Jökull - 01.12.1989, Page 15

Fig. 7. Grímsvötn, subgla-
cial topography. Contour
interval 50 m. The topogra-
phy of the area north and
east of the main caldera is
based on Bjömsson (1988).
The area covered by the
subglacial lake in June 1987
is shaded. Dashed lines
mark the cross-sections on
Figs. 6a-c.
Mynd 7. Grímsvötn, botn-
kort. Bil milli hœðarlína
50 m. Skyggingin afmarkar
það svœði sem vatnið náði
yfir í júní 1987. Slitnu lín-
urnar sýna legu þversnið-
anna á myndum 6a-c.
Furthermore, comparison of the velocity data of the
earlier surveys with that of the present survey sug-
gests that no significant changes in the velocity
structure of the ice shelf have taken place. If it is
assumed that the elevation of the lakefloor has not
changed, the thickness of the ice shelf and the water
layer can be calculated.
The present survey covers the areas where sta-
tions III and IV of the 1955 survey were located.
The lakefloor elevation at these places is 1070-
1100 m and 1100 m respectively, but 812-847 m and
922 m according to Martin’s calculations. An ice
shelf thickness of 120 m at station III and 150 m at
station IV at the time of Martin’s survey would be in
agreement with unchanged elevation of the
lakefloor.
Bjömsson (1988) presented data on the thickness
of the ice shelf in Grímsvötn for the period 1958-
1987, based on measurements of the ice surface
elevation above the water level. The results show
that the ice thickness was 200-230 m in 1958-1966,
but 250 m in 1980 and 1987. Furthermore, local
melting reduces the ice thickness to 150 m in 1958
and 1964. Even though the figures from the rein-
terpretation of the 1955 survey must be treated with
caution, they clearly suggest a substantial thicken-
ing of the ice shelf, in agreement with Björnsson’s
results.
DISCUSSION
The evidence from the seismic lines suggests that
parts of the present caldera floor are covered with
lava flows, and also that similar lava flows or sills
are present, buried by sediments and other lava
JÖKULL, No. 39, 1989 13