Jökull - 01.12.1984, Síða 160

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(In Icelandic with English summary).
Helgi Björnsson 1974: Explanation of Jökul-
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SUMMARY
THE GRÍMSVÖTN
ERUPTION IN 1933
Thorarinsson (1974) wrote a comprehensive
account of the volcanic activity of the Grímsvötn
volcanic system in the last 1100 years. He reports
two eruptions in this century, in 1922 and 1934,
respectively and a probable eruption in 1938. In
the light of the small eruption in late May and
early June 1983, Jóhannesson (1983) has
reviewed the volcanic history of Grímsvötn this
century. He presented evidence for 8 eruptions in
the Grímsvötn volcanic system, in the years
1902-05, 1922, 1933, 1934, 1938, 1945,1954 and
1983 and probable eruptions in 1939, 1941 and
1948.
In this paper the evidence for the 1933 eruption
is presented.
Newspaper reports from late November and
early December 1933 suggest volcanic activity in
the northwestern part of the Vatnajökull ice cap.
The reports include descriptions of columns of
fire, eruption (vapour or ash?) columns and light-
nings, along with minor ashfall. The first sight-
ings of the eruption were in the afternoon of
November the 30th but the eruption may have
started as early as the 28th of November. The last
sighting was in the early morning of December
the 8th.
The location of the eruption site was deter-
mined by means of aims towards the eruption
from four different places. The aims intersect
about 11—12 km north of the southeastern rim of
the Grímsvötn caldera (Fig. 1) and the eruption is
supposed to be within the fault swarm which
stretches northeast from the Grímsvötn caldera
(Fig. 2). No jökulhlaup (glacial burst) accompa-
nied the eruption. A short distance west of the
supposed eruption site, a small cauldron was
formed in the course of the 1938 Grímsvötn
jökulhlaup, most likely due to a subglacial erup-
tion which did not break through the ice cap.
The 1933 eruption may have been a forerunner
to the major eruption in March-April 1934 which
was accompanied by a jökulhlaup. By compari-
son with the rifting event in the Krafla volcanic
system, the 1933 eruption may represent a minor
rifting episode.
158 JÖKULL 34. ÁR