Jökull - 01.06.2000, Blaðsíða 32

Surface and bedrock topography of Mýrdalsjökull
ted flood waves in coastal waters. During 18 of the
20 documented eruptions the associated jökulhlaups
flowed southeast down to the Mýrdalssandur outwash
plain (Figure 1), but in two cases jökulhlaups flowed
southwest to the Sólheimasandur outwash plain; the
third route, northwestwards into the river Markarfljót,
was taken by a jökulhaup in 1600 B.P. (Haraldsson,
1981). During the jökulhlaups, a mixture of water,
ice blocks, volcanic products and sediment, frequently
hyperconcentrated, surges over the outwash plain.
Velocities of 5-15 m/s, peak discharge of 100-300,000
m


/s reached in a few hours, and total volumes of 1-8
km


have been suggested (Jóhannsson, 1919; Þórar-
insson, 1957, 1959, 1975; Hannesson, 1934; Maizels,
1993; Tómasson, 1996; Karlsson, 1994). These jök-
ulhlaups, along with heavy fallout of tephra, make
the Mýrdalsjökull volcano the most hazardous one in
Iceland.
In this paper we present detailed ice surface
and bedrock topography data from Mýrdalsjökull,
and describe the morphology, size and shape of the
Mýrdalsjökull volcano with reference to its erupti-
on history. Furthermore, we present new informati-
on regarding the location of eruptive vents, ice and
water drainage basins and subglacial flowpaths of jök-
ulhlaups during eruptions.
Previous mapping of the glacier surface and
bedrock topography
The first maps outlining with some accuracy the co-
verage of Mýrdalsjökull were surveyed by the Dan-
ish Geodetic Institute in 1904-1907 (the southernmost
margins) and during 1937-1938 (the main ice cap).
The maps were published in a scale of 1:100,000
(Nørlund, 1944). However, the indicated ice-surface
elevation on these maps was not based on the survey-
ing data, except along the glacier edge. Instead, the
contour lines were drawn with reference to oblique
air photos and show, according to Nørlund (1944), the
shape of the ice surface rather than its elevation. In
later editions of these maps, the position of the glacier
edge has been revised using aerial photographs.
The first triangulation surveying on the ice cap
was carried out in 1943 by Steinþór Sigurðsson,
who compiled a surface map, later published by
Rist (1967a). The next maps of the ice cap were
produced by the U.S. Army Map Service in a scale
of 1:50,000 on the basis of aerial photographs taken
in 1945-46, and the triangulation system previously
surveyed by the Danish Geodetic Institute. The ice-
surface contours along the marginal areas of the ice
caps (some few km up from the edge) were compiled
from aerial photographs; but higher up, the contours
were identical with those of the Danish Geodetic
Survey maps. The surface maps showed the gener-
al shape of the ice cap, the outlet glaciers and the
caldera depression surrounded by the higher domes
of Háabunga and Goðabunga (bunga means dome in
Icelandic).
Exploration of the ice thickness of Mýrdalsjök-
ull and its subglacial topography began in 1955 when
seismic reflection soundings were carried out at 9
locations on the ice cap, showing an ice thickness of
300-400 m (Rist, 1967a). In 1977 a few radio echo-
sounding profiles on Mýrdalsjökull showed consi-
derable variations in bedrock topography. An ice
thickness of 500-600 m was observed in the central
part of the ice cap (Björnsson, 1978) confirming the
presence of a deep depression (caldera) beneath the
central part of Mýrdalsjökull. ERTS Landsat-images
from the early 1970’s also revealed surface forms
which strongly suggested that the Mýrdalsjökull ice
cap covered a prominent volcanic caldera (Sigbjarna-
son, 1973; Sæmundsson,1982).
SURFACE AND BEDROCK
TOPOGRAPHY OF THE ICE CAP AS
MAPPED BY RADIO ECHO SOUNDING
In May in 1991 the ice surface and bedrock elevations
of Mýrdalsjökull were mapped in detail. Ice thickness
profiling was carried out by continuous radio echo
sounding (Figures 2 and 3). Navigation on the ice
cap employed GPS and Loran-C and position of the
sounding equipment was logged at 50 m intervals
with an accuracy of 50-100 m. Most of the sound-
ing lines run along longitudes or latitudes, but some
were placed perpendicular to the trend of the buried
subglacial structures in order to minimize lateral ref-
lection. Crevasses prevented sounding on the steepest
outlet glaciers, flowing to the east, south and north-
JÖKULL No. 49 31