Jökull - 01.12.1991, Page 34

Fig. 12. Distribution of ash on the surface in Grímsvötn 1945-1984 as interpreted from vertical air photos. The
thick line marks the southern caldera rim. Útbreiðsla gjósku áyfirborði Grímsvatna 1945-1984 samkvœmt
loftmyndum.
photos from the early summer of 1935 (Áskelsson,
1936; Nusser, 1948) show that the crevassed slope at
the northern margin of Grímsvötn is covered with
tephra while fresh snow and firn cover the lowstand-
ing areas and the glacier surrounding Grímsvötn. The
photos from the summer of 1938 (Figs. 7-8) show lit-
tle tephra in May, in June the area had a light grayish
colour and in late August the surface of the depres-
sion was covered with dark ash.
In August 1942 the western part of Grímsvötn was
covered by dark ash and the snow in the eastern part
was coarse and dirty (Sigurðsson, 1942; 1984).
Vertical air photos have been taken several times
in Grímsvötn. The first set of photos were taken on
August 30, 1945, one month prior to a jökulhlaup
(Þórarinsson, 1974). The northwestern part of Gríms-
vötn was covered with dark ash and the firn in the
southern and eastern parts has a grayish colour. The
photos from September 19, 1946 (Fig. 11), show the
area to be covered with the same ash layer as in
August 1945. This can be seen by comparing the
margins and the surface pattern of the ash layer (Fig
12). On both sets of photos the margins of the ash
layer are clear, shifting from dirty firn to continuous
ash cover. Apparently the ash layer is overlain by
firn.
The northwestem corner of Grímsvötn was cov-
ered with ash on July 22, 1947 and August 28, 1950
as well as the edge of the northern slopes (Figs. 13a
and 13d). In contrast, no ash layers could be seen on
Febmary 22, 1948 (Figs. 13b, c). However, the sur-
face in the northwestern part of the depression is
uneven (Fig. 13c) because dirt cones are only partly
covered with the winter snow.
32 JÖKULL, No. 41, 1991