Jökull - 01.12.1989, Blaðsíða 85

Fig- 8. A quartz tholeiitic dyke intruded parallel to
the almost vertical iiow structures in the upper zone
of a rhyolite at Skúmhattardalsbrík. The dyke is
0.50 m thick.
Mynd 8. Þóleiítískur gangur sker rýólíthraun sam-
hliða lagskiptingu hraunsins.
Kœkjuskörð rhyolite — The Kækjuskörð rhyolite
(Fig. 10) is apparently the most voluminous of the
silicic lavas from the volcano and occurs in the area
°f Orustukambur, Kækjuskörð, Skúmhöttur, Þriggja-
hnjúkafjall, Miðfjall and an unnamed hill with a
height of 643 m above sea level (Fig. 4). The
Kækjuskörð rhyolite has a maximum thickness of
300 m, an area about of 12 km2, and it probably has
a volume of about 3 km3. This is rather large com-
pared with other silicic lavas in Eastern Iceland,
whose thickness rarely exceeds 250 m (Walker,
mctrcfl
600
400
200
0
mainly olivine tholeiite
Herfell Ignimbríte
FlataQall Dacite
mainly ollvine tholeiite
ÞríggjahnjúkaQall Rhyolite II
ÞríggjahqjúkaQall Rhyolite I
Kækjuskörð Rhyoiite
Orustukambur co-ignlmbríte breccia II
mainly quartz tholeiite
Orustukambur co-lgnlmbríte breccia I
mainly quartz tholeiite
4)
Fig. 9. Schematic stratigraphic section through the
Kækjuskörð rhyolitic volcano showing its assumed
total thickness.
Mynd9. Einfaldað þversnið í gegnum Kœkju-
skarða-eldstöðina.
1966). The main conduit of the Kækjuskörð rhyolite
could been have been situated on the mountain ridge
between Þriggjahnjúkafjall mountain and the Skúm-
höttur mountain, because here the Kækjuskörð rhy-
olite reaches its maximum thickness and thins out to
the west and northwest.
The typical zonal structure of a rhyolite lava as
shown in Fig. 5a is well preserved in the Kækju-
skörð rhyolite. Ramp structures or "squeeze-up"
extrusions are found frequently.
Þriggjahnjúkafjall rhyolite I—The mountain
Þriggjahnjúkafjall is situated close to the assumed
centre of the Kækjuskörð rhyolitic volcano, and here
two other rhyolitic lavas are exposed above the
Kækjuskörð rhyolite. From the morphological
JÖKULL, No. 39, 1989 83