Jökull - 01.12.1975, Síða 9

can also explain this in terms o£ overburden
pressures relative to the datum level z0 (Björns-
son 1974). The curve cp = 0 represents the ice
overburden pressure P( = pj g (zg — zw) at the
top of the subglacial water layer (and the gla-
clier bedrock outside the reservoir). The bed
curve of the glacier represents the water pres-
sure P = pw g (z0 — zb) - at the glacier bed if
there were a hydraulic connection between the
reservoir and the glacier bedrock, see Fig. 2 for
the notations. As long as the curve cp = 0 lies
beneath the glacier bed, the reservoir is sealed
by a pressure barrier P( — P. The barrier is
reduced to nil when the hydrostatic water pres-
sure at the reservoir can match a threshold
value for the ice overburden pressure. The de-
pression is underlain by both ice and water.
Therefore, the pressure barrier is removed be-
fore the height of the surface depression has
matched the height of the glacier at the thres-
hold. A jökulhlaup occurs before the depres-
sion has been filled with ice.
An ice cauldron will be formed when water
drains out of the cupola. The shape of the
reservoir at the start of the jökulhlaup is given
by Equation (8). The cupola is situated be-
neath the centre of the depression. The width
of the cupola is about one tenth of the width
of the depression. The reservoir may become
sealed again before it is empty. After the jökul-
hlaup water will continue to accumulate beneatli
the ice cauldron. The depression is now shal-
lower than the original depression because ice
has continuously moved into the depression.
The next jökulhlaup will be triggered at a
lower cupola than the former jökulhlaup. Ice
will continue to flow into the depression and
the water cupola will gradually decrease in
volume. The jökulhlaups will cease when the
depression is filled with ice.
JÖKULHLAUPS FROM THE INTERIOR
OF GLACIERS
Jökulhlaups can be expected from the in-
terior of temperate glaciers if a depression is
present in the glacier surface. A depression
must be created by a high local melting rate
at the bed of the glacier. The melting may be
caused by a permanent geothermal area or by
a subglacial volcanic eruption.
Jökulhlaups from a geothermal area
Many geothermal areas in Iceland are covered
by glaciers. Some of the areas are situated at
the edge of the glaciers, for example at Kverk-
fjöll, Torfajökull and Sólheimajökull, Fig. 1.
Meltwater drains continuously from these areas
into the glacier rivers. However, melting at a
geothermal area beneath a glacier can create a
permanent depression in the glacier surface.
Meltwater may be trapped in situ beneath the
depression. Ice and water will flow into the
depression. At a permanent depression, the in-
flow of ice equals approximately the melting
of ice at the bed of the glacier. A water re-
servoir will grow beneath the depression until
a jökulhlaup results. The frequency of the
jökulhlaups depends upon the filling rate of
the depression. Melting of the ice cover itself
does not reduce a given pressure barrier. Only
the rate of ice flow into the depression deter-
mines how fast the pressure barrier is reduced.
Two examples, both from Vatnajökull, will be
described.
a) Jökulhlaups in the river Skaftá
Eleven jökulhlaups have occurred in recent
time in the river Skaftá, the first one in 1955.
The jökulhlaups are accompanied by formation
of an ice cauldron some 10 km north-west of
Grímsvötn (Fig. 1 and air photo p. 1). Sofar, no
explanation has been given of the cause of these
jökulhlaups. Analysis of hydrological data for
the jökulhlaups favours the hypothesis that the
burst water in Skaftá is melted by a permanent
geothermal area (Björnsson in preparation).
Around 1955 a change seems to have taken
place in the Grímsvötn water basin causing
somt water to flow towards Skaftá (Björnsson
1974).
Fig. 4 shows a cross-section of Skaftárjökull
just after a jökulhlaup at t0. An ice cauldron has
been formed in the centre of a depression in
the glacier surface. The bed topography is not
known in details. As far as is known, the glacier
may be 500—600 m thick beneath the ice caul-
dron. Fig. 4 shows a model which predicts a
jökulhlaup when the centre of the water cupola
has risen about 100 m. This is a steady state
model in the sense that the thickness of the
glacier remains constant with time. The inflow
of ice into the depression equals the melting
at the geothermal area. The glacier surface
JÖKULL 25. ÁR 7