Jökull - 01.12.1984, Side 3

Eruption in Grímsvötn 1983;
course of events and chemical studies of the tephra.
Karl Grönvold
Nordic Volcanological Institute
University of Iceland, Reykjavík
Haukur Jóhannesson
Icelandic Museum of Natural History
PO Box 5320, Reykjavík, Iceland
ABSTRACT
A short eruption took place in the Grímsvötn
volcano in May — June 1983. The eruption most
probably started on May 28th but was first
observed in the morning of May 29th. Activity
was last observed on June lst and by June 5th it
was certainly over.
The Grímsvötn volcano is situated within the
western part of the Vatnajökull ice cap. It is
almost totally ice covered and the caldera lake is
covered by an ice shelf about 200 metres thick.
The eruption site is within the caldera near the
southern rim and a lake, about 500 m in dia-
meter, formed in the ice shelf with a small island
in the middle. The eruption was subaquatic and
intermittent ash explosions were observed in the
lake. Usually these were about 50—100 m high
and a steam column rose up to about 5000 m
height a. s. 1.
Three small ash fans formed on the surround-
ing ice sheet; two by explosions, one to the south
early in the eruption and another to the east most
likely on June lst; the third to the north within
the caldera was most likely caused mainly by an
avalanche from the overhanging caldera wall into
the lake.
The glass phase of the ash was analyzed in a
number of samples and found to be evolved
basalt with a uniform chemical composition but
minor variations are indicated. Samples from the
1934, 1922 and 1903 Grímsvötn eruptions were
analyzed for comparison and show very similar
chemical composition as the 1983 ash. This com-
position is also very similar to that of the glass
phase of the eiju^tij^a^^e^lp^ki craters 1783-
378146
ÍStANOS
The Grímsvötn volcano is also the site of a
major geothermal system, estimated at 5000
MW. The heat source of this system is assumed to
be magmatic intrusions, most likely with the
same compostion as the ash. It appears unlikely
that the heat extraction takes place in the same
parts of the magmatic system as the evolution of
the basalt.
INTRODUCTION
In May 1983 a short, small and little observed
eruption took place in the Grímsvötn volcano in
the western part of Vatnajökull ice cap. The
eruption was mainly subaquatic but managed to
build up an ash cone that reached the surface
through a small opening formed in the ice shelf
which floats on the caldera lake. Explosions
formed two very thin ash fans on the surrounding
ice and a third ash fan is presumed to have
formed mainly by a wave caused by a snow
avalanche into the lake. The present report
summarizes observations made during the erup-
tion and gives preliminary results on the chemical
composition of samples of the ash. For compari-
son, samples from earlier eruptions 1934, 1922
and 1903 were included in the study.
GEOLOGIC SETTING
The fact that Grímsvötn volcano and the sur-
rounding area is covered by the Vatnajökull ice
cap makes our knowledge of the geologic setting
uncertain. The tectonic fissures and crater rows
which enter the ice cap from the south have a
north-easterly direction but where they emerge
JÖKULL 34. ÁR 1