Can geophysical modelling predict the end of the Soufrière eruption?
D Elsworth, G
Mattioli, J Taron, B Voight and R Herd, Implications of magma transfer between
multiple reservoirs on eruption cycling.
Science (2008) 322: 246-248.
Science is about
data and models. First you collect a mass of data. Then you spend anything up to
a lifetime trying to devise a model that satisfactorily explains your facts. If
you doubt the word “lifetime”, remember Charles Darwin and evolution. But the
goals of science models are well worth the effort put into devising them
because, if correct, they bring some order into the apparent chaos of a mass of
data and may lead to the predictions about the future of a phenomenon.
eruption of Montserrat’s
Soufrière Hills volcano (SHV) has produced many data sets with unparalleled
detail and continuity. In turn, these data sets have encourage people to
generate many models about aspects of how the volcano has behaved so far, and
therefore might do in the future. The instances when the eruption has shown
cyclic activity have especially fascinated modellers. For example, lava domes on
SHV and other explosive volcanoes often have periods of repetitive cyclic ground
deformation, degassing, seismicity (earthquakes), lava extrusion and explosive
eruptions -- the cycles taking hours to days.
Lensky, Sparks, Navon and
Lyakhovsky have recently (2008) modelled these distinctive cycles in terms of a
cyclic “dance” between the resistance of a rigid frozen lava plug capping the
lava conduit, bubbly magma beneath this cap and gas-bearing magma deeper in the
volcano that can foam as soon as the pressure above it drops.
cyclic behaviour of SHV since 1995 has recently attracted the attention of a
team of modellers based on both sides of the Atlantic (USA, UK) and Australia.
The volcano has alternated between 2-3 year periods of extruding lava on and
around its summit (with or without explosions, dome collapses etc) and quiet
periods for 1.5-2 years, when it just sits there innocently and invites
foolhardy islanders and tourists to try to approach it too closely.
causes this cyclic behaviour? Elsworth and his colleagues recently published their ideas
in the eminent journal Science. Their starting point was two
sets of data: (1) estimates of the volume of lava extruded (peacefully as domes
or violently in explosions) at the SHV summit; (2) the movements of GPS
monitoring stations all over Montserrat during the same period.
GPS (Global Positioning System) stations on the island in 2003-5 are shown on
Figure 1. They were all, of course, the sorts of ultra-accurate instruments
that can record positions to within 1 mm or so.
Fig. 1. Sites of GPS stations on Montserrat in 2003-5.
The red triangles are long-term stations and the blue diamonds are
temporary ones. The arrows at some stations show how they moved
horizontally during that period (note scale below).
volcano was quiet during that period, with no lava extrusion. But meanwhile the
GPS data show that the magma reservoirs below were continuously inflating, like
a balloon under a blanket. Put some ink marks on the top of the balloon and blow
it up. You will see the marks move both
from each other and
as the balloon inflates. This means that the GPS data (Fig. 2) show that the
during quiet (no lava/explosions) periods and
when surface lava extrusion is active.
Fig. 2. The top panel shows the cumulative erupted
magma throughout the eruption. The lower two panels show vertical
movements at two of the GPS stations during the same period. The volcano
clearly slowly “breathes”!
next point that Elsworth and his colleagues had to fix for their modelling was
the number of magma reservoirs beneath the volcano. The GPS data alone cannot
give a firm answer to this question but other published studies can help. During
the early years of the eruption, research into seismic (earthquake) signals from
beneath the volcano agreed with other lines of enquiry, such as the compositions
of minerals in the erupted andesites and also in samples of powdered rock heated
experimentally to various temperatures at various pressures. All this evidence
pointed to a single large (about 4 km3) magma reservoir only about 5
km below the summit of SHV.
the focus of research on the erupted rocks turned to the small (up to a few cm)
scattered “dark blobs” seen in many samples. Although the lava surrounding them
the chemical composition of the blobs is
(lower SiO2 and higher MgO than andesite; see Topic 1 of this
website – “General introduction: the science view”). Basalts are relatively hot
magmas that trickle into the base of the Earth’s crust beneath volcanoes such as SHV, having formed by “wet” melting in the mantle above the subduction zone
underlying the Lesser Antilles. In the long term they develop, by fractional
crystallisation, into andesite (see section 1) but they also play a short-term
role, by reheating reservoirs of earlier semi-crystalline andesite and thus
triggering eruptions. The growing consensus of opinion is that this magma-mixing
reservoir at SHV is deeper that the main andesite one, at about 10-12 km depth.
