Jökull - 01.12.1965, Page 7

the precipituous NW face of Svíahnúkur eystri
— to the open water at the foot of Depill (cf.
the ntap Fig. 1). A precise levelling of this pro-
íile was carried out on June 10, 1958, by Baldur
Jóhannesson and is publishecl here as Fig. 4.
Usually, however, these levellings have been
carried out by driving along a straight line
between Gríðarliorn and Depill in a wiesel,
measuring the height of the surface every 200
to 500 meters with the Kollsman sensitive alti-
meter. The results of rhese measurements be-
tween the last two jökulhlaups are shown in
Fig. 5. Tliese measurements which are in good
agreement with jóhannessons precise levelling,
are exact enough to prove that the shape of
the profile is nearly the same from year to year
even after a sinking caused by a jökulhlaup.
The central area is always practically flat (Fig.
6). The sloping at both ends of the profiles is
mainly due to increased ablation caused by the
black caldera walls. The height difference be-
tween the central area and the lake surface at
Depill is nearly constant, averaging 21 to 22
metres. Thus the entire firn floor of the caldera
rises at the same rate as the water level. This
is easiest to explain by assuming that the firn-
ice mass within the caldera is floating. Assum-
mg the specific weight of this mass to be about
0.9, the thickness of the ice in the central part
of the caldera would be 210 to 220 m. The
specific weight could possibly be somewhat
higher as tephra from the 1934 Grímsvötn erup-
tion and previous eruptions is embedded in the
lower part of the ice and this would mean a
somewhat greater thickness of the floating ice
mass.
THE RESULTS
OF THE SEISMIC SOUNDINGS.
Convincing as the assumption of floating icc
mass may seem in the light of the results of
the annual levellings, tliis assumption is not in
good agreement with the results of the seismic
soundings that have been carried out within
the Grímsvötn area.
The French-Icelandic expedition in March—
April 1951 (Eythórsson 1951, Floltzscherer 1954)
sounded the thickness of Vatnajökull in a great
number of places spread nearly all over the
glacier. These measurements were carried out by
an experienced French geophysicist, Alan Joset,
but the results were worked out by another
French geophysicist, J. J. Holtzscherer (op. cit.)
as Joset was killed by an accident on the In-
landice of Greenland on August 4, 1951.
Most of the soundings gave reliable results,
but the efforts to measure the ice-thickness in
the area NE of Svíalinúkur eystri were without
success. Within the Grímsvötn caldera the thick-
ness was measured at two points. At the eastern
point, in the SE part of the caldera floor (cf.
Holtzscherer 1954, Fig. 9) the thickness of the
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