Jökull - 01.12.1973, Blaðsíða 13

tlie area of Heinabergsfjöll, 40 km to the east
of Esjufjöll (Fig. 3).
STRUCTURAL, VOLCANIC,
AND GLACIOLOGIAL FEATURES
WITHIN VATNAJÖKULL
Within the icecap proper are perhaps the
most interesting aspects of the ERTS-1 image.
Some of these features are, of course, known;
others apparently are not. It should be noted
that although seismic sounding techniques liave
been used to measure the thickness of Vatna-
jökull along certain profiles (Holtzscherer, 1954),
and hence the gross configuration of the sub-
glacial topography is known, sucli sounding
gives only the broadest outlines of the sub-
glacial landforms and precious little on the
geologic structure. Unfortunately, rnuch of our
view and knowledge of the cliaracter of such
landforms and structure is wholly or partly ob-
scured by glacial ice and snow. The ERTS-1
tmage, because of its portrayal of regional rela-
tionships and subtle features within the icecap,
shows some important structural features.
Note, for example, that the trend of the
crater rows and hyaloclastite ridges continues
right under Vatnajökull on its southwestern
margin. This helps to explain, among other
things, that Sídujökull, Skaftárjökull, and
Tungnárjökull surge independently of each
other (Thorarinsson, 1964 and 1969).
Depressions and linear higher features in
Skaftárjökull, Tungnárjökull, Jökulkambur,
and Köldukvíslarjökull appear to reflect the
underlying structure. Of particular interest,
also, is the area around Kerlingar because of
the circularity of the structure. There is a pro-
nounced line of depressions which connect the
subglacial features at Hamarinn and Grímsvötn
(Fig. ]). The main depression (the easternmost
°ne) in the line between Grímsvötn and Ham-
arinn was first observed by Thorarinsson and
Hannesson during a flight over Vatnajökull on
7 September 1955. The flight had tlie objec-
tive of detecting the source of a jökulhlaup
(glacier outburst flood) on the Skaftá river,
tvhich had begun on. 2 September 1955 and
ended 9 days later. The depression sighted on
this flight had obviously been formed as the
result of the jökulhlaup, because its gaping
concentric crevasses were quite fresh. In addi-
tion, the depression was not there when Thor-
arinsson flew over the area on 27 August 1955
(Thorarinsson and Rist, 1955).
Since that time jökulhlaups have occurred in
Skaftá about every second year (the last one
took place in July 1972), and the depression
has never had time to be quite filled up be-
tween. The depth of the depression after each
jökulhlaup has amounted to 100—150 m.
The subglacial course of the 1955 Skaftár-
lilaup (subsequent jökulhlaups have followed
the same course) which led to the Skaftá river
and not, as one would have expected, to the
Tungná river, led Thorarinsson to suggest that
the same type of topography as exists on both
sides of Langisjór, viz NE—SW trending hyalo-
clastite ridges, extends northeastward under
Vatnajökull. Thus, hyaloclastite ridges as an
extension of either Fögrufjöll or the ridges west
of Langisjór form the drainage divide between
Skaftá and Tungná. The ERTS-1 image de-
finitely confirms this suggestion, and it also
explains the fact revealed by the studies of H.
Tómasson and E. G. Vilmundardóttir (1967)
that Tungná was a clear-water river a few
hundred years ago, when the glacier border was
5 to 10 km inside the present one. The sub-
glacial ridge extending towards the southwest
from Kerlingar, has then diverted the glacier
water of Tungnárjökull to Langisjór and Skaftá.
The ERTS-1 image also indicates a sub-
glacial channel extending 3 km towards the
south-southwest from the easternmost depres-
sion in the area between Hamarinn and Gríms-
vötn. This channel is probably new, having not
been observed before. The shallow depression
farther west has become more marked in recent
years than it was before, but it has not been
proven that it is connected with the jökul-
hlaups which empty into the Skaftá. It may
possibly have a connection with Kaldakvísl
along a subglacial course south of Hamarinn.
The ellipsoidal outline of Bárdarbunga (1988
m) is suggestive of a central volcano such as
that which underlies Tungnafellsjökull or
Kverkfjöll particularly when its size and eleva-
tion is also considered. Drilling in the icecap
of Bárdarbunga at 1880 m altitude showed the
ice to be more than 415 m thick. There appears
to be a depression in the center of Bárdar-
JÖKULL 23. ÁR 1 1