Jökull - 01.01.2005, Page 36

Árni Hjartarson
Figure 2. The Ágúll rhyolite dome, a cross-section along the strike in Ábær river gorge. 1) Olivine tholeiite
lavas. 2) Acid tephra. 3) Basal breccia. 4) Rhyolite dome. 5) Dyke. 6) Till cover. – Ágúllinn við Ábæ. Þversnið
í strikstefnu. 1) Ólivínbasalthraun. 2) Súr gjóska. 3) Botnbrexía. 4) Gúll úr líparíthrauni. 5) Gangar. 6)
Jökulruðningur á yfirborði.
dome. In some places this breccia forms huge pillows
rising many meters up into the lava itself (Figure 2).
The lowest 60 m of the lava are excavated in the
gorge. The upper part is more or less capped by screes
and till but reaches 500 m a.s.l. in the mountain slopes
east of the gorge. The texture of the lava is fine-
grained with small plagioclase phenocrysts that make
up 30% of the rock volume. The rock is dense, con-
sisting of irregular flow layering and flow foliations
and forming coarse polygonal columns. Cracks and
fissures are most often perpendicular to the layering.
The acid lava can be traced for 3 km upstream, along
the Ábæjará river, where it disappears because of the
local tectonic tilt (10◦ SE). Along the Jökulsá river it
also can be traced 3 km upstream where it disappears
below the bottom of the valley. A landslide covers
the western part of the lava dome but it most probably
terminates below the western part of the slide. The
dome has not been found on the south slopes of the
Austurdalur valley.
Instead of it, there is a thick sedimentary layer,
consisting perhaps of reworked material from the
dome and is its lateral equivalent. The surface brec-
cia of the dome, or the contact between its top and the
country rock, is not exposed. The acid dome is cut
by numerous basaltic dykes related to the overlying
basaltic lava pile.
The dimensions of the lava dome are not clear be-
cause only the northern part of it is exposed. The
southern part is hidden below the bottom of the valley
due to the regional dip. At the surface it can be traced
for 5 km from east to west and 3 km from north to
south. Its thickness reaches at least 250 m. The base
of the domemight either be semi-circular or elongated
(which is more likely), with the long axis running
north-south according to the dominating tectonics. If,
however, it is assumed to be circular, 5 km in diam-
eter and with an average thickness around 100 m, its
volume would be 0.6 km3.
The Ábær tholeiites make up a thick pile of thin-
layered lavas that bank up against, and cover, the
Ábær dome. The thickness of the lava pile is 250–
300 m. All interbeds are thin and seem to indicate
intensive and continuous volcanism. The lavas most
probably originate in flank eruptions inside the juve-
nile central volcano.
The Tinná lignite sediment is a fine-grained, light-
coloured sand-siltstone, rather soft and with layers
rich in acid volcanic tephra indicating contemporary
acidic eruptions in distant central volcanoes. In sev-
eral localities the layer contains lignite seams (Magn-
ússon 1980, 1981). It extends through much of the
area with a maximal thickness, 30 m, in the Goðdala-
dalur canyon. Its thickness varies from 3 to 30 m from
36 JÖKULL No. 55