Jökull - 01.12.1983, Side 18

Fig. 3. The depression north of Grímsvötn, 28th May 1938. Photo Pálmi Hannesson.
3. mynd. Umbrotin norðan Grímsvatna við Skeiðarárhlaupið 1938. Myndina tók Pálmi Hannesson úr Jlugvél 28. maí 1938.
(1973) has described this process for the formation
of Tertiary volcanoes in East Iceland. Further, this
process has been observed in the Krafla area in
North Iceland (Bjömsson et al. 1979, Pálmason et al.
1979).
INCREASED THERMAL ACTIVITY
NORTHWEST OF GRÍMSVÖTN
At the same time as the strength of the Grímsvötn
geothermal area has declined from 5000 MW to
4000 MW thermal activity has increased in an area
10— 15 km northwest ofGrímsvötn (Fig. 1). In this
area some thermal activity has been reckoned íbr
many decades (Bjömsson 1977), but increased acti-
vity since 1955 has resulted in 16 jökulhlaups in the
river Skaftá (Thorarinsson and Rist 1955, Rist 1973,
1947—1980, personal communication, 1983). Fig.
4 shows the volume of the jökulhlaups. The jökul-
hlaups originate from geothermal areas beneath
two ice cauldrons. The western one was first ob-
served in 1971.
The average heat flow from the eastem area can
be estimated at 800 MW and 200 MW for the
western one. We have assumed that all meltwater
drained out in the jökulhlaups and water melted by
meteorological processes was amply estimated at 20
x lCPm3yr-1 for the easternarea and 5 x lO^m^yr-1
for the other one. (Eastern cauldron drainage area S
= 50 km2, western cauldron S = 10 km2). The heat
output can be explained by the same process as at
Grímsvötn, that is, penetration of water into a
magma body. The solidification front may move 5
m yr“1 down a surface area of 1 km2 beneath the
eastern cauldron and 0.3 km2 beneath the western
one.
The thermal activity northwest of Grímsvötn
may be caused by magma intrusions after 1950. The
heat output increased continuously up to 1968 be-
neath the eastern cauldron but dropped when the
western cauldron was formed in 1971. Still, a new
depression in the ice to the west of the two cauldrons
was observed in 1982 (H. Bj'ómsson, in prepara-
tion.). The reduction in power from 1977 to 1979,
shown in Fig. 3 for the eastern cauldron, can now
be seen to have been temporary.
Looking back, we point out that since 1938 in-
trusive activity has migrated from the Grímsvötn
area towards Hamarinn (Fig. 1). This has occurred
at the same time as the thermal activity at Gríms-
vötn declined.
CONCLUDING REMARKS
Observations ofjökulhlaups from Grímsvötn ind-
icate fluctuations in the heat flux. First, a decline
has been observed in the heat output for the last 80
16 JÖKULL 33. ÁR