More recent geophysical data also fit this view.
“magmatic plumbing” beneath SHV used by Elsworth and his colleagues for their
modelling is shown in Fig. 3.
Fig. 3 “Magmatic plumbing” used by Elsworth and his
colleagues in their modelling calculations. Don’t be confused by the
umpteen symbols referring to equations in the article. The essentials of
this diagram are listed in the text below.
two magma reservoirs are drawn spherical because there is no detailed evidence
yet of their real shapes. The magma reservoirs are labelled I (shallow) and II
(deep). The connections between the reservoirs (and also both from the Earth’s
mantle below and up to the surface above) are considered to be narrow enough to
contain little magma, compared with the main reservoirs. Two long arrows from
the upper magma reservoirs point to two GPS stations (1 and 2). Small arrows
above each of these show how the slight surface bulging caused by magma pumping
into reservoirs several km below cause the surface of Montserrat to move
slightly upwards and away from SHV (and then to return when the magma leaves the
reservoir). Obviously similar arrows can be drawn from the deeper reservoir to
GPS stations 1 and 2.
stage at SHV is now set for the geophysical modellers to do their mathematical
magic. The mathematical direction Elsworth and his colleagues took is called
inversion, where you measure effects of a process and use maths to figure out
what caused these effects. Inversion can be thought about simply by imagining a
bomb going off somewhere in a village. Ask villagers which direction they heard
the explosion come from and draw all these directions as lines on a map of the
area. The crater will be where the lines all converge. Now repeat the experiment
with an earthquake somewhere beneath an area dotted with seismometers, each
recording the arrival of the earthquake shock waves at different times. A
seismologist can then use inversion maths to pinpoint the source of the
the SHV “magmatic plumbing” Elsworth and his colleagues take the accumulated
records of the volumes of magma erupted, and the gyrations of the island-wide
GPS stations over 12 years, and calculate the volumes of magma (as rates of flow
in metres per second) moving in and out of both reservoirs throughout the
eruption. At this point you may ask; “Fascinating stuff but so what?” Most of
the detailed conclusions, such as those shown on Fig. 4, are indeed strictly
for “magmatic plumbers” (who may be Jo in the USA but more likely
the UK!) to think about but
one that really matters to all Montserratians is shown in Fig. 5.
Fig. 4. Some of the plots summarising the calculated
amounts of magma entering (red) and leaving (blue) the two reservoirs
(magma chambers) during the eruption. (Efflux here means the eruption
periods of SHV).
Fig. 5. The graph that matters! Again hard to follow
but the dotted line shows how an elastic pressurised body (e.g. a
balloon) would shrink as it deflated (very slowly!). The lower (deeper)
magma reservoir is doing this stepwise. E=eruption; P=pause. For this
model the diameter of the “pipe” (conduit) between the two reservoirs is
only 30 metres. It is probable that the new Gages Vent – source of the
vicious 28 July 2008 explosive eruption – is much the same diameter
below the surface.
appears that, although the upper (shallower) reservoir is the direct source of
all the erupting lava, this has not changed significantly in volume (~4 km3)
over the 12 years of the eruption (to 2007). Therefore
the lower (deeper) reservoir that fuels the activity.
For instance, the lower reservoir continues to receive magma from below at a
steady rate of about 1 cubic metre per second, whether or not there was/is
surface eruption. Furthermore, during the last twelve years the lower magma
reservoir has been steadily
Projecting this process into the future, Elsworth and his colleagues estimate
95% of the magma that is going to come out of SHV during this long eruption has
now done so.
course these conclusions may not be gospel. It will take the labours of other
equally well qualified groups of geophysicists and mathematicians to check every
detail of the present model and its possible errors or alternatives. That is how
science slowly progresses. But, for the moment, let us take the results of the
model at their face value. Does this mean that the end of the eruption is nigh?
It is extremely tempting to jump to this conclusion but the trap is to assume
that, because about 95% of the magma
has been erupted, therefore only about 5% of the
of the eruption remains. Oh that volcanoes were so simple! The eruption might
end even sooner, if some critical part of the magmatic plumbing freezes, or
later, if that last 5% takes its time oozing or blasting out. We shall see;
watch the MVO website!
In 2010 this group
updated their work in a short report that
re-estimated the depth of the shallow magma reservoir at about 5 km depth and
the deeper one at about 17 km depth. The deeper one appears to change volume
about 20 times faster than the shallower one and therefore dominates in the
fuelling of the eruption.