rainfall after 11 December may have triggered several small
ash emissions, depositing red-brown ash on the upper W-flanks. The
ash presumably consisted of non-juvenile material, from rock
avalanches sloughing off the new dome, and some hot juvenile ejecta
from small explosions vented in or around the new dome.

Although quantitative SO2 flux measurements were lacking, as of
early December related damage to vegetation extended ~3 km downwind
and 1.5 km laterally. Tree damage was severe on the upper W flank.
Gases sampled at three of the established fumaroles (soufrieres)
around the volcano showed no change in composition. Although gas
and acid aerosol production had been at enhanced levels from
mid-November to early December, air sampled in Plymouth during
early December contained very little SO2.

Dome growth. Beginning on 30 November, good visibility allowed
observers to watch a single dome develop from two smaller bodies
(figure 5). One body was NW of the September cryptodome (an
intrusion that produces a surficial bulging), and the other at Vent
1. The evolving dome had a rough blocky carapace that initially had
some small (<5 m high) protruding spines. Two of these spines
became taller in subsequent days; others failed and broke off the

A prominent spine on the new dome's E side grew in height until 7
December when it began to collapse. The spine's maximum vertical
growth rate was estimated to be 5-8 m/day. Further dome growth at
a slower rate occurred until 9-10 December, and slower growth, or
a possible halt, continued as late as 13 December. On 13 December
a small, radial crack on the N side of the new dome emitted steam
and ash for most of the day. At least two columns reached in excess
of 500 m above the crater rim.

A new batch of extruded material reached the surface on 15
December. On the 17th, in addition to widespread incandescence
radiating from the new dome, observers saw a new ~40-m-tall spine.
Between the 17th and 20th the spine grew vertically at 7 m/day, and
the adjacent dome also rose, but at a slightly slower rate. The
spine's growth rate during some undisclosed intervals reached up to
20 m/day. On 17 December observers also saw a narrow crack in the
dome within Vent 1 that emitted glowing ejecta. Many small ash
releases sent columns up to ~1.1 km above the summit.

During the week ending 27 December, several spines grew 5-10 m/day
then subsequently collapsed. One spine had grown to ~15 m higher
than Castle Peak (summit elevation ~910 m) prior to failing late on
25 December.

Explosions on 21 December produced a mildly convecting ash cloud
that rose ~1.5 km above the volcano. Ash fell to the N, reaching
the N portion of the island. Although apparently phreatic events
took place in early- to mid-November, this was the most vigorous
explosion since then and it may have been driven magmatically.
Steam production remained constant during 21-27 December, feeding
a plume that sometimes carried small amounts of ash. From 28
December to 3 January there was relative quiet and slow dome
growth. Only 3 m of dome growth took place during the week, and for
a least a few days after about 1 January, the dome may have ceased

Deformation. Data from two electronic tiltmeters showed no
significant changes during the crisis. Despite their stability,
around 10 November deformation in the upper part of the volcanic
edifice was recorded by EDM and GPS measurements at Castle Peak
Dome and Chances Peak. Four days of significant deformation were
followed on 15 November by intense seismic activity (see below).
These were followed on 17 and 18 November by an upward extension of
the dome that formed in September. The dome also appeared to have
extended slightly towards Chance's Peak. Although visibility was
poor for the next 10 days, glimpses through steam and cloud cover
suggested further doming and rock avalanching. These processes
influenced a wide area on the NW side of Castle Peak Dome,
including the edge of Vent 1.

From mid-November until about mid-December, the rate of deformation
remained very low, with daily shortening on the order of a few
millimeters along most lines, even those aimed at the presumably
less stable upper flanks.

The EDM data for 10-12 December showed lengthening of the lines to
Castle Peak--a deflation of the edifice. Around this time, a longer
interval of GPS data also showed their lines had lengthened by >1
cm overall (with some shorter-term variability). This rate was
equal to or greater than the average rate during the month of
October. Late December deformation measurements using GPS and EDM
techniques suggested either a return to slight inflation (14-20
December) or stability (21-27 December).

