Jökull - 01.12.1991, Page 27

Fig. 4. The eruption of 1983 under the southem caldera rim (photo Helgi Bjömsson). Grímsvatnagosið 1983.
Svíahnúkur vestrifyrir miðri mynd.
fans towards south and east were also seen. The erup-
tion was detected by seismometers (Einarsson and
Brandsdóttir, 1984) but was not seen from the inhab-
ited areas to the south of the glacier. The 1983 erup-
tion was much smaller than that of 1934.
Since 1983, conditions at the eruption site have
been observed during the annual expeditions of the
Iceland Glaciological Society and other expeditions
to Grímsvötn (Bjömsson and Guðmundsson, unpub-
lished data). The opening in the ice shelf around the
crater remained open through 1984 but open water
was not observed in the crater in 1985 and 1986.
However, it is evident from the crevasse pattem on
air photos taken on September 12, 1986 (Table A1 in
Appendix), at the end of a jökulhlaup, that the ice
shelf in the crater area was very thin. During the sum-
mer of 1987 the mound formed in 1983 was clearly
visible and the opening in the ice shelf reappeared. A
much smaller opening was observed in 1988. By
1989 the mound had disappeared under the rising
water level and open water was not detected.
Inspection of the crevasse pattern formed at the end
of the 1991 jökulhlaup shows that a substantially
thicker ice shelf covered the crater of 1983 than at the
end of the jökulhlaup in 1986.
Fig. 5 shows a cross section through the southern
caldera rim, including the two eruption sites. It is
based on the results of recent radio-echo soundings,
air photos and seismic reflection (Guðmundsson,
1989). The mounds formed during the eruptions of
1934 and 1983 can be seen. The mounds rise slightly
above the lake level at the time of the eruptions. The
mounds are presumably the rims of the craters active
during the eruptions and Fig. 6 is a schematic cross
section showing conditions during an eruption like
the two described above. The dashed lines show the
form of the ice shelf before the eruption.
The volume of the material erupted in 1934 and
JÖKULL, No. 41, 1991 25