Jökull - 01.12.1991, Page 29

Fig. 7. The subsided area north of Grímsvötn on May 28, 1938. Grímsvötn in the upper right hand comer (photo
Pálmi Hannesson). Sigdœldin norðan Grímsvatna 28. maí 1938. Fjœr sér til Grímsvatna.
dence of the ice surface was somewhat less than else-
where (Fig. 7); suggesting reduced basal melting at
the border area. According to a map drawn by G.
Gestsson (Þórarinsson, 1974) (Fig. 9a) on the basis of
the photos from May 28, 1938, the volume contained
within the subsided area is 2 km3. Þórarinsson
(1953a; 1974) used the same data together with
rough mass balance estimates for the Grímsvötn
basin and obtained a value of the order of 3 km3.
Oblique air photos were used to draw the surface
contours of Vatnajökull on the maps of the Danish
Geodetic Institute (Nprlund, 1944). These photos
were taken in 1937 and 1938. A photo taken over
Dyngjuháls (50 km to the north of Grímsvötn) on
August 28, 1937 shows that the depression to the
north of Grímsvötn observed in 1938 did not exist at
that time. Three sets of photos exist from 1938 that
show Grímsvötn, the depression and the cauldrons.
The first set was taken on June 23 from a plane over
Köldukvíslarjökull, about 25 km to the northwest of
Grímsvötn (Fig. 8a). The second set of photos was
taken on July 1, at a point 40 km to the southwest of
Grímsvötn. The third set is taken from a point about
25 km to the northeast on August 30 (Fig. 8b). The
northern part of the subsided area is covered with a
dark layer on the July and August photos while it
cannot be decided with certainty whether such a
layer existed on June 28.
Þórarinsson (1974) considered it likely that a sub-
glacial eruption had occurred to the north of
Grímsvötn causing rapid melting of ice, forming the
depression and the cauldrons. Björnsson (1988) car-
JÖKULL,No. 41, 1991 27