Jökull - 01.12.1973, Síða 12

is formed, thus producing the marvelous array
of glaciological features.
It is perhaps with some understatement, then,
that one can say that the most striking and
impressive aspect of the ERTS-1 image is the
regional portrayal of geologic structure, glacio-
logical features, volcanic landforms — particul-
arly the large structural and volcanic features
which are partly or wholly buried under the
glacial ice of Vatnajökull. Let us first discuss
the areas outside the present margins of the
great Vatnajökull icecap.
STRUCTURAL AND VOLCANIC
FEATURES OUTSIDE VATNAJÖIÍULL
In the area between the outlet glaciers, Skeid-
arárjökull and Sídujökull, which is called Kálfa-
fellsheidi, are a pronounced series of NE—SW
lineations which are structurally controlled
(Fig. 2). These strong lineation are a south-
east continuation of the striking belt of crater
rows and hyaloclastite ridges which occur far-
ther northwest. The reason why the lineation
is not so marked in the topography as in the
adjacent belt to the northwest, is simply that
the latter belt consists of postglacial and
historical crater rows (Raudhólar, Lakagígar,
etc.) and subglacial ridges from the last glacial
epoch. The more southeasterly belt, on the
other hand, was built up during older parts of
the Pleistocene ice age and has thus been much
more eroded. The glacial erosion has mainly
been in a direction at right angles to the main
tectonic and constructional volcanic lineation.
[.Kjartansson (1962) shows glacial striae with a
NW—SE to NNW—SSE direction in this area.]
Tlius, glacial erosion has tended to destroy the
original volcanic morphology. The bedrock in
this area consists of an almost horizontal sequ-
ence of basalts, breccias, and conglomerates of
Quaternary age formed during the Matuyama
and Brunhes epochs [Old Gray Basalts on
Kjartansson’s map (1962)].
The circular structure in the Kálfafellsheidi
area is the result of glacial and fluvial erosion,
perhaps acting along a secondary tectonic trend
which shows up as faint NW—SE lineations
across the primary structural grain of the area.
This NW—SE trend in Iceland has only re-
cently been recognized outside the Snæfellsnes-
1 0 JÖKULL 23. ÁR
Borgarfjördur area and is no doubt tectonic in
origin. Tltere is a marked NW—SE lineation
apparent in the area between Mýrdalsjökull
and Vatnajökull to north of Grænafjall east of
the active zone. Preliminary geological mapping
(G. Sigbjarnarson, oral communication) already
shows the presence of faults having this trend.
In fact, they are also present across the active
zone to the west (Sigurdsson and others, 1972),
even though they do not show up on this
particular ERTS image. This may indicate that
differential glacial erosion is needed to bring
them out morphologically. The tectonic signi-
ficance of this “secondary” tectonic trend will
be discussed at length later in this paper.
The volcanic landforms and geologic struc-
ture of the area to the southwest of Vatnajökull
are striking. The shadows cast by the serrated
crests of the hyaloclastitje ridges, the craters of
the crater rows, and grabens cause these land-
forms to stand out sharply at the low sun
angle. The image also shows very clearly how
extremely straightlined some of the serrated
ridges are, and how for long stretches tliey
show no tendency to the en échelon pattern
so characteristic for many fissure systems in
southwest Iceland. There is no question that
a study of the regional topography of this area
could have a strong bearing on the understand-
ing of the submarine topography of the Mid-
Atlantic ridge as shown on Heezen’s map
(Heezen and others, 1959).
Also of particular interest in this area are
the X-shaped intersections of some of the fea-
tures. The X-shaped intersection southeast of
Hofsjökull has a 35° angle (Fig. 1). One feature
strikes N. 70° E.; the other strikes N. 35° E. The
trend of the crater rows, grabens, and hyalo-
clastite ridges between Lakagígar and Vatna-
öldur varies between a strike of N. 37° E. and
N. 55° E., with several X-intersections of fea-
tures.
In the area between Skeidarárjökull and
Öræfajökull a distinct circular structure can be
seen in the bedrock topography. A central vol-
cano of probable early Quaternary age is known
to be present in this area The deep erosion of
this central volcano by rivers and glaciers
appears to have been controlled to some extent
by the circularity in its structure. Another ex-
ample shows up at the edge of Vatnajökull in