Jökull - 01.12.1984, Síða 48

Grímsvötn lake is estimated to be 0.60-0.83 1011
kg/yr, and 0.24-0.34-1011 kg/yr of steam. The
mass of ice melted in the lake is estimated as
4.0-4.2T011 kg/yr. The total thermal power of the
Grímsvötn area is estimated 4700-4900 MW, of
which 2100-3000 MW are derived from steam and
1900-2600 MW from water. No marked changes
have ever been observed in the geothermal activ-
ity on the mountain ridge. Inside the depression,
however, the activity has been reduced since 1960
as expressed by ice cauldrons and fumaroles. But
in the autumn of 1983 melting of ice in places due
to increased thermal activity on the northeastern
slopes of Grímsfjall may have opened waterways
into the lake and triggered a jökulhlaup. An
odour of hydrogen sulphide was detected from
the river Skeidará for weeks before the jökul-
hlaup commenced. The jökulhlaup occurred at a
water level 20-30 m lower than the critical level
required for triggering the jökulhlaups that are
discharged from the bottom of the Grímsvötn
lake. Such a mechanism may have triggered the
small jökulhlaups in 1939,1941,1945 and 1948.
Sulphurous odour for long periods may warrant a
forecast of such premature jökulhlaups.
Grímsvötn is one of the few geothermal sys-
tems where active volcanism is observed and
where there is a direct interaction between
magma and geothermal water. Evidence of volca-
nic activity was found by the high content of
sulphate and the presence of iron in the jökul-
hlaup of December 1983. Volcanic activity in
1983 has increased the S04 concentration in the
Grímsvötn lake. The high S04 content in the
jökulhlaup in 1965 may suggest volcanic activity
in Grímsvötn at that time. The iron content in the
jökulhlaup in December 1983 indicates eruption
of magma into the geothermal fluid.
A NOTE ADDED IN PROOF.
In June 1984 the authors visited Grímsvötn.
Then, a water pool was observed at the foot of
W-Svíahnúkur, in the crater of the 1983 eruption.
The pool was 40—50 m wide and 300—400 m long.
The water level was at 1385 m a. s. 1. Hot springs
with temperature up to 80°C were located along
the entire southem bank of the lake and vigorous
emanation of gas was observed at the lake floor.
The temperature at the lake surface was 4—6°C.
Steam emanated from loose tephra south of the
pool and sulphur was precipitated at the surface.
Sulphurous odour arose from the crater as well as
from an open crevasse east of E-Svíahnúkur. A
30-50 m deep trench in the ice surface was obser-
ved along the eastern slopes of Grímsfjall. The ice
cauldron that collapsed in December 1983 (Fig. 4)
was situated in this trench.
ACKNOWLEDGEMENTS
Sveinbjörn Björnsson reviewed the paper and
suggested several improvements, especially on the
section of the geothermal mass fraction. Jón Orn
Bjarnason read the manuscript and suggested
many improvements. The selection of data in
Table 3 for the Krafla and Námafjall geothermal
areas was discussed with Halldór Armannsson
and Magnús Ólafsson. The Public Roads Author-
ity financed the collection and analyses of chemi-
cal data since 1983. The laboratory work was done
by Kristján H. Sigurdsson. Sigfús J. Johnsen
kindly prepared and supervised the analyses of
samples for bI80 at the Geophysical Isotope
Laboratory in Copenhagen. Expeditions to
Grímsvötn have been organized by members of
the Iceland Glaciological Society and supported
by the National Power Company of Iceland and
the Public Roads Authority.
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46 JÖKULL 34. ÁR