Jökull - 01.12.1963, Page 14

Influence of Geothermal Activity on the Chemistry of
three Glacier Rivers in Southern Iceland
BY
GUDMUNDUR E. S I GVALDASON
UNIVERSITY RESEARCH INSTITUTE, RF.YKJAVÍK
INTRODUCTION.
On October 1961 a volcanic eruption occur-
red in the Askia caldera in NE-Iceland. Few
weeks before the eruption a hydrothermal fielcl
developed on a fault line cutting perpendi-
cular to the later volcanic fissure. The study of
this thermal activity (Sigvaldason 1964) indi-
cated a strong emission of gases such as HoS
an CO2, which were partly absorbed in the local
ground water. Continuous observations of gas
emission from thermal fields adjacent to dor-
mant volcanoes might therefore in favorable
cases serve as a means of predicting a volcanic
eruption. This would be of special interest in
volcanic areas where other methods of predict-
ion such as tiltmetry are not applicable. A
number of icelandic volcanoes fall under this
category, the most important being Katla in
South-Iceland. Eruptions from this volcano,
which is covered by a thick sheet of ice have
caused considerable damage and even devastat-
ion, to nearby farmsteads, besides cutting off
the main traffic lines to the southeastern part of
the country. Destruction is primarly caused by
glacier bursts, which cause temporary flooding of
the lowlands, where everything is swept away by
the current, and by ash fa.ll, which may affect a
large part of the country. Effective predicting of
a natural catastrophe of this magnitude woulcl
therefore be most helpful in saving life ancl
property by taking necessary precautionary steps
in time.
During the past centuries Katla has erupted
more or less regularily two times each century
(Thorarinsson 1959). The last eruption occurred
in 1918 and a new eruption is of growing con-
cern to all those which might in some way be
affected by its violence. Because of the glacial
cover on Katla the only method of prediction,
which has been applied is seismic recording.
Seismographs under dayly observation are situ-
atecl in two nearby villages and a warning
systeni has been established, which will be put
in effect if unusual tremors should be observed.
Increased seismicity may, however, not be re-
corded until at a very short notice and the
necessity for a prediction method, which could
indicate the coming eruption days or weeks
ahead is obvious. The possibility of increased
emission of volcanic gases prior to the eruption
might provide means to prediction some time
in advance.
Thermal areas are known to exist beneath
the ice cover of Mýrdalsjökull, but none is
directly accessible. Rivers flowing to the south
and fed by the glacial melt water do however
contain components derived from these thermal
areas. One river especially, Fúlilœkur (The Foul
River), smells with hydrogen sulfide, which is
present in solution in amounts up to 2 ppm
H2S. A continuous observation of the chemistry
of these rivers would reflect major changes oc-
curring in the emission from the thermal areas.
Such changes have actually occurred in con-
nection with volcanic events under glaciers in
South Iceland as indicated by strong H2S and/or
SO2 odors of river waters, which normally clo
not carry detectable amounts of these com-
pounds.
The purpose of the present study is to deter-
mine the normal concentration levels of various
compounds in the river waters over a periocl of
one year. Furthermore an attempt will be made
to explain the origin of each component founcl
in the water, whether it is derived from the
thermal areas or from other sources.
GENERAL GEOLOGY.
Katla is a part of a large volcanic massive,
which is covered by two separate glacial sheets,
JÖKULL 1963
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