Jökull - 01.12.1974, Blaðsíða 6

Fig. 2. A map of Gríms-
vötn surveyed by Gunn-
ar Thorbergsson in June
1960 (Rist, 1961). The
lake is situated north and
below a 300—400 m ver-
tical wall on Grímsfjall,
but faced at the westside
with sloping liillrocks at
Vatnshamar. Open water
and display of natural
steam holes are found at
some places alongside
these mountains, but else-
where the lake is covered
with glacier ice.
Mynd 2. Kort af Gríms-
vötnum mœlt af Gunnari
Þorbergssyni i júni 1960.
foundly affected the Icelandic environment.
Repeated tremendous bursts of water, the jökul-
hlaups, originate at the lake. Thirty to forty
jökulhlaups have been traced, two of them
back to the early 14th century (Thorarinsson,
1974). Many of the jökulhlaups have been ac-
companied by volcanic eruptions. The jökul-
hlaups run down the broad Skeidarársandur
to the sea, attacking vegetation and temporari-
ly deterring fish from entering coastal waters.
An important effect has been the disruption
of the Icelandic road system. The next jökul-
hlaup can be expected in 1977—78 with un-
certain consequence for the expensive new roacl
across Skeidarársandur. It is therefore important
to investigate whether artificial interference in
the jökulhlaup mechanism is possible. To do
that one must fully understand the glaciological
problem.
The cause and process of these jökullilaups
has been a much clebated topic during the last
forty years (Áskelsson, 1936a, 1936b, 1959; Niel-
sen, 1937a, 1937b; Thorarinsson, 1939, 1953,
1965; Glen, 1954). Recently, Thorarinsson
(1974) has written a major work on the history
of jökulhlaups from Grímsvötn and has given
a review of the present problem. So far the
mechanism has been unexplained but various
clata have been gathered about the process and
about conditions within Grímsvötn. On the
basis of a review of all available data this paper
4 JÖKULL 24. ÁR
presents an explanation of the mechanism of
jökulhlaups from Grímsvötn.
A general description of Grimsvötn
and the jökulhlaups
The Grímsvötn lake is situated inside a vol-
canic caldera of about 30—40 km2 in the in-
terior of the accumulation area of Vatnajökull,
Fig. 1 and Fig. 2. A high temperature hydro-
thermal area is situated inside the caldera. A
water basin of about 300 km2 stretches to
warcls Bárdarbunga. Both the surface ablation
and the ice flow toward the geothermal area.
During the last tliree decades jökulhlaups have
occurred at five to six years intervals. The water
level has dropped 80—100 m in a fortnight’s
time. The water escapes through a 50 km sub-
glacial waterway down to Skeidarársandur. The
jökulhlaup thus removes the ablation accumu-
lated over five to six years from a 300 km2
area of Vatnajökull. After a jökulhlaup the
water level in the lake gradually rises again
(Fig. 3).
A cross section of the ice cap and the sub-
glacial geothermal area is shown in Fig. 4. The
presence of the geothermal area is obviously a
very important factor for the hydrology of the
ice cap. Its melting of ice affects the surface
slope of the ice cap and is therefore respon-
sible for directing ice and water flow into the
caldera. If no geothermal area were present the