Jökull - 01.12.1983, Síða 16

Fig. 1. YVestern Vatnajökull.
1. mjnd. Vesturhluti Vatnajökuls.
the time prior to the next jökulhlaup would be
shortened and the rate of melting overestimated for
that period. The result would be a low succeeded
by a peak. If, on the other hand, the level fell
deeper than normal the melting rate would be
overestimated for that period and may be under-
estimated for the next one. Such irregularities
would, however, equal out in the long run and not
distort calculations of average values for the melt-
ing rate.
VARIATIONS IN MELTING
AT GRÍMSVÖTN
Fig. 2 shows a base heat flux which was at about
5000 MW from year 1850 to about 1900, but has
since then declined continuously. Bjömsson et al.
(1982) have suggested that this base heat flux
could be explained by heat extraction of water
which penetrates into hot rock boundaries of
magma at shallow depths. The decline in the heat
flux from 5000 MW to 4000 MW in 80 years may
reflect continuously deeper penetration of water
into an underlying magma body and reduced heat
exchange with increased depth. With a similar
trend the geothermal activity would disappear in
300 years. We may, however, expect continuing
intrusive activity so the geothermal system will be
maintained.
Four peaks rise above the base heat flux of
4000—5000 MW shown in Fig. 2. They reflect
exchange of heat at a rate which is two to three
times higher than for the base flux. Such peaks
may be caused by eruption of magma to the glacier
bed and subsequent rapid cooling through direct
contact with ice or water in Grímsvötn. Observa-
tions suggest that this may apply for at least two of
the peaks (in 1938 and 1867). All the peaks are
connected with jökulhlaups which were unusual in
some respect.
The highest peak on Fig. 2b is prior to the jökul-
hlaup in 1938. This jökulhlaup was unusual. The
hydrograph rose to a maximum in three days and
receded slowly in two weeks. The typical hydro-
graphs for jökulhlaups havea different shape. They
rise to peak value in a fortnight and recede in a few
days. Further, the jökulhlaup in 1938 was unex-
pected as it occured only four years after the jökul-
hlaup in 1934. A depression was formed in the
glacier surface at the northern slopes of Grímsvötn
in connection with this jökulhlaup. The depression
was 7 km long, 2 km wide and 200 m deep, with a
400 m deep circular crater at the northemmost end
(Fig. 3, Thorarinsson 1974). The depression was dis-
covered from the air but no expedition went to the
area. About 3.5 km3 of water must have drained
from this area into Grímsvötn and triggered the
jökulhlaup. An intrusion of0.3 km3 oflava up to the
glacier bed could have produced the meltwater.
About 20 m thick lava flow spread over 15 km2
would have cooled down from 1100°C to 0°C in a
few weeks or months.
The rate of accumulation of water in Grímsvötn
was rather high during the first year after the jökul-
hlaup in 1938. Evidence recently provided by Ragn-
ar Stefánsson (personal communicadon 1982, 1983)
has verified the occurrence of a previously un-
authenticated jökulhlaup in Skeiðará in June of
1939. This had a sulphurous smell which disproves
the former hypothesis that the flood originated
from draining of a marginal lake. Furhter, he esti-
mates the total volume to have been similar to that
of the jökulhlaup in 1982 — that is ofabout 1 km’.
The second peak on Fig. 2b is related to the
jökulhlaup in 1867 which also had an abnormal
14 JÖKULL 33. ÁR