Jökull - 01.12.1974, Page 14

Fig. 10. An ERTS-1
image from 31 January
1973 of the Vatnajökull
area (Thorarinsson et al.,
1973). Key: 6: Grímsvötn,
3: Bárdarbunga, 2: Kverk-
fjöll, 8: Skeidarárjökull.
Notice several cauldrons
on a line leading from
the east side of Gríms-
vötn down to Skeidarár-
jökull. These cauldrons
were presumably formecl
by the collapse of a sub-
glacial tunnel after the
jökulhlaup in March ’72.
They were first observed
from Grímsfjall under a
low sun angle immediate-
ly after the jökulhlaup in
1972 (Björnsson and Hall-
grimsson, 1973). The
image suggests that the
Grímsvötn depression
consists of two calderas
(Thorarinsson et al., ’73).
Mynd 10. Mynd tekin úr
gervitungli 31. jan. 1973,
sem sýnir sigkatla, sem
líklega mynduðust við
jökulhlaupið i mars 1972.
the gravity survey (Sigurdsson, 1970). The sug-
gested thickness of the ice cover corresponds
rather well to that of a floating ice shelf in a
theoretical model presented by Weertman
(1957).
The topographical model presented is not
detailed in many respects ancl must be subject
to future revision, but it proves to be accurate
enough for the present purposes. It demon-
strates that the glacier is well above the Gríms-
vötn level north of the lake and well below
the Grímsvötn level south of the lake. Also, the
topography of the glacier bottom is well known
just southeast of the lake.
from the geothermal area required to melt the
ice will be estimated.
Tlie extent of the water basin
The Grímsvötn water basin is defined as the
area feeding ice and water into the subglacial
lake. The area contributing ice flow to the lake
is entirely determined by the glacier surface
topography and turns out to be about 300 km2.
The subglacial drainage of water presumably
takes place both in a thin water sheet and
through some subglacial tunnels. The drainage
basin of a subglacial water sheet can be deter-
mined for the topographical model. As discuss-
ecl by Weertman (1972), the pressure gradient
WATER AND ENERGY BALANCE
The flow of subglacial water and ice into the
Grímsvötn will now be described and the energy
(1) VPw = piS V hs + (Pw - Pi) g V hb
drives a subglacial water sheet with velocity v
given by the viscosity equation
1 2 JÖKULL 24. ÁR