Jökull - 01.12.1984, Síða 47

behaviour cannot be predicted. They might
become catastrophic if the activity prevented the
ice overburden from closing the tunnels out of
the lake. A volcanic eruption within the lake
would melt the floating ice cover but would not
cause a rapid rise in the lake level because of the
lag in flow of ice into the lake from the surroun-
ding glacier. We would not expect such an erup-
tion to spark off a jökulhlaup immediately if the
lake was somewhat below the critical level
(assuming, of course, that the activity did not
change the triggering mechanism and the critical
level remained the same; see Björnsson 1974).
The eruption in Grímsvötn in May 1983 is an
example of such an event. Increased melting in
the lake, however, may accelerate the rise of the
water level (especially if an opening is formed in
the ice cover along Grímsfjall and the ice can
flow freely into the lake unimpeded by the moun-
tain). That would hasten the occurrence of a
jökulhlaup if the area of the lake increased only
slowly in response; the resulting jökulhlaup
would have no substantial increase in volume
compared to jökulhlaups before the melting
increased. But the increased inflow of ice to the
lake would only be temporary as it is limited by
the mass balance of the drainage basin. The
floating line of the ice cover would then move
outwards as the ice cover and the surrounding
glacier became thinner. The area of the lake
would increase and we would expect less frequent
and more voluminous jökulhlaups.
An eruption north of the lake would immedi-
ately drain meltwater to the lake and cause a rise
in the lake level that might trigger a jökulhlaup
(Björnsson 1974). This happened in 1938.
Steinthórsson and Óskarsson (1983) discussed
the effect of increased geothermal activity and
suggested that the volume of jökulhlaups would
increase but their frequency remain constant. We
can agree to the suggestion of increased volume
but not to that of constant frequency. Their
suggestion of constant frequency seems to be
based on a model of a steady state flow of ice into
a lake of constant area. The steady state assump-
tion is a valid approximation in the long run but it
is questionable whether the area of the lake
would remain constant if the geothermal activity
increased. Their suggestion of increased volume
is based on the observation that more water
would be drained out of the lake if the ice cover
was thinner. We could agree if it implied that the
area of the Iake was larger, thus increasing the
volume discharged from the lake. But if the area
of the lake is to be constant, the drained water
volume is the same whether the floating ice cover
is thick or thin. Therefore their model seems to
imply that the lake is drained empty in the
jökulhlaups. But that is not suggested by the
authors and all observations indicate that the ice
cover on the lake is floating at the end of jökul-
hlaups.
CONCLUSION
Geothermal activity in the Grímsvötn area is
expressed by depressions in the surface of the ice
cap; a number of small depressions are superim-
posed on the main Grímsvötn depression. Ice is
diverted to the depression where it melts and
water accumulates in the Grímsvötn lake. The
lake is covered by a floating ice shelf. Waterpools
are observed along the slopes of Grímsfjall and
occasionally hot springs have been reported when
the surface of the lake is at low levels. The
geothermal activity observed on Grímsfjall is
minimal in terms of steam outlets and alteration
of ground. The water level is 200-300 m below the
observed steam outlets on the mountain and by
condensation and evaporation of local water all
H2S has been washed out of the steam that
escapes from Grímsfjall. The oxygen isotope data
are in agreement with the chemical data and
show extensive fractioning and depletion of lxO
relative to leO. Chemical studies of the vapour
from the fumaroles yield little information about
the deep reservoir fluid.
Information about the geothermal fluid at
Grímsvötn is obtained from the rivers on Skeidar-
ársandur during jökulhlaups. This information is
not easy to interpret because of complications
involving water-rock interaction in the lake; this
applies to the soluble cations (and Na/K thermo-
meters are not applicable) but to a lesser extent
to other elements like silica, carbonate, chloride,
fluoride and sulphate as well. Silica solubility
data and assumptions about the likely tempera-
ture in the geothermal reservoir, however,
enables one to estimate the mass of the geoth-
ermal fluid discharged into the lake. The geoth-
ermal mass fraction is estimated 14-16% of the
total mass in the jökulhlaups. The mass and
energy balances require that steam is 20-35% (by
mass) of the geothermal fluid that enters the
lake. The mass flow of geothermal water to the
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