Jökull - 01.12.1984, Page 30

Shallow intrusions of magma are the source of
heat for the geothermal area. Meltwater perco-
lates down towards the magma intrusions and
heat is transferred upwards by hydrothermal con-
vection. Björnsson et al. (1982) have suggested
that this heat flux can be explained by penetra-
tion of water into hot boundaries of magma at
shallow depths. Assuming an upper surface area
of 10 km2 for a magma body under Grímsvötn,
water penetrating into that body would have to
propagate at an average rate of 5 m/yr to yield the
observed flux of 5000 MW. The geothermal activ-
ity, however, is not limited to the lake but scat-
tered over a larger area, estimated to be up to 100
km2.
The Grímsvötn area is one of the most (if not
the most) powerful geothermal systems in Ice-
land. So far the heat transfer in the geothermal
system upwards to the lake has not been studied.
The circulating fluid is probably liquid domin-
ated. As the system is situated within the active
volcanic area the fluid probably attains a base
temperature of 300 to 340 °C and is presumably at
the boiling point when it flows as water and steam
through vents into the lake. The bottom of the
Grímsvötn lake is situated at about 1000 m a.s.l.
(Björnsson 1974). The pressure at the lake floor
varies from 30 bar at the end of jökulhlaups to 40
bar when the water level has risen to the critical
level. Hence, the boiling temperature at the lake
floor varies from 235 to 250 °C. The transfer of
heat to the lake by steam and water is discussed
in the paper.
The heat transfer within the lake has not been
studied either. Boiling water and steam would be
injected into the lake through hydrothermal
vents and thermal plumes ascend into the lake.
Convection in the lake brings heat up from the
floor to a level where the temperature is 4 °C.
From that level heat is conducted in a thin layer
to the melting ice cover. Meltwater at 0 °C is
continuously added to the top of the lake from
the ice cover and mixed with warmer water. The
conducting layer can hardly remain stable. The
hydrothermal vents that inject boiling water to
Grímsvötn are located on fissures and scattered
spots, which are believed to cover only a small
part of the lake floor. The floor between the
vents is heated by conduction and is much cooler.
Chemical analyses, presented in the paper, show
that the concentration of magnesium in water
from Grímsvötn is similar to that of cold ground
water. The magnesium was leached out of hyaloc-
lastites at the lake floor. The leaching cannot
have occurred at temperatures exceeding 30-40
°C and the water has not been exposed to higher
Fig. 3. Variations of water level in Grímsvötn.
2. mynd. Vatnshœð í Grímsvötnum frá 1954.
28 JÖKULL 34. ÁR