Jökull - 01.12.1973, Page 16

The other known elliptical feature which cir-
cumscribes the Kverkfjöll area can be discerned
on the 1:100,000 base map (Blað 85, Kverkfjöll,
1954), and is well shown on the ERTS-1 image.
This partially subglacial caldera has a long axis
which is oriented about N. 30° E. and is bound-
ed by bedrock outcrops on the northwest in
the Hveradalur area and on the east and south-
east. Tlie caldera, as has been described, is
breached on tlie north by Kverkjökull.
The Esjufjöll area of Vatnajökull (Fig. 3)
has, because of its remoteness, been difficult
to study. Geological reconnaissance (G.
Sigbjarnarson, oral communication) showed
that there is a central volcano located in this
area. The Esjufjöll area of Vatnajökull lias a
particular form on the ERTS-1 image. Another
snow- and ice-filled elliptical area is faintly
visible on the north side of tlie east-west and
northeast-trending arcuate ridge. Several more
erosional ridges extend from this ridge to the
southeast. The elliptical area may very well be
another subglacial caldera with dimensions
which are about 5 km X 8 km. In fact, the
entire Esjufjöll area has the appearance of a
buried Öræfajökull volcano complex, except
that the central caldera is larger. If the Esju-
fjöll area is a subglacial central volcano com-
plex, it should be regarded as extinct because
of the marked glacial erosion.
Another structural feature that appears on
the ERTS-1 image is the tectonic line wliich
strikes in a N. 35° W. direction from the south-
east tip of Grænafjall to Thórdarhyrna and
probably continued along the base o£ Öræfa-
jökull on the southeast and to Hamarinn or
even beyond Hágöngur on the northwest. The
northwest-trending structural features in Ice-
land are related to the direction of the “frac-
ture zones”, three of which (Ward, 1971; Sœ-
mundsson, 1974) have been suggested in Ice-
land.
The NW—SE trending feature is part of a
lineament which separates two markedly dif-
ferent tectonic regimes. To the southwest of
this line is an area dominated by volcanic fis-
sures, faults, hyaloclastite ridges, and crater
rows of unusual length and linearity. Northeast
of this line, within Vatnajökull proper, is an
area dominated by central volcano complexes.
This distinction is also apparent northwest of
1 4 JÖKULL 23. ÁR
Vatnajökull — with ridge topography south-
west of Hágöngur, but a central volcano com-
plex farther north, including the Tungnafells-
jökull. North of Vatnajökull previously publish-
ed work (Sœmundsson, 1974) and ERTS-1 imag-
ery shows a mixture of linear volcanism and
shield volcanoes spread out along a pronounced
arcuate structural pattern until a fault zone is
reached which runs through the Tjörnes area.
The area covered by Vatnajökull, lying between
this pronounced tectonic line north of Græna-
fjall and the northern edge of Vatnajökull, is
a region of central volcano complexes in which
the volume production of eruptive rocks has
concentrated in a fairly restricted region, thus
creating a series of highland areas in the sub-
surface of Vatnajökull (Eytliorsson, 1952 and
1960; Holtzscherer, 1954). These densely spaced
central volcano complexes provided one of the
chief factors for the growth of the world’s larg-
est temperate icecap, Vatnajökull. Another
feature which reflects the significance of this
tectonic lineament is seen where the Öræfa-
jökull and Skaftafellsfjöll volcano complexes
are juxtaposed against the broad sandur plains
to the southwest of it. The strata which make
up the highlands along the eastern edge of
Vatnajökull are abruptly truncated at this NW-
SE-trending feature (Fig. 4). It should perhaps
be regarded as a left lateral transcurrent fault,
and as suclr a segment of the Snæfellsnes Frac-
ture Zone along which the axial rifting zones
in Iceland are readjusted (Sigurdsson, 1970; Sce-
mundsson, 1974). The position of Öræfajökull
according to this interpretation is similar to
that o£ Snæfellsjökull at the western end of
the fracture zone. This feature, then, separates
two markedly different tectonic regimes with
differing quantity and loci of eruptive pro-
ducts, thereby resulting in strikingly different
volcanic landforms. A clue to some of the sub-
glacial topography, structure, and type of rocks
within Vatnajökull may be represented by the
area surrounding Tungnafellsjökull. This
glacier, which is visible on the image between
Bárdarbunga and Hofsjökull, is a large central
volcano over 1500 m high. There is a NE—SW
elongated caldera within it which is occupied
by a glacier. This is visible on the image as
well as an eruptive ring fissure which encircles
the southern part of the volcano. To the south