Seismicity. Montserrat seismic activity falls into four categories:
1) tremor, 2) long-period events, 3) volcano-tectonic earthquakes,
and 4) regional earthquakes.

After 15 November, elevated seismicity prevailed with relatively
few quiet periods. The pattern appeared very similar to that seen
in late September associated with the formation of a cryptodome and
possibly associated with the later extrusion of a spine. The
elevated seismicity was inferred to be due to a high-level magmatic

After 27 November there was a loss of discreet, locatable events.
Low-amplitude tremor became intermixed with intervals of intense,
low-amplitude, long-period events; these arrived at rates of up to
5/minute but were recorded only on the closest seismic station
(MGAT, Upper Gages, figure 6). In early December tremor increased
somewhat at other stations farther from the crater (MLGT, Long
Ground, and MBCT, Bethel); at this time amplitudes of events at
Gages also increased and the RSAM seismic index rose as high as it
has been since 15 November.

Until 9 December there were also small, frequent, long-period
earthquakes. These were accompanied by low-to-variable amplitude
tremor at the Gages station, but tremor disappeared from all other
stations by 8 December. The number of locatable earthquakes dropped
to 1-2/day, the lowest observed during this crisis. Located
earthquakes were mostly volcano-tectonic and at slightly greater
depths (0-5 km) than the long-period and hybrid-type earthquakes
that had dominated since 24 November. High-amplitude,
high-frequency tremor was recorded at station MGAT for several
hours during 10-11 December; this was probably due to an increase
in steam venting from several areas on Castle Peak.

The dome grew during the week ending on 13 December, with few
accompanying earthquakes early on 6 December. In contrast, during
14-20 September there were 2-20 locatable earthquakes/day, many
with epicenters along the N flanks at depths of 0-6 km. During the
week ending on 20 December all stations registered earthquakes with
emergent onsets and a dominant frequency of 2.2 Hz; these took
place 5-15 times/day. Some of the earthquakes corresponded to small
explosions. Heavy rains on 16-19 December triggered floods and
dilute mudflows who's acoustic signals were detected by the seismic

Information Contacts: Montserrat Volcano Observatory, c/o Chief
Minister's Office, P.O. Box 292, Plymouth, Montserrat; Seismic
Research Unit, University of the West Indies, St. Augustine,
Trinidad (URL:

Table 1. Summary of the daily behavior of Soufriere Hills, 1
November through 11 December 1995. The table omits most geophysical
and geodedic observations, however, "eruption signal" refers to
seismically determined eruptions, and "mudflow signal" refers to
seismically determined mudflows. Courtesy of MVO.

DATE  EVENT & COMMENTS (local time)

