Jökull - 01.12.1974, Blaðsíða 20

the Torfajökull area but twice as powerful with
a flux density of ip = 45—50 W/m2.
As volcanic eruptions have been frequent in
the Grímsvötn region, a high flux density of
geothermal energy is to be expected. According
to Sigvaldason (1965), the presence of SO2 in
the water of the jökulhlaups indicates that the
base temperature beneath Grímsvötn is higher
than that observed in any other higli-thermal
area in Iceland.
Friedman et al. (1972) estimated the strength
of the Kverkfjöll geothermal area to be 1200—
2300 MW; that is 14 to 14 of the present esti-
mate for Grímsvötn. The Kverkfjöll thermal
area is not more than 10—20 km2 so this esti-
mate would give an average flux density for
Kverkfjöll which is still higher than the present
one for Grímsvötn.
THE MECHANISM OF JÖKULHLAUPS
A model of the topography of the Grímsvötn
water basin and Skeidarárjökull has been dis-
cussed and some understanding of the import-
ance of the geothermal area in filling the lake
with water has been obtained. The mechanism
of jökulhlaup triggering will now be considered.
The triggering of a jökulhlaup by lifting
In Grímsvötn continuous accumulation of
water goes on until the water level has risen
about 100 m (Fig. 3). Then a sudden cata-
strophic flood occurs.
Consider the distribution of the ice over-
burden pressure p( = pj g H4 around the lake
and the distribution of the subglacial (hydro-
static) water pressure which would exist if there
were a hydraulic connection between the glacier
bed and the lake P = pwg (hw — hb). The sym-
bol Hj represents the glacier thickness, hw re-
presents the water level in Grímsvötn, hb re-
presents the elevation of the glacier bottom,
pj and pw denote the density of ice and water,
respectively, and g represents the acceleration
of gravity. It appears that the ice overburden
pressure ps is greater than the possible sub-
glacial water pressure P, see Figs. 13 and 14. The
lake is therefore sealed and water can accumu-
Iate inside Grímsvötn.
Consider an ice cap with a subglacial lake at
its centre, as in Fig. 4. Somewhere on the
periphery of the ice cap the ice surface eleva-
tion must be less than the water level in the
lake. The distribution of p^ ancl P in such a
model will now be considered. Since the ice
cap gets thinner as the edge is approached the
ice overburden pressure pj must attain its
maximum value at some distance from the lake
and then decrease toward the edge and finally
vanish. If a hydraulic connection exists be-
tween the glacier bed ancl the lake, the hydro-
static subglacial water pressure is equal to P =
pwg (hw — hb). So long as the subglacial
water pressure P does not exceed the over-
burden pressure p4 the lake will be sealed.
However, if the water level rises to such an
extent that the overburden pressure pj is ex-
ceeded, the glacier will be lifted. The differ-
ence between p^ and P represents a potential
barrier around the lake. This pressure barrier has
a certain width and a certain threshold value.
As the water level rises in the lake, the hyd-
raulic potential increases. If the water rises
high enough, the thresholcl value of the poten-
tial barrier will be exceeded and some water
will escape through the barrier.
Consider the topographical model of Gríms-
vötn shown in Figs. 6, 13 and 14. A charac-
teristic feature of this model is that the glacier
surface is well above the Grímsvötn level for a
large area north of the lake and well below
the Grímsvötn level south of it. The width of
the potential barrier is smallest at the south-
east side of the lake. When the water level in
Grímsvötn is at its lowest height, (hw) min, the
barrier extends only 5 km to the head of the
subglacial Skeidarárjökull valley. Due to the
effect of Bárdarbunga and Kverkfjöll on
the surface topography of the glacier, the
shortest width of the barrier north of Gríms-
vötn is 20 km north-east of Grímsfjall at
the head of the subglacial Dyngjujökull valley.
The topography of the glacier bottom ancl the
glacier surface near the Skeiclarárjökull valley
is therefore of primary interest for the investiga-
tion of the potential barrier. Fortunately, the
topography of this area is well known.
Consider the portion of the seal south-east
of Grímsvötn (Fig. 13). As the water level rises
in Grímsvötn the width ancl the height of the
potential barrier will be recluced. It is obvious
that in our model there is no objection to the
water level rising high enough for the barrier to
1 8 JÖKULL 24. ÁR