Jökull - 01.12.1989, Blaðsíða 4

Fig. 1. Western Vatnajökull.
Mynd 1. Vesturhluti Vatnajökuls.
has been carried out on the glaciology and history of
jökulhlaups and eruptions (Þórarinsson, 1953, 1965,
1974), on the glaciology and jökulhlaup mechanism
and the nature of the geothermal area (Björnsson,
1974, 1983, 1988; Bjömsson and others, 1982;
Bjömsson and Kristmannsdóttir, 1984; Steinþórsson
and others, 1983) and the recent volcanic and
seismic activity (Grönvold and Jóhannesson, 1984;
Einarsson and Brandsdóttir, 1984; Brandsdóttir,
1984; Jóhannesson, 1983, 1984).
Grímsvötn is one of the most powerful geother-
mal areas in Iceland with a heat release of 4000-
5000 MW thermal (Björnsson, 1974; Björnsson and
Kristmannsdóttir, 1984; Bjömsson, 1988). The
geothermal heat melts the ice within the caldera,
forming the subglacial lake which is covered by a
floating ice shelf. The ice north of the caldera flows
into the lake, where it is melted. The meltwater
accumulates in the lake. The mechanism of the
drainage of the lake is discussed by Bjömsson
(1974, 1988) and Nye (1976). Ice melting causes
the water level in the lake to rise, and when a criti-
cal level is reached, the pressure at the bottom of the
lake is sufficient to open a subglacial waterway to
the edge of the glacier some 50 km to the south and
the lake is drained in a jökulhlaup which lasts
between one and three weeks.
Since 1934 jökulhlaups have occurred once every
4-6 years. Before that time the jökulhlaups were less
frequent (approx. one every 10 years) and more
voluminous (Þórarinsson, 1974, Björnsson, 1983,
1988). The total volume of water drained in each
jökulhlaup before 1938 is estimated to have been 5-
7 km3 (Þórarinsson, 1974; Björnsson, 1988). Since
1938 the volume of the jökulhlaups is estimated to
have ranged from 0.55 to 3.5 km3 (Rist, 1955, 1984;
Kristinsson and others, 1986).
Askelsson (1934) was the first to suggest that
Grímsvötn is a caldera. Þórarinsson (1974) esti-
mated the size of the Grímsvötn Caldera as 35 km2,
from the glacier surface topography. He also sug-
gested that it was composed of two calderas, the
main caldera, and a second smaller caldera in the
northwest part of the area. Sæmundsson (1982) sug-
gested, also from the surface topography, that a third
caldera is situated in the northeast comer of the
depression. Finally, Björnsson (1988) gave a
description of the subglacial topography of the area
based on radio-echo soundings. He uses the term
Grímsvötn Caldera for an area 6 to 10 km in diame-
ter, bordered by the mountain ridge Grímsfjall to the
south and subglacial mountain ridges to the north
and east. Furthermore, he concludes that the caldera
is divided into two main parts. The eastem part is 4
to 6 km in diameter and has its long axis striking
NE. The westem part can be divided into two ellipt-
ical areas. The southem elliptical area corresponds
to Þórarinsson’s main caldera and the northem area
2 JÖKULL, No. 39, 1989