Jökull - 01.12.1984, Side 31

temperatures for any length of time. Furth-
ermore, we may add, as the water emerges on
Skeidarársandur during jökulhlaups, the temper-
ature has been measured 0 °C (Rist 1955). The
temperature of the lake has neither been mea-
sured nor estimated by theoretical models but we
believe the average temperature is close to the
melting point.
Water in the Grímsvötn lake is a mixture of
meltwater from the glacier and fluid discharged
from the geothermal system. The chemical com-
position of the two components is rather diffe-
rent. The circulating geothermal fluid attains a
base temperature of around 300 °C and obtains
its chemical composition by interaction with the
ambient rocks. When water from the geothermal
area in Grímsvötn has reached Skeidarársandur
in jökulhlaups, the chemical composition of the riv-
ers changes drasically. The first sign of a jökul-
hlaup is a strong sulphurous (H2S) odour from
the river Skeidará that can be noticed some days
before the discharge begins to increase. The col-
our of the river changes, sometimes even before
the jökulhlaup commences. All jökulhlaups on
Skeidarársandur start in Skeidará, and as they
proceed the flood may emerge in Sandgígjukvísl
and finally in Súla in the largest burst.
Considerable information about the Grímsvötn
area has been obtained by studies of the jökul-
hlaups on Skeidarársandur. The discharge rate
and total volume of the jökulhlaups have been
estimated (Rist 1955,1973,1976, see Thorarinsson
1974). The sediment transport has also been esti-
mated, both the total load and chemical composi-
tion (see Thorarinsson 1939,1974, p.165, Rist
1955, Tómasson 1974, Tómasson et al. 1982).
Furthermore, chemical studies of water from the
river Skeidará have been presented by Rist
(1955), Sigvaldason (1965) and by Steinthórsson
and Óskarsson (1983).
The present work presents chemical data (col-
lected by the National Energy Authority and the
Science Institute) for the rivers on Skeidarársand-
ur during jökulhlaups in 1976,1982 and 1983 and
in the period between the last two jökulhlaups.
The data show the normal chemical concentra-
tions in the rivers in 1982/83 when they are not
influenced by jökulhlaups. With this background
we can distinguish the various changes in the
chemical composition that appear in the rivers
during jökulhlaups. In addition, we can distingu-
ish certain chemical changes in jökulhlaups from
Grímsvötn when the geothermal area is influ-
enced by volcanic activity. On the basis of gene-
ral knowledge about high temperature geoth-
ermal areas we can explain the chemical composi-
tion of water in Grímsvötn in terms of interaction
with highly reactive basaltic glass in the lake.
However, the concentration of some substances
like silica is not changed by water-rock interac-
tion and by using data for those we estimate the
fraction of the geothermal fluid in the water
mixture from Grímsvötn.
The heat flux in Grímsvötn shows sporadic
increases, which may be of the order of 2 to 3
times the base flux of 4000 to 5000 MW
(Björnsson 1983). During the last 120 years these
peaks constitute about 10% of the total heat flux.
They are believed to be caused by intrusions of
magma to the base of the glacier. Altogether the
inflow of magma into the Grímsvötn area is esti-
mated to be about 50T06 m3/yr of which 10% is
erupted to the surface and 90% is solidified in the
upper crust and cooled down by the hydrother-
mal convection (Björnsson et al. 1982, Björnsson
1983).
No volcano in Iceland has shown a higher
eruption frequency than the Grímsvötn volcano.
Thorarinsson (1974) attributed at least 50 erup-
tions to the volcano in historical times and
Jóhannesson’s (1983) recent studies of historical
documents add still more eruptions to the list.
The latest eruption in Grímsvötn occurred in
May-June 1983. Grönvold and Jóhannesson
(1984) describe the course of events and chemical
studies of the tephra and Einarsson and
Brandsdóttir (1984) analysed the seismic activity
leading up to the eruption. The present paper
adds information about the recent volcanic activ-
ity based on geochemical studies of water in the
jökulhlaups.
DESCRIPTION OF GEOTHERMAL
ACTIVITY
The mountain Grímsfjall marks the southern
rim of the Grímsvötn caldera (Fig.2). The two
highest peaks of the mountain form nunataks
where hyaloclastic rocks are exposed to the sur-
face: W- Svíahnúkur (1700 m) and E-Svíahnúkur
(1725m). The northern face of the mountain
beneath the two nunataks forms almost vertical
cliffs, 300 to 400 m high. There hyaloclastic rocks
with intermediate basalt layers are exposed. Gla-
JÖKULL 34. ÁR 29