Jökull - 01.06.2000, Page 34

Surface and bedrock topography of Mýrdalsjökull
proportional to the velocity of the receiver sledge,
measured with a bicycle wheel odometer. The velocity
of electromagnetic waves in ice was assumed to be
169 m/ . s. This same value was also used for the
surface firn layer, as its maximum thickness is on-
ly about 20-30 m. The vertical thickness of the ice
was computed from a digitized sounding record using
general inversion techniques as described by Harri-
son (1970). The sounder sees a strip along the bed
of width typically 100-200 m (defined by the first
Fresnel zone for a pulse length of 34 m, Björnsson,
1988). Along each sounding line the recorded data
represents a moving average of the real bed profile on
a strip beneath the line. The accuracy of the absolu-
te ice-thickness measured along the sounding lines is
considered to be , 15 m or 2%, whichever is greater.
Echo returns were obtained over the whole glacier,
however, they were faint in some places adjacent to
and south of the northeastern caldera rim, Austmanns-
bunga, where the ice thickness reaches 600-680 m.
Data from Mackintosh et al. (2000) were used to
compile the map of the Sólheimajökull outlet glacier.
The bedrock elevation was obtained as the difference
between the ice surface altitude and the ice thickness.
Map compilation
Digital elevation maps (DEM) of the bedrock and
surface topography, with equal grid spacing of
100x100 m, were compiled by interpolating data from
our soundings and existing geodetic maps of the
area surrounding the ice cap. Outside the surveyed
area, the final glacier surface map is based on the
DMA-series of the Iceland Geodetic Survey (1990).
The residual between elevations on our survey lines
and the DMA-map was calculated and a new map
produced by adding the calculated residuals to the
DMA-map. The outlines of the glaciers are the same
as on the DMA-series.
Due to the large spacing between the sounding
lines (typically 500-1000 m), the topographic map
does not fully reproduce features smaller than 1-2 km
across, but local detail is described along the sounding
lines. However, relative resolution of the bedrock data
with respect to topographical features is considera-
bly better. Volcanic and tectonic structures of vertical
extension of the order of 10 m and larger can thus be
resolved, e. g. hyaloclastite ridges and major normal
faults but we are unable to delineate fissure zones with
small vertical displacements.
The maps are presented in conformal conical
Lambert-coordinates with coordinate axes originating
at 65 / N and 18 / W. The rows and columns in the
matrix implicitly define the geographic coordinates.
Smoothed contour maps were drawn from the digital
matrix.
The glacier surface map
The central parts of the ice cap form a plateau at
an elevation of about 1,300 m (Figures 3 and 4),
surrounded by higher rims at Háabunga (1497 m)
and Goðabunga (1505 m), the nunatak Austmanns-
bunga (1377 m) and Kötlukollar (1320 m). This is the
surface expression of the Mýrdalsjökull caldera. Steep
outlet glaciers flow in narrow valleys down to 100-800
m on the southern and western flanks. Broader outlets
drain eastward down to 200-400 m, and one large ice
lobe covers the northern flank down to 600-650 m.
The surface map shows 12 small depressions in
the glacier surface that have been created by su-
bglacial geothermal activity. These ice cauldrons are
typically 20 to 50 m deep and their diameter is 500 to
1000 m.
The most striking difference between our map and
the 1938 map of the Danish Geodetic Institute is that
we describe the sharp elongated shape of the ridges
Háabunga and Goðabunga. At Háabunga the elevati-
on of the old maps was wrong by up to 200 m, mainly
due to misplacement of the dome. In contrast our map
is very similar to that compiled by Sigurðsson in the
1940s (Rist, 1967a).
Bedrock terrain and geological structures
The most prominent landform beneath the ice cap is
a large volcano with a circular base of a 20 km dia-
meter at 700 m elevation and 30-35 km at the base.
The mountain rises up to rims of 1300-1380 m that
surround a 650-750 m deep caldera, reaching down to
an elevation of about 650 m (Figures 5, 6 and 7). The
area of the depression, girded by the highest points on
the rim, is 100 km

. The arcuate ridges reaching about
1300 m elevation, form the caldera rims beneath Háa-
bunga, Goðabunga, and between Enta and the nuna-
JÖKULL No. 49 33