Jökull - 01.12.1984, Síða 33

Fig. 4. An ice cauldron NE of Grídarhorn that has collapsed and changed into a circular cylinder about
100-200 m in diameter. Photo: Helgi Björnsson, December 1983.
4. mynd. Sigketill norðaustan við Gríðarhorn sem breyst hefur í sívalningslaga pytt, 100-200 m í
þvermál. Myndina tók Helgi Björnsson meðan hljóp úr Grímsvötnum í desember 1983.
odour emanated from the hole. Áskelsson sug-
gested that a small eruption had occurred at the
end of the jökulhlaup in late September covering
a large area in the western part of Grímsvötn with
ash. Further, in 1945 Áskelsson (1959) observed a
cauldron close to the western crater of the erup-
tion of 1934. During the jökulhlaup 4-22 July
1954 Thorarinsson (1974, p. 188-191) observed
from the air a crater at the southern slopes of
Svartibunki. The diameter was about 200 m and
loose materials had been thrown around by an
explosion. In June of 1955 Thorarinsson (1955)
described the sink hole in an expedition to the
area. He also reported a dark layer at 1.5-2 m
depth in snowpits. This layer was presumably
spread by an explosion when the sink hole was
formed during the jökulhlaup in July of 1954.
Thorarinsson (1956) reported a circular depress-
ion of 200 m diameter in the SW corner of
Grímsvötn that was not there in 1955. Eythórsson
(1960) observed a hole 2-3 km northeast of
Vatnshamar 5 months after the jökulhlaup during
8-25 January. The opening was circular, 25-30 m
in diameter. Steam and sulphurous odour arose
from the hole. Thorarinsson (1974) suggested
that all the sink holes were formed in steam
explosions as the overburden pressure was
released during the jökulhlaups. He suggested
that the dispersed loose material might have been
sand rather than new volcanic products. An
explosion of that kind had been reported from
the Kverkfjöll area in 1959 (Jóhantisson 1959).
However, Tryggvason (1960) reported earth-
quakes of magnitude about 3 in the Grímsvötn
area on 17th and 21 July 1954 and 21, 22 and 24
January 1960 that may indicate volcanic activity.
Since 1960 sudden changes of the thermal
activity have not been reported in Grímsvötn
until the eruption in late May 1983 from a crater
beneath W-Svíahnúkur (Fig.2). During the
jökulhlaup in Dec. 1983 an ice cauldron east of
Grídarhorn collapsed and changed into a circular
cylinder some hundred metres deep and about
100-200 m in diameter (Figs. 2 and 4). So far
(February 1984) this sink hole has only been
studied from the air. No signs of explosions were
observed. We refer later to this cauldron in the
discussion of the triggering of the jökulhlaup in
December 1983.
Geothermal activity on the Grímsfjall
mountain ridge
On the Grímsfjall mountain, direct surface
expressions of geothermal activity can be
JÖKULL 34. ÁR 31