Jökull - 01.12.1974, Síða 9

Fig. 5. A characteristic
hydrograph of a jökul-
hlaup. The jökulhlaup in
1954 (Rist, 1955). Accord-
ing to Rist, the accuracy
is ± 20%. Notice that the
flow is cut off in a few
hours time.
Mynd 5. Vatnsrit jökul-
hlaupsins 1954.
of the glacier. The thicker the glacier the less
frequent and more voluminous the jökulhlaups.
And he suggested that the release mechanism
was the same as he had earlier (Thorarinsson,
1939) suggested for glacier dammed lakes, that
is lifting of the glacier. But later Thorarinsson
(1965, 1974) concluded, on the basis of available
data on the bottom topography, that the re-
lease mechanism must be more complicated as
it seemed doubtful whether the rise in water
level could lift the glacier.
Glen (1954) pointed out that if a water-filled
hole in a glacier were 200 m deep, the dif-
ferential pressure between the ice and water
would cause plastic yielding of ice. He advanc-
ed the hypothesis that the water escaped from
Grímsvötn by causing plastic deformation of
the ice. Thorarinsson (1965) questioned the
validity of this theory. The fact that the pre-
jökulhlaup water levels in Grímsvötn were 50—
100 m higher in the early parts of this century
than they are at present could not be explained
by the model of a water-filled hole.
Addition of new data to the description of
the Grimsvötn topography has now made it
possible to obtain a model where lifting is to
be expected and the resulting jökulhlaups can
be explained.
TOPOGRAPHY of GRÍMSVÖTN,
the water basin,
and skeidarárjökull
The subglacial and surface topography of
Grímsvötn’s water basin and Skeidarárjökull
will now be described. The evidence for this
picture will be discussed later.
T opography
Fig. 6 shows the bottom topography of the
western part of Vatnajökull and Fig. 7 gives a
detailed map for Grímsvötn. The bottom of the
Grímsvötn depression lies at about 1000 m alti-
tude and is surrounded by a rim rising up to
1100 m on the north and east side but ascend-
ing 500 m to 700 m above the bottom on the
west and south side at Vatnshamar and Gríms-
fjall.
The bottom of the water basin north of
Grímsvötn has the form of a sadclle with a
ridge running relatively steeply down from
Bárdarbunga (1850 m), with the saddle point
at about 1050 m in a fairly flat area, and with a
ridge rising slowly up to 1100 m on the northern
part of the rim of the Grímsvötn caldera. The
trough running across the ridge has a flat bot-
tom inside the water basin but joins the sub-
glacial Skaftárjökull valley at the other end.
The deep and long subglacial Skeidarárjökull
valley runs close to the southeast side of Gríms-
vötn. The jökulhlaups flow down this valley.
The glacier surface falls continuously and
with fairly parallel contours from nearly 2000
m at Bárdarbunga down to 1400—1500 m at
Grimsvötn; south of the lake it drops further
down to 100 m at Skeidarársandur. The ice
thickness increases from 150 m at Bárdarbunga
to about 600 m above the water basin’s saddle
point but decreases to 400 m at the northern
rim of the caldera, and there is a 220 m thick
ice cover on Grímsvötn. The depression, at an
elevation of between 1000 m and 1100 m, is
permanently filled with 2—3 km3 of water. East
and south-east of Grímsvötn the ice thickness
increases and Vatnajökull reaches its greatest
JÖKULL 24. ÁR 7