Jökull - 01.12.1984, Síða 32

cier tongues flow from the mountain between the
nunataks down to the caldera lake. The western
rim of the caldera is formed of gently sloping
hyaloclastic hills which are covered by ice. The
main hill, Vatnshamar, rises about 100-200 m
above the ice cover of the lake. Rocks are visible
at some places on the eastern slopes of the hills.
The northern and eastern borders of the caldera
are covered with thick glaciers. However, the
glacier surface topography indicates an extension
of the depression towards Svartibunki in the
northwestern area. Further east, the northern
border is located south of Svartibunki.
Three craters were active during the eruption
in 1934. The largest crater, 600 m in diameter was
located northwest of the foot of W-Svíahnúkur.
Two smaller craters were situated in the SW
corner of the Grímsvötn depression (Fig.2). In
May 1935, more than one year after the eruption
was over, the largest crater still issued gas and
smoke at intervals.
Signs of geothermal activity
in the Grímsvötn depression.
Indications of geothermal activity can be
observed along the northern walls of the moun-
tain Grímsfjall and at the eastern slopes of
Vatnshamar. Wadell (1920, fig. 4 and 6, p.
309-312) described open water at several places
along the walls of the caldera. Almost every
expedition has reported a waterpool at Vatns-
hamar and frequently along the slopes of Gríms-
fjall beneath the two nunataks. The water level is
at the same height in all the waterpools (Thor-
arinsson 1953a, p. 15; 1957, p. 46; 1958, p. 3;
1974). The water is usually cold (near to 0 °C) but
when the surface of the lake is at low levels warm
springs have been observed at Vatnshamar.
Thorarinsson (1953a) observed seven hot springs
situated between basalt dykes 1 to 15 m above the
lake level at about 1350 m a.s.l. Their total
discharge was 5 to 10 l/s and the largest one
gushed water at 87.5 °C continuously up to about
40 cm height. Samples from the largest spring
contained 52 mg/kg of Si02, and 91 mg/kg of total
carbonate as CaC03 and had a pH of 7.3. The
ground beneath this spring was covered by a 5
mm thick layer of calcium carbonate. In 1942 an
expedition reported open water in a 30-100 m
wide strip at the foot of W-Svíahnúkur. Hot
springs were situated along the cliff and emerged
both from underneath the water and the glacier
tongues (Steinthór Sigurdsson, this issue).
Further, in 1954 luke-warm water was observed
in a depression beneath the walls northeast of
Grídarhorn (Holtzscherer 1954, fig.21, p. 25). In
1957 this pool was covered by ice but with a small
opening (Thorarinsson 1957, p.46). Sulphurous
odour is usually not noticable in the Grímsvötn
depression. However, Thorarinsson (1953a)
reported a distinct odour of sulphur in the area
and a faint odour could be detected from open
water near Grídarhorn.
A number of ice cauldrons or circular depress-
ions in the surface of the glacier bear witness to
subglacial geothermal activity on the northern
slopes of Grímsvötn (Fig.2). The main cauldrons
north and northeast of E-Svíahnúkur are always
visible but others appear just after jökulhlaups
when water has been discharged from them. The
largest ice cauldrons are up to 100 m deep and 1
km in diameter.
Considerable changes in the thermal activity
have been reported by several expeditions. Some
of the changes may even be considered as volca-
nic. During the jökulhlaup in May 1938 drastic
changes occurred in the Grímsvötn area, which
may be interpreted as caused by volcanic activity
that did not break through the glacier. A recon-
naissance flight revealed that a large area of the
ice surface to the north of the Grímsvötn depress-
ion had subsided (shown by Pálmi Hannesson’s
(1958) photos, see for example Thorarinsson
1974, fig. 41, p. 166 or Björnsson 1983, fig.3 p.16,
and Gísli Gestsson’s map, see Thorarinsson 1974,
fig. 43, p.168). Water was released to the caldera
lake. A jökulhlaup resulted. The effects of the
events in 1938 could be observed for more than
10 years as a depression in the glacier surface
north of Grímsvötn (Fig.2). In the summer of
1945 an expedition led by Skarphédinn
Jóhannsson reported increased fumarolic activity
along the foot of Grímsfjall where no such activ-
ity had been observed in 1944 (Thorarinsson and
Sigurdsson 1947, Áskelsson 1959). In late
September of 1945 a jökulhlaup occurred from
Grímsvötn. A reconnaissance flight reported
steam rising up above the slopes of Vatnshamar
and ash spreading over the western part of
Grímsvötn (Hannesson 1958). An expedition led
by Áskelsson (1959) in October observed an
explosion-like crater or a sink hole, maybe 100 m
deep, near Svartibunki in the northwestern part
of the Grímsvötn area. A strong sulphurous
30 JÖKULL 34. ÁR