Jökull - 01.12.1989, Page 3

The Grímsvötn Caldera, Vatnajökull:
Subglacial Topography and
Structure of Caldera Infill
MAGNÚS T. GUÐMUNDS S ON
Department ofGeological Sciences
University College London
Gower Street, London WCIE 6BT, UK
ABSTRACT
A shallow seismic reflection survey was carried
out on the ice shelf covering the suhglacial lake in
the Grímsvötn Caldera, Vatnajökull, in 1987. The
survey showed that at the relatively low water level
9 months after the jökulhlaup in 1986, the area of
the subglacial lake was 10 km2 and the volume of
the lake was 0.5 km3. The ice shelf was 240-260 m
thick in most parts and the water layer 40-90 m
thick. Comparison and reinterpretation ofa seismic
survey conducted in 1955 suggests about 100 m
increase in the thickness of the ice shelf over the 32
year period. The size of the main caldera is about
20 km2 and the elevation of the caldera floor is
1060-1200 m a.s.l. The caldera floor dips slightly
from south to north and the southern and
southwestern parts are believed to be covered with
lava flows. In the northern and eastern parts, the
lakefloor is believed to be covered with sediments.
Interfaces could be seen below the lakefloor in the
northern and central parts. These reflections are
believed to arise from lava flows or sills within a
sediment pile. It is suggested that the caldera infill
is composed of a pile of lava flows and volcaniclas-
tic sediments. Lava flows compose the greater part
of the pile in the southern part but sediments are
predominant in the northern part. This suggests that
eruptions have been more frequent in the southern
part of the caldera. The existence of the lava flows
suggests that eruptions onto the lakefloor have been
more voluminous than previously believed. It is
suggested that the observed drop in geothermal
power of the area in recent years, is caused by the
reduced volcanic activity after 1940.
INTRODUCTION
In June 1987 a shallow seismic reflection survey
was carried out in the ice covered Grímsvötn Cal-
dera, Vatnajökull (Fig. 1). The purpose of the sur-
vey was to map the bottom of the subglacial lake
within the caldera and in that way obtain informa-
tion on the size and volume of the lake, as well as
the structure of the caldera. The caldera has been
highly active in the past, and numerous eruptions are
known to have occurred in Grímsvötn over the last
400 years (Þórarinsson, 1974).
The area has been the focus of interest for Earth
scientists since 1934 when an eruption and jökul-
hlaup (glacier burst) prompted the first scientific
work (Askelsson, 1936). The phenomena of vol-
canic and geothermal activity in the glacial environ-
ment are of great interest, as many of the present
landforms observed in Iceland were formed in the
subglacial eruptions during the last glaciation. The
effects of the geothermal area on the glacier and the
jökulhlaups are of great glaciological interest as
: well 'ás. beiríg important in estimating the flood
danger. As a consequence, a great deal of research
JÖKULL, No. 39, 1989 1