Jökull - 01.12.1974, Page 44

clase porphyritic type and proved a most valu-
able marker horízon together with the directly
underlying tillite. The rest o£ the lavas are
tholeiites. Between individual flows are usually
found thin layers of reddish aeolian dust and
ashes sometimes filling only the slaggy tops of
the flows. A second tillite-conglomerate inter-
bed of 6—10 m thickness occurs 40 m below
the top of this lava succession (number 2 of
Plate 1).
2.2 THE HÚSAFELL CENTRAL VOLCANO
The activity of the Húsafell central volcano
is best described in terms of three phases each
representing a copious acidic volcanism. During
the intervals the acid products were buried by
basaltic lavas. The two first phases culminated
in very large ignimbrite eruptions. The third
was exceptional in that is was accompanied by
the intrusion of acid and composite intrusions.
The Húsafell volcano, as can be seen from
Fig. I. Buried topographic expression of the
Húsafell central volcano shown by contour-lines
drawn on the base and top. The prominent
curving of the base contour-lines indicates sagg-
ing of about 200 m due to the weight of the
volcano. The curving of the top contour-lines
on the other hand is due to its high relief of
at least 300 m above its surroundings.
Myncl 1. Á kortinu kemúr Húsafells-eldfjallið
fram sem þykkildi í jarðlagastaflanum. Jarðlög
frá upphafstima þess sveigjast i boga vestur,
Uklega vegna sigs undan fargi þess. Jarðlög frá
endalokum eldfjallsins sveigjast hins vegar i
boga austur, sem sýnir að Húsafellseldfjallið
hefur staðið a. m. k. 300 m hátt yfir umhverfið.
42 JÖKULL 24. ÁR
Fig. 1, grew to an important shield-like struc-
ture as a result of its greater lava production
relative to its surroundings. The floor of the
volcano was suppressed due to the excess load
and the top stood up above the surroundings
for some time after its activity ceased. Besides
producing acidic rocks the volcano erupted also
basalts which screened the slopes at intervals
between the acid phases. Among those basalts
are found tliin tholeiitic lavas with little or no
sedimentary interbeds, but also olivine tholeiites
of lava shields.
2.2.1 The first acid phase.
This phase is represented by very conspicuous
platy dacite lavas cropping out between the
rivers Reykjadalsá and Hvítá. The thickness of
these lavas amounts to possibly more than 100 m.
Another large outcrop was found in a tributary
valley, the Bæjargil, northeast of Gilsbakki.
An ignimbrite sheet, also of dacitic composi-
tion, follows upon the dacite lavas but overlies
them only marginally. The ignimbrite extends
for more than 15 km along the strike. The
maximum thickness is about 40 m in the area
between Asgil and Gilsbakki. It thins towards
west to only 5 m in a small gorge east of Búr-
fell. The clegree of compaction decreases in the
same direction. We propose for it the name
Hraunfossar ignimbrite as it is well exposed in
the picturesque waterfalls of tliat name. In the
northern part of the area the ignimbrite con-
sists of two flow units each beginning with an
airfall tuff 1—2 m thick. The lower unit is
thicker and the only one present south of
Reykjadalsá. The ignimbrite is a rather loose
pumiceous or agglomeritic rock of red or gray
colour, which is typically nonwelded. A higher
degree of compaction which comes close to
welding was observed near the middle of the
ignimbrite unit in the area of greatest thick-
ness. The red coloured variety is more wide-
spread in the area of greatest thickness. The
reddening is due to hematite which probably
formed as a result of oxidation by exsolved gases.
The ignimbrite is composite consisting of two
varieties of glass, the more basic one being less
abundant. The light coloured component is
particularly abundant in the air fall tuffs. The
refractive index of the two glass varieties was
determined at about 1.52 for the light variety