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MENACE OF 'EXTINCT'
VOLCANOES (Continued)
Rip-Van-Winkles
and legends of
Sleepy Hollows
10
circumLPacific magmas do not stretch
round the whole globe in a continuous
layer beneath the earth's crust but form
pockets within the crust, that because
of their viscosity, their mobility is
extremely low, and that they contain
a high quantity of dissolved gases, we
can understand why seismographs and
tiltmeters cannot, as in the case of
basaltic volcanoes, clearly warn of the
approach of an eruption.
If such be indeed the case, the
magma would normally be quite near
to the surface and the seismic effects
accompanying any possible rise of the
magma would not be distinguishable,
as in the case of Hawaii, by their focal
depth from the tremors due to various
causes which are constantly occurring
in the upper kilometres of any active
volcano.
Moreover, this magma is often so
viscous that the speed of its rise is
greatly reduced, if not nil. The seismic
effects connected with the rise of the
nragma may thus be lost among ordi¬
nary earth tremors, making it very dif¬
ficult if not impossible for the seismo¬
logist to distinguish genuine foreshocks.
Tiltmeter readings would be
equally useless: the volcano will ob¬
viously not swell unless matter is rising
up
inside, it.
How do these volcanoes erupt at
all if there is little or no rise of the
lava from the magma chamber towards
the surface? It may be that the action
of the gases alone is responsible.
Years or centuries may pass and
as yet we have no means of telling
from the surface of the earth that this
slow concentration of endogenic ener¬
gy is going on. As a result, such a
crater will soon come to be classified
as belonging to an extinct volcano
and we know the terrifying conse¬
quences which. this may have.
In these circumstances, how can
we forecast a renewal of activity? In
the first place, at the risk of repeat¬
ing myself, I would say that we must
get it into our heads that whatever the
type of volcano, magma or activity con¬
cerned, we shall never be able to pre¬
dict anything with any accuracy unless
a constant watch is kept by a specia¬
lized team.
Once this has been established, and
accepting the theory that the violent
explosions of volcanoes of the circum-
Pacific type are in fact the result of
the accumulation of gases under the
roof of the chamber, it would seem
logical to look for significant signs in
possible changes in the fumaroles
which the crater exhales to a grea'ter
or less extent and which have their
origins inside the pocket of incubating
lava. Changes discovered in this way
may not always be easy to interpret
in so far as they can be interpreted
at all but logically they must hold a
clue to what is going on down below.
T HE temperature of some
fumaroles has been recorded for a long
time back, on the logical assumption
that the temperature will rise as an
eruption approaches. However, with
acid volcanoes at least, this method of
detecting an eruption has had practical¬
ly no success. This is not surprising if
we accept the theory that explosive
eruptions are the result of the building
up of gas pressure and not of the rise
of magma, since it is essentially the
latter which determines the rise in
temperature.
We are thus left with the chemical
composition of fumaroles, which ought
to depend on the deep-lying processes
mentioned above. The reflection of
these processes in the chemistry of
the fumarole gases should provide
valuable information.
Observation of a dormant volcano
may not require analyses at very close
intervals, but the development of the
chemical composition and pressure of
the fumaroles should at least be follow¬
ed step by step. Since only gases are
involved, this alone might yield warn¬
ing signs, however slight, by which to
detect renewed volcanic activity. But
the best hope for a better understand¬
ing of volcanic activity, and, hence, of
developing volcanological forecasting
is to make a close study of the varia¬
tions, both sudden and gradual, in
the gases given off from the mouth of
an active volcano sampled at a fixed
point.
This is the job with which we have
been particularly concerned; to try and
analyse the volcanic gases as nearly
continuously as possible, and to look
for warning signs in the variation in
their composition and in the compari¬
son between this variation and varia¬
tions detected by other means such
as the seismograph and the clinometer.
The first samples of gas taken by
our group were analysed in a labo¬
ratory by Dr. Marcel Chaigneau, direc¬
tor of the Gas Laboratory at the Centre
National de la Recherche Scientifique
in Paris, using the Lebeau and Damiens
method over a mercury trough. The
results were extremely accurate but
the operations took so long that, with
the resources at our disposal (i.e.,
without special volcanological staff or