Jökull - 01.06.2000, Page 37

Helgi Björnsson et al.
0
4 0 0
8 0 0
1 2 00
E
le
va
tio
n
(
m
a
.s
.l.
)
0 5 0 00 1 0 00 0 1 5 00 0 2 0 0 00 2 5 00 0 3 0 00 0 3 5 00 0 4 0 00 0
H o rizon ta l d is ta nce (m )
D E
0
4 00
8 00
1 20 0
E
le
va
tio
n
(m
a
.s
.l.
)
A B
0
40 0
80 0
12 00
E
le
va
a
tio
n
(m
a
.s
.l.
)
C B
A
B
C
D
E
Figure 6. Sections across the Mýrdalsjökull caldera.
AB: Goðabunga to Kötlukollar, CB: Fimmvörðuháls
to Kötlukollar, DE: Háabunga to Sléttjökull. – Snið
yfir öskju Mýrdalsjökuls.
Several glacially eroded passes cut the caldera
rim but the elevation of their deepest points has not
been exactly determined. The lowest pass seems to
lie at about 740 m between Háabunga and Kötlukoll-
ar, facing southeast to the glacier outlet Kötlujökull.
Sólheimajökull drains southwest through a 1050 m
breach between Háabunga and Goðabunga. A pass in
the northwest at about 1100 m heads toward Entu-
jökull, and a northeastern pass toward Sandfellsjök-
ull. A narrow gorge trending northeast is located just
west of Austmannsbunga. This gorge may be a tect-
onic feature subsequently eroded by water and ice. All
these breaches in the caldera rim are potential path-
ways for jökulhlaups from subglacial lakes at geot-
hermal areas and during volcanic eruptions.
Outside the caldera, several linear structures are
prominent on the bedrock map. The topographic
ridges that strike outward from the central volcano,
presumably consist of hyaloclastites, and crater rows
built up on volcanic fissures. On the western side of
the caldera margin an E-W trending ridge, Fimm-
vörðuháls, connects Goðabunga to the neighbouring
Eyjafjallajökull volcano. The Kötlukollar ridge on the
eastern side of the caldera has the same trend. Entu-
jökull flows between two parallel NW-striking ridges
named Enta and Entukollur. In contrast, no ridges
strike toward south in the direction of propagation
of the rift zone. A ridge strikes N45 / E from Aust-
mannsbunga toward Öldufellsjökull, separating Sand-
fellsjökull and Sléttjökull, and the deep and narrow
V-shaped gorge north of Austmannsbunga. This gor-
ge is 200-250 m deep and 1.5 km wide, and bounded
by steep slopes. This roughly linear structure is a
continuation of the Eldgjá fissure, which produced
a lavaflow of 14 km


in 934 A.D. and is of tect-
onic origin. However, it may subsequently have been
eroded by jökulhlaups. Beneath Sléttjökull and Botn-
jökull several isolated peaks bear witness to recent
volcanic activity that has created new mountains at a
rate, which keeps up with glacier erosion.
Beneath the deeply-eroding Sólheimajökull the
bottom dips 50 m below sea level, which is 100 m
lower than the terrain in front of the glacier outlet, and
is indeed the lowest observed elevation under Mýr-
dalsjökull (Mackintosh et al., 2000). The region under
Kötlujökull, however, has not been sounded.
Ice thickness
The ice thickness of Mýrdalsjökull is highly variable
(Figure 8). The maximum ice thickness of about 740
m, is found in the northern part of the caldera where
an area of 12 km

is covered with more than 600 m
thick ice.
Outside the caldera the greatest ice thickness of
450 m was measured on the Eldgjá fissure. The
thickness of the ice capping caldera rims is 150 to
200 m at Háabunga and Goðabunga. The main part of
Sléttjökull has an ice thickness of 200-300 m, as has
Sólheimajökull. The ice thickness of Kötlujökull is
unknown. The distribution of the glacier surface area
and ice volume for given elevation shows that about
20% of the bedrock of Mýrdalsjökull and 55% of its
ice surface lies above 1000 m (Figure 9). The total
volume of ice on Mýrdalsjökull is about 140 km


and
the average thickness only 230 m. Inside the caldera
an area of 17 km

is below 740 m elevation containing
0.7 km


. The area and volume of ice inside the rims
of the caldera, is 100 km

and 45 km


, respectively.
36 JÖKULL No. 49