Jökull - 01.12.1977, Síða 78

0.60 km3/yr (0.50 km3/yr i'rom Grímsvötn and
0.10 km3/yr from the cauldrons).
Consider the cause of the increased runoff
towards Skaftá since the early 1950ies. Thermal
activity may have migrated across the water
divide of the Grímsvötn water basin as a result
of small fluctuations in the surface activity of
the geothermal field. Such fluctuations can
result frorn intrusive activity at shallow levels
and from tectonic events. However, according
to the liydrological data, the total geothermal
power in the central part of Vatnajökull did
not increase as the runoff towards Skaftá in-
creased in the 1950ies. The strength of the geo-
thermal field seems to have remained fairly con-
stant. If the Grímsvötn geothermal field has
expandecl, this has taken place at the expense
of the average lieat flux density.
Alternatively, the Grímsvötn water basin may
have diminished in area and an increased part
of the geothermal field been left outside the
water basin. The ice cauldron which was formed
in 1955 is situated in an area where the direc-
tions of runoff are fairly sensitive to changes
in the glacier surface topography. The glacier
surface contours turn from east-west to north-
west direction at this site (Fig. 1). Tlie general
shrinkage of Vatnajökull in the first lialf of this
century has changed the glacier surface topo-
graphy and affected the drainage of meltwater
at tlie glacier bottom. In 1938 an extensive de-
pression was formed north of Grímsvötn (7 km
long, 2 km wide, 200—300 m deep) (Fig. 1).
At about 1950 the depression liad been refillecl
with ice. In 1955 the eastern ice cauldron was
formed about 4 km west of the site where the
depression was forrned in 1938.
THE ACCUMULATION
OF MELTWATER AND TRIGGERING
OF THE JÖKULHLAUPS
Björnsson (1974, 1975) and Nye (1976) have
explained how water can be accumulated at a
glacier bottom if melting by geothermal activity
has created a depression in the glacier surface.
Ice will flow towards the geothermal area where
it melts. The meltwater does not drain con-
tinuously away, but becomes trapped at the bed
beneath the depression. The water storage is
sealed off by a pressure barrier because the over-
burden pressure is lower beneath the depres-
sion than around it. The water pressure will
increase beneath the depression as a water
cupola rises above the glacier bed. A jökul-
hlaup occurs if the pressure barrier is reduced
to zero.
Björnsson (1975) presented a model which
predicts a jökulhlaup from the present water
cupola when the ice surface has risen about
100 m. If all water drains out of the cupola
during the jökulhlaup, an ice cauldron witli a
diameter of 2 km will be formed. The agree-
ment with observations is fairly good. In the
model, the ice thickness above the cupola is
constant, that is to say, the ice melted at the
geothermal area equals the inflow of ice.
The geothermal activity beneath the ice
cauldron has not been constant in time. The
production of meltwater has increased on the
average since 1955 (Fig. 4). The inflow of ice
to the geothermal area may be variable, there-
fore, one can expect some variations with time
in the thickness of the ice above the water
cupola. Hence, fluctuations in the height of the
water cupola are to be expected. A jökulhlaup
occurs if a given value for the ice overburden
pressure outside the depression is matched by
the total overburden pressure of water and ice
at the cupola. If the ice thickness above the
cupola is decreased, the cupola itself would
have to rise liigher than before for a jökulhlaup
to be triggered. Botli the volume of the water
cupola and the time interval between the jökul-
hlaups would be increased. This may explain
the trend of both increased water volumes of
jökulhlaups and increased intervals between jök-
ulhlaups from 1955 to 1965 (Fig. 4). An increase
in the ice thickness above the cupola, on the
other hand, may explain the trend of decreased
volumes of jökulhlaups from 1965 to 1970. A
variation in tlie volume of cupolas from 100-10°
m3 to 250 • 10° m3 corresponds to a variation
in the heights of water cupolas from 90 m to
120 m.
No simple model can describe tlie accumula-
tion of meltwater ancl triggering of the jökul-
hlaups after 1970. This may result from the
formation of the western ice cauldron.
76 JÖKULL 27. ÁR