Montserrat Volcano Observatory
Overview of the Eruption
at Soufriere Hills Volcano, Montserrat
29 March 1997
Introduction | Observations | Expert Elicitation | Concluding Comments
Introduction | Observations | Expert Elicitation | Concluding Comments
Introduction
A general review of the activity of the Soufriere Hills Volcano, Montserrat
has been carried out with the principal purpose of providing the
Governments of Montserrat and the UK with a synopsis of the state of the
volcano and its scientific evaluation. The overview is based on a meeting
at Bristol University on the 3rd March, a meeting at the Montserrat Volcano
Observatory on the 6th March and an expert elicitation involving 5
scientists in the UK and 10 MVO scientific staff on Montserrat. The major
findings to date are summarised. Some concluding remarks are given on the
extent of success amongst the scientific team in anticipating the course of
the eruption and the prognosis.
Observations
Since the onset of dome growth in mid-November 1995 approximately 77 x 106
m3 of andesite magma has erupted as of 27th March 1996.. Experimental and
analytical studies of the magma constrain its origin from a source region
at between 5 and 6 km depth with an initial temperature of between 820 and
850oC. The magma has been intermingled with higher temperature magma,
likely derived from a deeper source and responsible for a heating event
that has affected the andesite magma. The magma lost much of its gas on the
way to the surface and crystallised extensively. As a consequence the lava
erupts with very high viscosity. However it is clear that the lava does not
lose all its gas efficiently and that high pressures can develop in the
erupting magma. Evidence for high pressures include September 17th
explosive eruption and the long period earthquakes are attributed to
movement of pressurised fluid along fractures.
Since the beginning of the eruption the volume of erupted magma has been
documented with determinations of volume being accomplished at intervals
varying from one week to a month. The rate of eruption has shown marked
variations on time scales of a few hours to a few weeks. Some of the
largest pyroclastic flow events and the 17th September 1996 explosive
eruption were associated with extrusion rates of up to 700,000 m3/day.
Despite marked fluctuations, inspection of the volume data since magma
first reached the surface indicates that the rate of eruption built up in
the first few months and has then maintained a fairly consistent rate of
about 200,000 m3/day ever since. Furthermore the height of the dome at the
time of this assessment is 942 m asl, which is approaching the greatest
height of 970 m asl in July 1996. A major conclusion is that the rate of
activity has remained at a high level for almost a year with no signs of
diminishing and no signs of any significant decay in the driving pressure.
These results are the basis of a consensus amongst the scientists that the
eruption shows no signs of finishing and is likely to continue for a
considerable time to come.
Seismicity has been a primary monitoring tool throughout the eruption and
the seismic network was enhanced in October 1996 when the broad-band
network was successfully installed. The seismicity has shown a rich
diversity of phenomena. Earthquakes have been predominantly shallow (ie
less than 2 km depth) and located within or beneath the dome, apart from
deeper earthquakes in the early phase of the eruption and for a few
isolated instances after the 17th September explosive eruption. Earthquakes
early in the eruption were more widespread, with locations beneath St
Georges Hill and along a broadly NE-SW line extending from Englishs Crater
to Long Ground and out to sea. Types of earthquake include classic
volcano-tectonic earthquakes, long period events, continuous tremor, events
related to rockfalls from the growing dome and the unstable Galways Wall
and shallow earthquakes known as hybrid events. The hybrid events have
dominated activity since the 17th September explosive eruption and are
characterised by both high frequency components similar to volcano-tectonic
earthquakes and long period components. The MVO is currently revising the
classification of the earthquakes. The earthquakes described here as hybrid
are reported as VT earthquakes in the daily and weekly scientific reports.
The hybrid earthquakes occur in swarms and in the October to early December
1996 period a remarkable anti-correlation developed between swarms and dome
growth. The earthquake patterns have been more complex since the New Year
began with the reappearance of banded tremor and episodes of increase in
the frequency of hybrid earthquakes merging into continuous tremor followed
by generation of rockfalls and pyroclastic flows activity on the dome.
These observations demonstrate that magma production, dome growth and
earthquake activity are intimately related. While present understanding of
these relations is very limited and inhibits their use as predictors of
short-term activity, they are the most important diagnostic that activity
is ongoing at depth.
Deformation studies, using the EDM and GPS methods and supplemented by
studies of fracture movements and a tilt station on Chances Peak, have
continued to generate important information on the eruption. The most
significant finding is that the deformation is almost entirely confined to
areas close to the dome. The GPS surveys show that locations further than
about 1km from the dome have shown almost no relative horizontal movements
since 1972, when the area was first surveyed, and during many repeated
measurements during the eruption. There is some evidence emerging of slight
subsidence on these distal locations which could prove significant. The
minor horizontal motions on some lines between measurement stations were
largest in the early phase of the eruption. Various measurements show large
movements of the ground close to the growing dome. The old Castle Peak site
on the SE margin of the dome has been monitored by EDM throughout the
eruption until 2 months ago when it was finally destroyed by the
encroachment of the dome. It typically expanded outwards from the dome at
several mm/day with a total movement of over 1.3 m. Other sites on Farrells
crater wall and Chances Peak have also shown evidence of outward motion.
The lack of a widespread deformation field is not consistent with a large
relatively shallow magma chamber deforming the crust elastically.
The most dramatic deformation of all, other than the dome itself, has been
that related to the deformation of Galways Wall, which began to crumble in
early October with development of extensive and deep fractures on the
southern shoulder of Chances Peak. Much of the upper parts of the wall have
now disintegrated and recently rockfalls from the still incandescent
pre-September 17 dome have moved over the wall down towards the Galways
Soufriere. Movement on the Chances Peak cracks have continued up to the
time of writing with movement rates being largest during earthquake swarms.
When the Galways wall first started to deform the most likely outcome for
the next three months was assessed to be slow crumbling and this has proved
to be the case. There still exists a fairly wide spectrum of scientific
opinion on the likelihood of a substantial failure of the wall and the
threat of a lateral blast. Generally there is a consensus that the threat
of a large-scale sudden failure has diminished. The most recent development
is minor signs of instability developing at the Gages Wall.
Gas studies have involved the now routine surveying of the plume by the
COSPEC method, some exploratory work with the FTIR method and information
on the gases in glass inclusions preserved from the deep source chamber.
These studies have confirmed that this is a very sulphur poor volcano and
that the major gas species is water with a concentration of about 4% in the
original magma. Studies in the early stages of the eruption indicate a
correlation of SO2 flux with seismicity. High rates of dome growth and
pyroclastic flow generation also approximately coincide with higher SO2
fluxes. Work by the Open University group has shown that the concentrations
of SO2 in the plume are affected by chemical reactions within the plume as
it moves away from the volcano, introducing added uncertainty to the
measurements.