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Montserrat eruption
Monitoring on Montserrat:
Montserrat, the Emerald Isle of the Caribbean, is not so green any more.
Simon Young reports on the eruption so far and prognoses for the future.
T
he erupting Soufrière Hills volcano has,
over the past 33 months, spread ash and
lava blocks over much of the southern
half of the island of Montserrat. People who
have not left the island entirely have been
forced to move towards the undeveloped
northern tip of the 39 square mile British Overseas Territory (figure 1).
Since surface activity began in July 1995, the
Montserrat Volcano Observatory (MVO) has
provided the base for monitoring operations,
using the widest variety of geophysical and
geological tools available. The British Geological Survey, in collaboration with UK academia
and regional colleagues at the Seismic Research
Unit (University of the West Indies) and Institute de Physique de la Globe at volcano observatories in the French Antilles, and with the
assistance of international experts from the
USA, Japan and Europe, has led efforts to document the eruption and provide hazard and
risk assessments. As well as providing scientific data, the monitoring effort has enabled considerable progress to be made in crisis management and the interaction between scientists,
authorities and the public in a natural disaster.
The current eruptive episode began with seismic unrest beneath the volcano between 1992
and 1994, similar in style but less intense than
three previous crises between 1897 and 1967.
For the first time in the documented history of
the island (colonized in 1632), surface manifestation of this enhanced seismicity began on 18
July 1995 and has gone on ever since. Four
months of phreatic activity – explosions without eruption of fresh magma – gave way to continuous dome growth. More than 250 million
cubic metres of andesitic magma have since
reached the surface. Early seismic activity was
dominated by volcano–tectonic earthquakes up
to 5 km beneath the volcano but, once the
dome began to grow, long-period seismicity 1
to 2 km down has dominated. Magma flux has
increased throughout the eruption (figure 2), an
unusual pattern. This increasing magma flux
from the 5 to 6 km deep magma chamber has
led to progressively more violent activity.
A chronology of the eruption
Violent activity was first manifest during the
spring and summer of 1996, when a series of
dome collapses occurred, generating pyroclastic flows out of the open side of the crater and
into the Tar River valley (figure 3). This series
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of collapses culminated in the first truly explosive event, on the night of 17 September 1996.
Pressure in the upper part of the feeder conduit
dropped rapidly after about 150 m of dome
overburden had been removed, and a short
explosion threw metre-sized blocks up to
1.5 km, generating a column of ash and
pumice 15 km high.
Increased magma flux was matched by
increased sulphur dioxide production and an
increasing rate of deformation in the upper
part of the edifice. Deformation became localized in November and December 1996, with
rapid degradation of the southwestern crater
wall, accompanied by intense hybrid earthquake swarms. A small explosion and pyroclastic flows in mid-December 1996 marked a
switch back to vigorous extrusion, generating
more pyroclastic flows in the Tar River valley
in January and in the White River valley to the
southwest in March and April 1997.
Increased rates of deformation on the lower
flanks of the volcano, as well as increasing gas
and magma flux, culminated in a large dome
collapse from the northern flank on 25 June,
generating pyroclastic flows and surges which
devastated several villages and led to the loss of
19 lives within the declared danger zone. High
magma flux was maintained throughout July,
accompanied by cyclic inflation and deflation
of the lava dome (measured by electronic tiltmeters on the crater rim) with hybrid earthquake and rockfall activity. The capital of the
island, Plymouth, was overrun by pyroclastic
flows, burying or burning vital infrastructure.
Some depressurization from continued dome
collapses, with the presence of magma rich in
gas high in the feeder system, led to a series of
13 short-lived, vulcanian explosions in early
August. These occurred within strong seismic
and deformation cycles and were remarkably
evenly spaced. Most generated pumiceous
pyroclastic flows in valleys around the volcano, as well as ash columns over 10 km high.
Similar, though longer-lived and less regular
explosive behaviour followed collapse of the
northern flank of the dome on 21 September. A
total of 75 explosions occurred, on average
9.5 hours apart, all but one generating fountain-collapse pyroclastic flows, and most generating ash columns more than 10 km high
(figure 4). Magma crystallization, forming a
cap on the conduit, so that pressure builds up
is likely to control these multiple explosions.
