Jökull - 01.12.1991, Blaðsíða 32

Fig. 10. Grímsvötn seen
from the east on October 5,
1945. The arrow points at a
slight depression that is
located above the eruption
site of 1938 (photo Steinþór
Sigurðsson, from
Þórarinsson and
Sigurðsson, 1947).
Flugmynd af Grímsvötnum
í lok hlaupsins 1945. Orin
bendir á grunna sigdœld
yfir gosstöðvunumfrá 1938.
complicated with several ridges and mounds. Figure
9 shows six east-west profiles across the area. A 70-
200 m high ridge is located beneath the subsided
area. The volume of the ridge is greatest in the north-
ernmost part where the width and depth of the sub-
sided area was greatest. The total volume of the ridge
is of the order of 0.4 km3. The form and size of the
ridge conforms reasonably well with that of the
depression and its volume is sufficient to account for
the ice melted by the eruption. This supports the sug-
gestion that this ridge was formed in the subglacial
1938 eruption.
A depression like the one formed in 1938 would
be eliminated by ice flow within a few years if it
were not maintained by melting by a subglacial heat
source. Apparently, by 1942 the larger part of the
depression had been eliminated by ice flow
(Sigurðsson, 1942; 1984). However, a slight depres-
sion was visible in 1945 (Fig. 10), and on vertical air
photos from September 1946, the northern margin of
the depression was marked by crevasses, indicating
that melting of ice at the bedrock was still taking
place eight years after the eruption.
COMMON EFFECTS OF
ERUPTIONS IN GRÍMSVÖTN
Before reviewing the various reports of observations
in Grímsvötn and their significance as indicators of
recent eruptions we summarize the effects of the
three eruptions in 1934, 1938 and 1983 which were
all thoroughly described.
1. Layers of tephra were spread on the ice sur-
face.
2. Openings were formed in the ice shelf or the
glacier directly above the crater, often con-
taining water bordered by vertical ice walls.
3. Open water or a depression in the ice at the
eruption site may persist for years, indicating
locally increased heat flux.
4. A pile (a mound or a ridge) of hyaloclastites is
formed in the bedrock at the eruption site.
30 JÖKULL, No. 41, 1991