Jökull - 01.12.1989, Page 10

1-5 2 3 4 5 6km
Fig. 5. a) Seismic line 1, time section. The spacing between traces is 20 m. ’a’ marks the ice-water
reflection, ’b’ is the lake bottom reflection and ’c’ is a reflection from below the lakefloor. Zero on horizon-
tal distance scale the same as on Fig. 6a.
Mynd 5. a) Endurkastsnið, mcelilína 1. ’a’ er endurkastið frá botni íshellunnar og ’b’ frá botni vatnanna. ’c’
er endurkast frá lagmótum undir vatnsbotninum. Núllpunktur á láréttum skala sá sami og á mynd 6a.
steep. The critical angle at the ice-water interface
for an upward travelling wave is only 23 degrees. A
wave reflected from a dipping interface below the
shotpoint is likely to have a larger angle of
incidence and would consequently be totally
reflected from the interface back into the water
layer.
Cross sections through the main caldera in Gríms-
vötn are shown in Figs. 6a-c. The sections highlight
the absence of any major topographic features in the
interior areas of the caldera. The ice shelf is in most
places 240-260 m thick and the water layer 40-90 m
thick.
The data make it possible to measure the volume
of the subglacial lake. The area of the ice shelf and
the lake at the time of the survey was about 10 km2.
The volume was calculated by estimating the thick-
ness of the water layer in all parts of the lake and
then summing. The result is a total volume of
0.5±0.1 km3.
The survey was conducted 9 months after the last
jökulhlaup from Grímsvötn, which occurred in Sep-
tember 1986. As a consequence, the water level at
the time of the survey was relatively low, or 1374 m
a.s.l. which can be compared with about 1350 m at
the end of the 1986 jökulhlaup and 1430 m just
before its start (Bjömsson, 1988). Fig. 7 shows the
subglacial topography of the main caldera. The
topography in the area to the north and east of the
caldera is based on Bjömsson (1988). The caldera
has a long axis at N80°E, and is slightly eggshaped
with the narrow end towards east. Its size is about
20 km2. The deepest areas are in the northem part
of the caldera where the lowest points have an
elevation of about 1060 m a.s.l. The depth of the
caldera from the highest rims (Eystri and Vestari
Svíahnúkur) is therefore about 650 m. The caldera
floor dips gently down towards north from the foot
of the southem caldera wall. The total height of the
southem wall is 500-550 m but the slopes bordering
the caldera towards north are 250-300 m high. The
height of the slopes on the westem margins is about
400-450 m. There are two subglacial openings
through the caldera walls into the caldera. A broad
pass with an elevation of about 1100 m connects the
main caldera to the north caldera. A second opening,
slightly higher, is towards northeast. It is through the
latter that many of the jökulhlaups are believed to
force their way out of the subglacial lake (Biörns-
son, 1988).
8 JÖKULL, No. 39, 1989