Jökull - 01.12.1991, Side 23

Eruptions in Grímsvötn,
Vatnajökull, Iceland, 1934-1991
MAGNÚS T. GUÐMUNDSSON and HELGI BJÖRNSSON
Science Institute, University oflceland
Dunhaga 5, 107 Reykjavík
ABSTRACT
Duríng the period 1934 to 1991 evidence has only
beenfoundfor three orfour volcanic eruptions with-
in the Grímsvötn volcanic centre, i.e. tlie directly
observed eruptions in 1934, 1938, 1983, andproba-
bly a small eruption in 1984, deduced from seismic
tremors. Tephra layers observed by visitors in the
northwestern part ofthe Grímsvötn depression in the
period 1934 to the 1960's have been misinterpreted
as signs of eruptions; the very same ash cover was
observed throughout tlie period. This ash cover dates
back to the eruption of 1934, but earlier Grímsvötn
eruptions may have contributed to its formation.
Reported openings in the ice shelf(1945, 1954, 1960)
are considered not to be signs of eruptions but could
be explained by either steam explosions of hydrother-
mal reservoirs sealed by impermeable caprock or by
increased upwelling of hydrothermal fluid in reser-
voirs of high permeability due to pressure release
during lowering of the Grímsvötn lake level in
jökullilaups. Frequent jökulhlaups in the period
1938-1948 can be adequately explained by high
melting rate at the site ofthe eruption of 1938.
The eruptions of 1934 and 1983 produced hyalo-
clastites of volume 30-40-106 m3 and 10-106 m3,
respectively. The eruption of 1938, on the other
hand, produced volcanic material of the order of
400-106 m3 andmay have been the third largest erup-
tion in lceland this century, after Hekla in 1947 and
Surtsey in 1963-1967. The volume ofmaterial erupt-
ed in Grímsvötn since 1600 AD has been estimated of
the order of 2.3 km3 and the total production may
have been 3-5 km3 in historical times.
INTRODUCTION
The history of recent volcanism within the Vatna-
jökull ice cap, SE-Iceland, has been studied by sev-
eral authors (Þórarinsson, 1974; Steinþórsson, 1977;
Larsen, 1982; Jóhannesson, 1983; 1984; Grönvold
and Jóhannesson, 1984; Björnsson, 1988; Björnsson
and Einarsson, 1990; Guðmundsson, 1992). About
80 eruptions have been attributed to the volcanoes
beneath Vatnajökull in historical times of which 63
are considered certain. Over the last several hundred
years, Grímsvötn (Fig. 1) has been the most active of
the volcanoes located within Vatnajökull. This vol-
canic centre has developed three calderas (Sæmunds-
son, 1982; Guðmundsson, 1992). The number of
eruptions in Grímsvötn over the last 1100 years has
been estimated to be between forty and fifty
(Þórarinsson, 1967). Björnsson and Einarsson (1990)
list 36 eruptions within or near Grímsvötn in their
compilation of known eruptions in historical times.
The volcanic history of the 20th century has been
a subject of some discussion in recent years. Three
eruptions are known for sure, in 1922, 1934 and 1983
(Þórarinsson, 1974). There is also a general agree-
ment that an eruption took place to the north of
Grímsvötn in 1938, causing a large jökulhlaup
(Þórarinsson, 1974; Bjömsson, 1988).
Jóhannesson (1983; 1984) reexamined records on
eruptions in Grímsvötn in this century and concluded
that eruptions had taken place in Grímsvötn in 1902-
1905,1922,1933, 1934, 1938,1945,1954 and 1983.
Moreover, he considered it possible that small erup-
tions had occurred in 1939, 1941 and 1948. His con-
clusions are based on reinterpretation of field obser-
vations in Grímsvötn, irregularities in the period
JÖKULL, No. 41, 1991 21