Jökull - 01.01.2012, Síða 135

Öræfajökull central volcano, SE-Iceland
ablation area) and satellite photographs (ASTER and
SPOT5), have become available.
The first survey of the bedrock underneath Öræfa-
jökull was carried out in 1985 with radio echo sound-
ing. It included profiles in the northern part of Ör-
æfajökull and within the Öræfajökull caldera reveal-
ing up to 500 m thick ice at the centre of the caldera
(Björnsson, 1988). Magnússon et al. (2007) have also
published a map showing the bedrock beneath the
lower part of the Kvíár-, Hrútár- and Fjallsjökull out-
let glaciers flowing eastwards from Öræfajökull.
Öræfajökull is situated in an active off-rift vol-
canic zone, some 50 km east of the Eastern Volcanic
Zone, (Figure 1). The off-rift zone is ∼120 km long
trending SW-NE, is represented as the Öræfi Vol-
canic Belt (Thordarson and Larsen, 2007). It is mostly
covered by the Vatnajökull ice cap and includes the
Esjufjöll central volcano in S-Vatnajökull and the
Snæfell central volcano north of Vatnajökull. Öræfa-
jökull rests between and partly covering two extinct
central volcanoes, Skaftafellsfjöll to the west and the
deeply eroded Breiðamerkurfjall to the east where in-
trusive gabbro and granophyre has been exposed. The
lower bulk of Öræfajökull is composed of horizon-
tally stratified basalt believed to have formed in the
rift zone about 10 –12 Ma ago (upper Miocene). This
Tertiary pile has been isostatically uplifted and largely
eroded by glacial activity over the last 3 Ma (Helga-
son and Duncan, 2001). The upper part of the vol-
cano, south of Svínafellsjökull and Fjallsjökull (Fig-
ure 1), piled up during the last 0.7 Ma and its struc-
tural composition varies from interglacial lava flow to
subglacial pillow lava, hyaloclastite tuff and breccia
(Thorarinsson, 1958; Prestvik, 1982; Stevenson et al.,
2006).
In this paper we present a detailed topographic
map of the bedrock beneath the ice cover of Öræfajök-
ull. This map in combination with an accurate map of
ice surface topography, improves our understanding
of how glaciological processes have shaped the bed
of this ice cap of extreme mass throughput and large
elevation range. We derive fundamental information
needed for risk assessment and evaluation of potential
ice-volcano interaction during an eruption in Öræfa-
jökull, including locations of water divides, ice thick-
ness and total ice volume within the caldera of Öræfa-
jökull. Furthermore, we identify topographic features
of the volcano which may be of relevance for the erup-
tive history of Öræfajökull.
DATA AND METHODS
Radio echo sounding data
The radio echo sounding (RES) data used in this
study (Figure 2–3) include profile measurements in
the northern part of our study area measured in 1991–
1993 during a survey of Breiðamerkurjökull (Björns-
son et al., 1992) and Skeiðarárjökull (Björnsson,
2009; Magnússon, 2008), point measurements ob-
tained mainly in the lower part of Öræfajökull in
1998–2006 (including data from Pálsson et al., 1998
and Magnússon et al., 2007) and profile measure-
ments carried out in the accumulation area of the
glacier in 2009 and 2012. Continuous profiles of
RES measurements were obtained by placing a trans-
mitter and a receiver on sledges at the centre of the
corresponding antenna, which together form a single
line. The setup is towed by a snow mobile or scooter
(e.g. Björnsson et al., 2000), equipped with a posi-
tioning system (Loran-C/GPS for horizontal location
and barometer for elevation in 1991–1993, differen-
tial GPS for both in 2009 and 2012). The radio echo
sounder used during the survey in 1991–1993 was a
mono pulse analogue system developed at the Sci-
ence Institute, University of Iceland (Sverrisson et al.,
1980). It has receiver bandwidth of 1–5 MHz and has
been used to map the subglacial bedrock of large parts
the Icelandic ice caps (e.g. Björnsson et al., 2000;
Björnsson, 2009). The profiles measured on Öræfa-
jökull in 1985 were obtained with this system but due
to inaccuracy in profile locations, and since later ob-
servations cover essentially the same area, the data
from 1985 are not used.
In 2009 around 170 km of RES-profiles were
measured in the accumulation area of Öræfajökull
(Figure 2). The profile locations were planned on
ortho-rectified SPOT5 images acquired in late sum-
mer 2006/07 in order to avoid crevasses. The ra-
dio echo sounder used in 2009 is a mono-pulse sys-
tem (Mingo and Flowers, 2010). By adopting the
JÖKULL No. 62, 2012 133