Jökull - 01.12.1989, Page 86

FLATAFJALL
n ftr i r c
Fig. 10. View over the Kækjuskörð rhyolite and
Flatafjall dacite from the southwest. Ramp struc-
tures in the rhyolite are visible ( —>) at Orustukamb-
ur cliffs to the left. Arrows (^-) point at the Or-
ustukambur ignimbrite II.
Mynd 10. Rýólíthraun Kœkjuskarða-eldstöðvar-
innar séð úr suðvestri þvert yfir Hraundal.
elevation formed by Kækjuskörð rhyolite, the lavas
of Þriggjahnjúkafjall rhyolite I and II can be traced
over a distance of about 2 km northward (Fig. 10).
The exposure of the lava flow is from south to north
due to N-S extension of the Þriggjahnjúkafjall
mountain. The profile in Fig. 4 also suggests N-S
flow direction of the lavas. The maximum thickness
of Þriggjahnjúkafjall rhyolite I is about 50 m. Ero-
sion prevents the reconstruction of the original
extension of the rhyolite. Ramp structures typical of
the central zone of a rhyolite flow also occur in this
rhyolite. The upper zone is shaped as a breccia with
a glassy pumiceous matrix.
Þriggjahnjúkafjall rhyolite II — The Þriggjahnjúka-
fjall rhyolite II is a lava flow with a thickness of
50 m. It is exposed up to a distance of 1.2 km in a
northerly direction from Þriggjahnjúkafjall. The
exposure suggests a flow of the rhyolite into a
depression or a valley in the flood basalts. Fig. 11
shows the remnants of a plug-shaped central vent,
probably the origin of the Þriggjahnjúkafjall rhyolite
II. The strike direction of a composite dyke at Or-
ustukambur with a thickness of 15 m points straight
Fig. 11. View from Miðfjall summit on Þriggja-
hnjúkafjall. Above basaltic lava flows (B), which are
just visible on the moss-covered valley side, the
northem end of Kækjuskörð rhyolite (KR) is
exposed. Þriggjahnjúkafjall rhyolite flow I (Þ I) cov-
ers Kækjuskörð rhyolite at the upper right hand
comer. The presumed central vent of the Þriggja-
hnjúkafjall rhyolite II (ÞII), and the depression into
which the lava flowed are discemible.
Myndll. Rýólíthraun í Þriggjahnjúkafjalli séð frá
Miðfjalli. Horft til austurs.
towards the Þriggjahnjúkafjall rhyolite II vent. This
composite dyke may be related to the vent as part of
an eruption fissure. The rocks of Þriggjahnjúkafjall
rhyolite II are very similar to the silicic rock, which
is part of the composite dyke. Flow foliation in the
downward flowing lava is conspicuous and espe-
cially evident where the rocks exhibit a zonal struc-
ture.
Flatafjall dacite—The Flatafjall dacite is the
youngest of the exposed silicic rocks of the Kækju-
skörð volcano and has a maximum thickness of
100 m. It is situated beneath the summit of Flatafjall
mountain and Skúmhattardalsbrík. The slightly
vaulted structure of the 1.5 km long lava flow can be
clearly seen from Kækjuskörð. The yellow-grey
dacite has a fine grained groundmass without macro-
phenocrysts. Dark lumps of iron oxides of a size up
to 1 cm in diameter can be discemed. Platy jointing
parallel to the interfaces of the glassy and finely
84 JÖKULL, No. 39, 1989