Fig. 3: Pyroclastic flow in the Tar River valley, spring 1996. Photo
he current eruption of the Soufrière Hills
volcano has lasted three years; it is likely
to continue and seems to be increasing in
violence. Seismic activity, ground deformation,
gas emissions and the composition of the ash
and lava erupted have all been monitored,
giving a data set valuable for understanding
the eruption mechanism as well as assessing
the hazards to local people. The eruption has
followed a pattern of increasingly explosive
T
Dome growth switched back to the southwest side of the crater in early November and
continued apace until Christmas Day 1997,
when frequent, large hybrid earthquakes,
merging periodically into continuous tremor,
preceded the most violent events yet at the
Soufrière Hills volcano. Collapse of the crater
wall on the southwest side produced a debris
avalanche in the White River valley, rapidly
followed by massive dome collapse and one or
more laterally-directed blasts generating violent pyroclastic surges which destroyed several
villages on the southwestern coast (figure 5).
About 45 million cubic metres of dome lava
were involved in this collapse, five times larger
than any previous single collapse event. A
small tsunami was generated as 20 million
cubic metres of material entered the ocean
April 1998 Vol 39
Montserrat eruption
the course of an eruption
1 The island of Montserrat,
situated towards the northern
end of the Lesser Antilles
island arc.
N
St John's
Centre Hills
W.H. Bramble
Airport
Salem
Harris
Cork Hill
St George's
Hill
S Young, BGS/NERC copyright.
Seismology of the eruption
Earthquake activity associated with the current
eruption began in 1992, reaching a peak in late
1994. Although poorly-located due to a limited seismic network at the time, these events
appear to show a progressive upward migration in focal depth. Surface activity began after
seven months of seismic quiescence and was
associated with swarms of volcano–tectonic
earthquakes both beneath the volcano and up
to a few kilometres away. Installation of a
dense short-period seismometer network
around the volcano brought better locations,
and hypocentres less than 5 km below the
April 1998 Vol 39
Tar River
2 Total volume of magma
erupted from the Soufrière
Hills volcano, measured by
dome growth and deposit
accumulation rates.
St Patrick's
4 km
2
volume (millions of cubic metres)
4.5 km from the dome; it had an estimated
wave-height of 1 to 1.5 m where focused in a
bay midway up the west coast of Montserrat.
Long Ground
Soufriere Hills
Volcano
Plymouth
eruptions, generating larger pyroclastic flows
that travel farther from the crater. This
appears to be a result of faster ascent of the
magma to the surface; more gas is held in the
magma until eruption, resulting in more violent
explosions. Of particular interest has been the
recognition of cyclic behavior of the volcano
and the coincidence of dome inflation with
enhanced seismicity, followed by rock falls or
pyroclastic flows.
English's
crater
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15 Nov 24 Jan 3 Apr 12 Jun 21 Aug 30 Oct 8 Jan 19 Mar 28 May 6 Aug 15 Oct 24 Dec 4 Mar
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eruption days
crater began to dominate. Phreatic explosions
and gas venting were associated with sharponset and tremor signals respectively. An array
of digitally-telemetered broadband instruments
was installed in October 1996 and, despite
losses during eruptions, a remarkable set of
data has been collected, recording dome-related seismicity in greater detail than ever before.
Hybrid earthquakes have dominated the seismicity since before the dome started to grow.
These are low-frequency events with impulsive
P arrivals, at shallow depth beneath the crater,
and probably caused by gas resonance within
the magma conduit. Long-period earthquakes,
with dominant frequencies of 1–2 Hz and
emergent onsets, probably have a similar
source mechanism but in a shallower location,
perhaps within the dome itself. Long-period
earthquakes are not common, but occur more
frequently on occasion, with up to 60 events
per day. The volcano–tectonic earthquakes that
dominated at the start of the eruption have
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Montserrat eruption
Formal hazard and risk assessments
PHOTOS S YOUNG, BGS/NERC COPYRIGHT
Hazard and informal risk assessments have
been carried out by the Montserrat Volcano
Observatory (MVO) throughout the crisis,
but a thorough re-assessment and formalization of the process was requested by the UK
Government following the prolonged period
of explosive activity at the volcano in September and October 1997. A meeting of 14
scientists, almost all of whom had been intimately involved in operations at MVO, was
held in early December. A review of scientific information relevant to the hazard assessment was given and every conceivable scenario for behaviour over the following six
months constructed. Through a formal
process of expert elicitation, a probability
tree was constructed for these scenarios, providing information on how likely a certain
event was and conditional probabilities on
hazardous phenomena reaching certain areas
on Montserrat.
