Jökull - 01.12.1973, Blaðsíða 14

bunga encirded by the high rim exposed up
to 1850 m on the west side. The flattening of
the ice sheet on Bárdarbunga and the steepness
of the outer slopes suggests a buried caldera.
Likewise, the high feature of the area to the
east of Hamarinn is likely a central volcano
complex. There is even a small central depres-
sion in it which suggests a central vent. This
“central vent” may also be related to the sub-
glacial geothermal or volcanic activity which
has caused the three prominent subsidence fea-
tures to the east.
The depressions seen on tlie ERTS-1 image
near the northern rim of the Grímsvötn caldera
form or deepen after eacli jökulhlaup from
Grímsvötn (cf. the American Army Map Service
map of Grímsvötn, scale 1:50,000, Sheet 5920,
II, 1950, Series C762*). They are most certainly
the result of permanent subglacial geothermal
activity and subsidence of the overlying ice,
following the withdrawal of meltwater. In one
such depression a large solfatara was observed
after the jökulhlaup from Grímsvötn in 1954
(Thorarinsson, 1955). The series of shallow de-
pressions forming a chain E to SSE of Gríms-
fjall probably show the subglacial course of
the floodwater from the March 1972 jökulhlaup
from Grímsvötn. The image also suggests that
the Grímsvötn depression consists of two cald-
eras.
The area around the nunataks, Geirvörtur
and Pálsfjall, and the areas of Háabunga,
Thórdarhyrna, and Hágöngur, are most inter-
esting from a structural viewpoint. Thorarins-
son’s map (1950) showed a possible structural
alignment of Geirvörtur—Grímsvötn, and on its
continuation to the northeast he suggested a
probable eruption center in Dyngjujökull. On
the map this center lies 10 km to the west of
Kverkfjöll. On the ERTS-1 image, however, a
volcano-tectonic lineament whicli strikes N. 45°
*) The aerial photography of the Grímsvötn
area, which was used to prepare the 1:50,000-
scale map of Grímsvötn was acquired by the
USAAF in Aug. 1945 and Aug.—Oct. 1946,
during the month before and year after a jökul-
hlaup. The unusual “composite” map is the
best map available of the morphology of the
caldera after a jökulhlaup, and thus is similar
to the situation portrayed on the ERTS-1 image.
1 2 JÖKULL 23. ÁR
E. can be traced from Hágöngur in the south-
west across Grímsvötn to Kverkfjöll in the north-
east where it cuts in half what appears to be a
caldera southwest of Kverkfjöll, and finally
merges with the trend of the solfatara valley
in Kverkfjöll. This structural lineament extends
at least 80 km from the southern to the north-
ern edge of Vatnajökull — extending from Sídu-
jökull to Kverkfjöll. It continues to the north-
east and to the southwest of Vatnajökull as the
line of demarcation between areas of active
volcanism to the west and extinct volcanism to
the east. The same relationship probably holds
true for tliat part of it which is covered by
the ice sheet.
In the Kverkfjöll area two elliptical features
are well exhibited. Both are about the same
size, 8 km x 5 km, but whose long axes are
oriented differently to each other. The long
axis of the elliptical structure southwest o£
Kverkfjöll is oriented about N. 30° W. An
apparent linear depression cuts N. 45° E. across
its northern side and merges with the 80 krn
long volcano-tectonic lineament previously dis-
cussed. Southwest of the structure this linear
depression is less distinct (with some parallel
linear features) but continues to the southwest
for at least 10 km. Just as the alignment of the
hot springs and solfataras of Hveradalur was
shown, through analysis of aerial thermography
(Friedman a?id others, 1972) to extend to the
northeast under Kverkfjöll, the ERTS-1 imag-
ery shows that the Hveradalur thermal activity
is also manifested to the southwest for a con-
sidererable distance.
Therefore, it is suggested that this northwest-
trending elliptical feature is the subdued sur-
face expression of a subglacial caldera. It is
also likely that this subglacial caldera is the
“possible eruption center” in Dyngjujökull
which was previously discussed by Thorarins-
son (1950). Thorarinsson suspected such a sub-
glacial volcanic center because of the occurrence
of historical jökulhlaups on Jökulsá á Fjöllum.
Fig. 3. Enlargement of eastern part of Vatna
jökull to 1:500,000 scale. [NASA ERTS-1 image
(MSS), 1192-12084.]
Mynd 3. Stœkkun af austanverðum Vatnajökli
(M 1:300 000).