1 Nov
      Ashfall (1129).
2 Nov
      Ashfall in Trails, Brodericks, and surrounding areas (0118).
      Explosions accompanied by light ashfall in Upper Gages and
      Chances Peak (1923).
3 Nov
      Mudflow (0254); Steam-and-ash emissions resulting in light
      ashfall in Parson's-Amersham and Plymouth (1122). Continued
      enlargement of Vent 1. Steam-and-ash emission (1122). No
      major changes in Castle Peak.
4 Nov
      Eruption signal (0247), no reported ashfall. Eruption
      signal; one eruption generated an ash plume reaching 2.5-km
      high; several millimeters of ash fell in Amersham-Plymouth
      and S of Plymouth (1725).
5 Nov
      Eruption signal (0139), no reported ashfall. Mudflow toward
      Fort Ghaut (0214). Minor eruption without visible ash or
      steam (1307). Eruption signal (2030).
6 Nov
      Minor mudflow (0410). Increase in the size of Vent 1.
      Ashfall, light (0347) in crater area and steam plume, 1.5-km
      high. Eruption signals (1044 & 1809), no ashfall.
7 Nov
      Eruption signal (0123), no ashfall. Ashfall (0815). Eruption
      signals (2018 & 2358).
8 Nov
      Eruption signal (0935).
9 Nov
      Ashfall, several millimeters accumulated in areas to the W
      and SW of the vent (Kinsale, Amersham, Plymouth, and
      Richmond) (0419).
10 Nov
      Eruption signals (0145, 0420, & 1348). Plume of ash and
      steam (1535), 1.5-km high, blown SW.
11 Nov
      Mudflows in Gages-Fort Ghaut areas (0548 & 0743). Eruption
      signal (0733), no ash emission.
12 Nov
      Eruption signal (0247), no ash emission. Steam emission from
      several new vents SW of main activity area. Old vent
      reopened S of Vent 1.
13 Nov
      Eruption signal (0600). Minor ash and steam (1603), blown N.
14 Nov
      Minor ash-and-steam emission (1610). Continued steam
      emissions from vents first observed on 12 November. Vent
      closest to Castle Peak greatly increased in size, surrounded
      by fresh ash.
15 Nov
      Minor ash-and-steam emission (0900-1000). Noise of breaking
      rocks, small landslides, venting heard from crater.
16 Nov
      Poor visibility but felt earthquakes, loud venting,
      rock-impact sounds, and light ashfall at Chances Peak
      (1500), with some drifting SW into the Broderick's area.
17 Nov
      Episodes of light ashfall in Amersham. Landslides had
      partially filled the Vent 1 crater. The September dome grew
      in height and extended toward Chance's Peak. Vigorous
      steaming at the two vents between Castle Peak and the dome.
18 Nov
      Occasional landslides at the edge of Vent 1.
23 Nov
      Noises heard from crater (rock breaking and small
      landslides). CO2 detected in the summit area for the first
24 Nov
      Noises heard from crater (as above).
26 Nov
      Confirmed emergence of a new spine adjacent to the September
      spine and close to Castle Peak.
28 Nov
      Sound of breaking rocks heard from crater.
29 Nov
      Sound of breaking rocks heard from crater.
30 Nov
      Confirmed lava dome within Vent 1.
1 Dec
      Dome slowly growing in Vent 1 crater; attendant ash emission
      and rock avalanches. A second area of dome growth identified
      NW of September spine. Two small ash clouds drifted towards
5 Dec
      Rapid increase in the size of and the number of cracks
      within the new (26 November) spine. Increased emission of
      steam and light ash of reddish color.
6 Dec
      Lava dome glowing, visible from the airport.
7 Dec
      Reddish ashfall (0929) accompanied a small explosion.
      Continued slow growth of lava dome.
8 Dec
      Lava dome had broken along cracks. Deformation continued in
      the area around the September and November domes. Ash cloud
9 Dec
      About 20 minutes of mudflow signal recorded at Gages seismic
      station (0434). Explosion with light ashfall (1419, 1520).
      Dome growth rate slowed.
10 Dec
      Mudflow signal recorded at Gages seismic station (2240).
11 Dec
      Rusty brown ash eruptions, ashfall W of crater (0910, 1455,
      1530, & 1604). No major dome growth detected. Steam emitted
      with variable intensity at a vent close to Castle Peak.

Figure 5. Topographic map of the crater area at Soufriere Hills
showing pre-eruption morphology (thin lines) and new features (bold
lines) as of 10 December 1995. Contour interval is 50 feet, values
shown are feet x 100 (3.28 feet =3D 1 m); coordinates shown are UTM.
CH indicates Chances Peak; CA indicates Castle Peak. Courtesy of

Figure 6. Montserrat seismic stations and epicenters shown in map
and cross-section views, 10 December 1995. The intersection of the
two cross sections is indicated by an asterisk. Epicenters are
shown with two symbols, indicating variations in data quality
(square, A and B quality; cross, C and D quality). Stations MSAT
and MPVF were off line; MVPZ and MSSZ were 3-component stations.
Courtesy of MVO.