Once probabilities were known and some
measure of uncertainty was assessed, the formal risk assessment took place. The population distribution of Montserrat at present
and in a number of different evacuation situations was used, along with expert assessments of the degree of impact of particular
hazardous phenomena on different areas (i.e.
the vulnerability of the population and infrastructure to the different phenomena).
Through a Monte Carlo simulation process,
the number of casualties for different scenarios was evaluated and a probability of one,
five, ten etc deaths in the next six months
assessed with different population distributions. In this way, the current societal risk
exposure for Montserrat was presented and
options for reduction of this risk exposure
presented by means of altering the population distribution. Individual risk was also
assessed for different populated areas.
Significant uncertainties surround any
quantitative assessment of risk due to the
inherent instability of the volcanic system.
However, proven statistical methods for
assessing low probability events and
accounting for large uncertainties can be utilized, and the end product of assessments
such as these has proven extremely useful for
decision makers in the Montserrat crisis.
Fig. 4: Vertical eruption column ascends over
Montserrat, as viewed from 15 miles west of the
island, October 1997.
Fig. 5: Devastation of the southwestern part of
Montserrat by debris avalanche, dome collapse
and lateral blast, Boxing Day 1997.
become less common, but occasional swarms
still occur, 2–4 km beneath the crater. These
are high-frequency events with P and S phases.
Other seismicity includes signals from rockfalls
and pyroclastic flows (high frequency, emergent signals with a characteristic cigar-shaped
profile) and explosions signals with associated
volcanic tremor (both phreatic and magmatic,
with dominant frequencies at 1–2 Hz).
Hybrid earthquakes have often been recorded in swarms and some show remarkable
sequences of near-identical events occurring at
regular intervals up to five times per minute for
many hours. These repetitive swarms sometimes develop into tremor as the individual
events merge into a continuous signal, with the
dominant frequency staying constant (figure
6). In the past year, occasional swarms of large
hybrid earthquakes have been recorded by seismic stations on neighbouring islands.
Hybrid and long-period events are thought to
be caused by gas under pressure within the
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dome and upper conduit area, and they are
often associated with changes in surface activity on a variety of time scales. Long-period
earthquakes often trigger rockfalls from the
dome, suggesting that they are caused by gas
venting locally. Perhaps the most remarkable
aspect of the seismic activity has been the association of hybrid earthquake swarms with
near-crater deformation recorded by tiltmeters,
and the generation of pyroclastic flows.
Electronic tiltmeters: pressure gauges
One of the clearest indicators of surface
processes on volcanoes is deformation of the
ground. The use of electronic tiltmeters to
measure ground deformation on volcanoes is
well established, since the May 1980 eruption
of Mt St Helens. Tiltmeters were installed at
three locations on the Soufrière Hills volcano
early in the crisis, but all were too far from the
growing lava dome. A tiltmeter was first
installed in December 1996 on Chance’s Peak,
within a few hundred metres of the lava dome,
to track deformation specifically associated
with degradation of the southwestern crater
wall. Harsh conditions at this location meant
that only a relatively limited data set could be
collected, but deformation associated with the
instability of the crater wall was recorded, as
were cycles of inward and outward tilt apparently centering on the lava dome and co-incident with swarms of hybrid earthquakes.
After replacement of tiltmeters on Chance’s
Peak in May 1997, a remarkable two-month
record of near-source deformation was collected (figure 7). Strongly cyclic behaviour of the
volcano was noted in the few days prior to the
large dome collapse on 25 June; cycles comprised a steadily increasing intensity of hybrid
earthquakes, which then dropped suddenly
from a peak, replaced by intense rockfall or
pyroclastic flow activity. Cycles repeated every
8 hours or so, and coincided with steady inflation of the lava dome during the hybrid swarm
and rapid deflation as the rockfall activity
began. On the basis of these well-developed
seismological and deformation cycles, warning
sirens were sounded prior to the onset of pyroclastic flows early in the afternoon of 25 June,
enabling Plymouth and the airport on the east
coast to be evacuated. Unfortunately, many
people had ventured too far into the danger
zone and could not hear the sirens; 19 lost their
lives and many others had lucky escapes.
