Jökull - 01.12.1991, Blaðsíða 37

Fig. 14. The sinkhole that formed in
Svartibunki near the end of the
jökulhlaup in 1954 viewed from the
northwest (photo Sigurður
Þórarinsson, August 28, 1954).
Strokkurinn sem myndaðist í
Svartabunka í lok hlaupsins 1954.
Horft úr norðvestri.
of the order of 500 mm/yr within the 160 km2
Grímsvötn basin. In order to estimate the surface
ablation in Grímsvötn in the period 1934-1960, we
have divided the drainage basin into three areas.
First, the 20 km2 ablation area where we estimate the
surface ablation to have been 2500-3000 mm/yr (ele-
vation 1350-1450 m a.s.l.). Second, we use a value of
1500 mm/yr in a 20 km2 surrounding area (elevation
1350-1500 m) and third, a 120 km2 area (elevation
1500-1700 m) with an ablation of 200 mm/yr, similar
to present conditions. The mean ablation within the
basin according to this model is estimated as 650-710
mm/yr or 30-40% greater than the present value.
OPENINGSIN THEICE SHELF
Volcanic eruptions can create openings or sinkholes
in the glacier but it has been a matter of some discus-
sion whether reports of such holes can be taken as
sure signs of volcanic activity. Depressions or caul-
drons in the ice surface are scattered over the area
surrounding the Grímsvötn lake (Fig. 1). These caul-
drons are created by subglacial geothermal activity.
Over the period of observations, these cauldrons
have varied in number, size and shape. Inspection of
the vertical airphotos (from 1945, 1946, 1954, 1960,
1972, 1984 and 1986, Table Al) shows, however,
that most of the cauldrons are observed over the
whole period. Several cauldrons are located under the
southern caldera wall and in the Svartibunki area, the
former northem margin of the subglacial lake in the
northwest corner of the depression (Fig. 1). Judging
from the size of the cauldrons, geothermal activity
under the southern caldera wall does not seem to
have decreased. On the other hand the cauldrons
located a few km north of Svíahnúkur Eystri have
decreased in size since 1960 and the same seems to
apply to the cauldrons in the Svartibunki area.
According to air photos and field observations
(Sigurðsson, 1942; 1984; Þórarinsson, 1953b) alarge
patch of open water existed in summer along the
western slopes of the caldera in the forties and early
fifties. This patch was 50-100 m wide and several
hundred metres long. This indicates intense heat
JÖKULL,No. 41, 1991 35