Jökull - 01.12.1989, Síða 84

Fig. 6. Autobrecciated rhyolite with discemable
layering in the upper part.
Mynd 6. Jaðarstykki úr rýólíthrauni, þar sem
hraunið hefur molnað vegna seigjumismunar þegar
það skreið fram.
is overlain and separated from the Orustukambur
ignimbrite II by 70 metres of quartz tholeiites.
The Orustukambur ignimbrite II is welded to a
compact mass and reaches a thickness of 80 metres.
It contains rhyolite fragments up to one metre in
size. This suggests an eruption conduit nearby. Prox-
imity to an eruption conduit is also suggested by the
strong propylitisation, which could, however, also
have been superimposed in a later stage of volcanic
activity. As the production of silicic lavas is mostly
initiated by explosive volcanic activity connected
Fig. 7. Flow foliated rhyolite. Microcrystalline parts
are in light colours, glassy parts are black. Section
shows vertical cut through the lava flow.
Mynd 7. Lagskipting í rýólíthrauni. Ljósu lögin eru
fínkristallað efni, dökku lögin úr gleri.
with eruption of pyroclastic material, the Orustu-
kambur ignimbrites I and II possibly incorporate this
first phase of the Kækjuskörð rhyolitic volcano.
Apart from these co-ignimbrite breccias only a
strongly weathered tuff layer, at least 10 m thick, is
encountered in a gully just west of the Orustukamb-
ur cliffs. The nature of this tuff, weathered to a white
to yellow muddy substance, cannot be determined
because it is only exposed here. It contains large
silicified tree stumps (up to 0.6 m in diameter).
82 JÖKULL, No. 39, 1989