Cyclic activity diminished in early July but
picked up again in late July and early August as
the next phase of intense activity began. The
single remaining tiltmeter was destroyed in a
series of vulcanian explosions on 5 August
1997; clear tilt cycles had been recorded prior
to this explosion, and it is thought that strong
tilt cycles would have matched the cyclic seismicity throughout these explosions.
April 1998 Vol 39
Montserrat eruption
Mick Murphy at Bristol University (Britain)
and Joe Devine at Brown University (USA) and
co-workers, have gained useful insights into
the volcano from samples collected as part of
the monitoring effort. The magma is a typical
Lesser Antillean andesite. It has spent some
time within the crust below the volcano: experimental work at Bristol and University of California, Berkeley, by Jenni Barclay has defined
the magma holding depth as at least 5 to 6 km,
with temperatures of between 830 and 850 °C.
Lava in older pyroclastic flow deposits at the
volcano, very like the lava currently erupting,
suggests a long-lived (>30 000 years) magma
storage region, periodically reheated and remobilized by injection of mafic magma. If enough
new magma arrives over a long enough period,
it may start surface activity and dome growth.
Petrological studies by Devine and Mac
Rutherford at Brown also show increasing
magma flux through the eruption. When
magma ascends to the vent, the outer parts of
hornblende crystals react to form rims of different composition. Rims thinned from
120 µm (implying 30 days ascent) to zero (less
than 5 days ascent) between the first erupted
magma (December 1995) and that erupted in
the summer of 1996. Since then, ascent rate
has been too fast for any reaction rims.
The increasing propensity for explosive eruptions involving highly vesicular pumice is consistent with less-complete gas loss during the
faster ascent of the magma. But there is as yet
little firm indication that the storage region has
changed. Near-surface phenomena, such as
sealing of the conduit, may reduce gas loss;
microlite crystallization within the feeder system also affects surface processes.
What next?
The long-term prognosis for the eruption is for
a continuation of similar styles of activity for
perhaps several more years, with impacts confined to the southern half of the island. The
inconvenience of ash and small clasts falling in
the northern, inhabited, part of the island will
April 1998 Vol 39
17.12
17.32
–13070
15083
17.52
–18545
22984
18.12
–16700
50510
18.32
Petrology of the Soufrière Hills magma
11620
18.52
A preliminary model for these cycles has been
proposed, and it is hoped that two tiltmeters
recently installed within a kilometre of the
dome will record similar cycles over a longer
time period. Gas-charged magma migrates in
batches up through the upper part of the conduit and dome, generating hybrid earthquakes
and inflating the upper conduit as pressures
increase. When there is high dome overburden,
the cycle peaks as the magma pulse is injected
into the dome, leading rapidly to increased ash
and gas venting and rockfalls. If there is little
overburden or if the magma is richer in gas, the
pressure is released more rapidly, through
short-lived vulcanian explosions.
–37353
44691
–26260
31933
–30909
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2
time (min)
4
6
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Fig. 6: Repetitive hybrid earthquakes merging into continuous volcanic tremor, December 1996.
–50
Chances 3 x-axis
–100
–150
–200
–50
–100
–150
Chances 3 y-axis
–200
2000
long ground RSAM
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1000
500
400
300
gages triggers/hour
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May
25
June
1
June
5
June
10
June
15
June
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June
25
Fig. 7: Strong cycles of seismicity and dome pressurization recorded by seismometers and electronic
tiltmeters close to the lava dome.
continue, but recent detailed hazard and risk
assessments by MVO suggest that risk in the
far north of the island is very low (see box).
Monitoring will continue at a high level and
constant revision of hazard and risk assessments will take place. MVO continues to provide accurate and timely information and
advice to the authorities on Montserrat and in
the UK as well as collecting a unique and
diverse set of geological and geophysical data
which will further volcanological science. ●
●
The first collection of papers on the
Montserrat eruption is to be published in Geophysical Research Letters later this year.
Reports by the MVO on current activity are
available on the Web at: “http://www.geo.mtu.
edu/volcanoes/west.indies/soufriere/govt”.
Dr Simon R Young works at the British Geological
Survey, Murchison House, West Mains Road, Edinburgh, and has been one of the Chief Scientists at
Montserrat Volcano Observatory since 1992.
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