Jökull - 01.12.1991, Page 24

Grímsvötn, lce Surface '
Legend
O Cauldron
1983 Eruption sites of 1934 and 1983
Subsided area formed in 1938
Contour interval 20 m
Surface elevation as measured in 1988
4 km
Fig. 1. Grímsvötn ice surface map. ESH: Svíahnúkur Eystri, VSH: Svíahnúkur Vestri.
Grímsvötn, yfirborðskort. Hœðartölur innan Grímsvatna samkvœmt mœlingum sumarið 1988.
between jökulhlaups from the Grímsvötn lake, and in
the case of the inferred eruption in 1933, on the sight-
ing of an eruption column.
Brandsdóttir (1984), analysing existing seismic
records for the period 1900-1982 concluded that
seismic activity similar to that observed during the
eruptions in 1934 and 1983 did not occur in the peri-
od between the eruptions. A burst of volcanic tremor
observed for about 1 hour on August 21, 1984 has
been interpreted as a small eruption that did not reach
the surface of the ice (Björnsson and Einarsson,
1990).
Due to the remoteness of Grímsvötn only the
eruptions of 1934 and 1983 have actually been
observed in the field (Askelsson, 1936; Grönvold and
Jóhannesson, 1984). The majority of eruptions in
Grímsvötn have been inferred from the sighting of
eruption columns and tephra falling on the lowlands
sun'ounding the glacier.
The first known visit to Grímsvötn was that of two
Swedish geologists, H. Wadell and E. Ygberg in the
summer of 1919 (Wadell, 1920). The next visit took
place during the eruption in 1934. Between 1934 and
1953 the area was inspected from the air or visited by
expeditions on the ground every one or two years
(Þórarinsson and Sigurðsson, 1947; Þórarinsson,
22 JÖKULL,No. 41, 1991