Jökull - 01.12.1984, Síða 4

Figure 1. Location of the Grímsvötn volcano in
the Vatnajökull ice cap.
Mynd 1. Grímsvatnaeldstöðin og næsta nágrenni.
to the north they have a northerly direction
(Figure 1). Possible relations between different
volcanic systems are elaborated further in the
discussion chapter.
The volcano itself forms a definite topographic
high with a central caldera subsidence. It has
been suggested that the Grímsvötn caldera con-
sists of three smaller, superimposed calderas
(Saemundsson 1982) while ice thickness measure-
ments by radio echo sounding suggest a single
caldera about 10 km in diameter (Helgi
Björnsson pers. comm.). The whole volcano is ice
covered except the southern caldera wall where
basaltic hyaloclastites are exposed in the cliffs.
The caldera lake is also covered by a 200 m thick
ice shelf (Björnsson 1982)
The Grímsvötn volcano has the highest erup-
tion frequency of Icelandic volcanoes. As it is
subglacial the interaction with the ice-cover and
melt water has marked effect on the eruption
behaviour and the eruption products (Thórarins-
son 1974). The quenching of the magma against
the melt water results in hyaloclastites, mainly
tuffs.
Within the Grímsvötn volcano is also a geoth-
ermal area, estimated at 5000 MW (Björnsson
1974, 1983), which continually supplies melt
water to the ice covered caldera lake. This causes
spectacular jökuihlaups (glacier bursts) every few
years in the river Skeiðará which drains the
Grímsvötn caldera lake. In most cases eruptions
in the Grímsvötn volcano are accompanied by
such jökulhlaups, the eruption under discussion
being a notable exception. Possible interaction
between the volcanic and geothermal activity
poses many interesting questions as the ultimate
source of the geothermal energy must be magma-
tic and possibly from the same system that feeds
the eruptions. It has also been suggested that in
some cases the sudden draining of the caldera
lake may trigger eruptions rather than the other
way around (Thórarinsson 1974).
VOLCANIC HISTORY
Available knowledge of the activity of the
Grímsvötn volcano and adjacent regions was
summarized by Thórarinsson (1974). The earliest
recorded possible eruption was in 1332 but the
remoteness of the volcano and the inadequacy of
the literary sources makes the early chronology
very uncertain. When sources improve in the
seventeenth century eruptions are mentioned at
frequent intervals, often one in a decade.
During this century eruptions in 1922 and 1934
were described, and a possible one in 1938.
2 JÖKULL 34. ÁR