Page 197 - Fundamentals of Physical Volcanology

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174 CHAPTER 11

provided with power in the form of batteries or
solar panels or both. In the event of an eruption, the
equipment may well be destroyed; and because the
instruments are left unattended they can be dam-
aged by animals or tourists (or simply stolen!). For
all these reasons, fewer volcanoes are permanently
instrumented than many volcanologists would like.
Nevertheless, a growing number of volcanoes have
permanent scientific stations – volcano observa-
tories – located on them, notable examples being
Kilauea in Hawai’I, Sakurajima in Japan, and Piton de
la Fournaise in Réunion. We now review the various
ground-based monitoring methods in common use.
Fig. 11.12 A compact, three-component broadband
We have seen that magma may be stored at shal- seismometer system. The three detectors sense the three
low levels in volcanoes or may ascend directly from spatial components of seismic vibrations over a wide range
great depths. As any dike propagates it causes frac- of frequencies and are stacked vertically in a ∼9 cm outside-
turing of the rocks at its tip. On the microscopic diameter stainless steel case. This type of instrument can
be set up in the surface vault of a volcano observatory, in a
scale this is a brittle, erratic process and produces
subsurface vault or in a vertically drilled hole. (Image
quite a lot of acoustic noise as rock surfaces fail and
courtesy of Dr Cansun Guralp, Guralp Systems Ltd.)
split, and this sound propagates as seismic waves in
the surrounding rocks. These waves decay in ampli-
tude as they spread out, and so a dike propagating
at great depth is harder to detect than one near the and uplift that can occur. A complication with
surface. Thus magma rising into a magma reservoir interpreting these kinds of data is that movement
from the mantle may well not generate any seismic of magma within a volcano may cause one part to
noise noticeable at the surface, especially if it is deflate as another part is inflating. All that is detected
rising through rocks that have been heated and soft- at the surface is the net effect of the movements,
ened by earlier dikes. However, the magma reser- and so a unique interpretation may not be possible.
voir will have to inflate to accommodate the new The arrival of new magma in a shallow reservoir,
magma, and this may cause new cracks to open, especially if it causes some deformation leading to

or old ones to move, around its boundary. This the opening of old cracks or the production of new
process also produces seismic signals. Finally, once ones, may lead to the enhanced release of volcanic
magma is moving through an open dike, its motion gases. Routinely monitoring gas release is a sensible
is not quite steady, and vibrations of the walls of the precaution at volcanoes that are located near inhab-
dike occur generating a particular kind of seismic ited centers or receive a lot of tourists, because
noise called harmonic tremor. Thus keeping even if eruptive activity is not imminent, changes
several seismometers (e.g., Fig. 11.12) permanently in gas flow can occur. The commonest gases to be
in place around a volcanic center is a standard way monitored are carbon dioxide and sulfur dioxide.
of monitoring both the potential for activity and This is because juvenile water vapor, although usu-
its onset. ally released in larger amounts than the other two
The fact that magma must accumulate in a shal- gases, is easily confused with atmospheric water
low reservoir, or rise into a shallow dike or con- vapor. Various methods are used. Gas samples can
duits, before an eruption can start provides another be sucked directly into containers from the soil or
technique for anticipating eruptions: monitoring from open fractures or fumaroles for return to a
the inflation of the volcano. The way that changes laboratory for analysis. The disadvantage of this
can occur in the tilt of the ground around an method is that the gases may be hot, making it
inflating magma reservoir was discussed in Chap- hazardous to approach the sampling site and intro-
ter 4, and Figs 4.7–4.9 illustrate the kinds of tilt ducing the risk of chemical reactions between the