Jökull - 01.12.1984, Page 23

Fig. 8. The eruption site seen from the ice shelf on Grímsvötn, view towards the south. The new island
is in the foreground, the southern caldera wall in the background. The island is about 80 m long.
Photo Jón Sveinsson, June 24, 1983.
8. mynd. Gosstöðvarnar og nýja eyjan séðar afíshellu Grímsvatna. Myndin er tekin til suðurs og má sjá
norðurhlíð Vestari Svíahnúks í bakgrunni. Eyjan er um 80 m löng.
eruption is likely to have begun between 11:47 h
when the last earthquake occurred and 12 h when
continuous tremor was first recorded. The first
direct observation of the eruption was not until
10:30 h the following day.
Volcanic tremor has been recorded during all
recent eruptions in Iceland. Where simultaneous
observations of the eruption sites are available,
there seems to be correspondence between the
amplitude of the tremor and the vigor of the
eruption or the rate of extrusion. Assuming the
same to be valid in Grímsvötn, one may conclude
that the eruption was most vigorous during the
first 26 hours. The extrusion rate then decreased
gradually and was considerably reduced on May
31 and June 1. It is interesting to compare this to
the few observations of the eruption. The erupti-
on was first seen in the morning of May 29,
during the last hours of its most vigorous period.
The eruption had then formed a 300 m wide
opening in the ice cover of the caldera lake, a thin
ash fan, about 5 km long, was visible on the
glacier to the south, and a short apron of tephra
and ice, apparently water borne, was on the ice
cover to the north of the opening (Fig. 7). In the
pit an occasional explosion was seen to break the
surface of the water, throwing ash a few tens of
meters up in the air and producing steam columns
a few hundred meters high. Low cloud cover
prevented direct observation during the following
days, but on May 31 and June 1 a column of
steam was seen from overflying aeroplanes, rising
several thousand meters through the clouds. The
explosivity of the eruption must have been consi-
derably higher during these days, in spite of
lower extrusion rate. After the eruption had ceas-
ed, observations revealed a small island in the
lake (Fig. 8). The eruption had built a cone on
the lake bottom, that finally reached the lake
surface. The higher explosivity during the last
days of the eruption is therefore likely to be
caused by lower water pressure as the volcanic
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