Jökull - 01.12.1989, Blaðsíða 79

Pig- 2. Silicic rocks in the Borgarfjörður Eystri/-
Loðmundarfjörður area.
Mynd 2. Súrt berg í Borgatfrði Eystra og Loð-
niundarfirði. Hraun og innskot eru skyggð, fiikru-
bergerdökkt.
1. Dyrfjöll central volcano, with eruption sites in
Njarðvík cove and Dyrfjöll mountain.
2. Breiðavík central volcano, with eruption sites
north of the mountains Hvítserkur and Leirufjall.
3- Kækjuskörð rhyolitic volcano, with eruption sites
in the mountain range between Borgarfjörður and
Loðmundarfj örður.
4. Herfell central volcano, with eruption sites north
of the mountain Herfell.
5. Seyðisfjörður central volcano.
1. The Dyrfjöll central volcano is currently being
investigated by Gústafsson (in prep.). The volcano
has a well developed caldera with a size of about
10 knr, comparable to the modern Öskjuvatn
caldera. The caldera became filled with a pillow
breccia, which now forms, along with a few basaltic
lava flows on top of it, the geographic elevation of
Dyrfjöll mountain. Since most of the top of the vol-
cano is eroded, only very little of the silicic rocks
that were produced during the caldera-forming erup-
tion, are preserved. Eruption sites at a lower strati-
graphic level have shaped the northem flank of the
volcano around Njardvík cove, where silicic
intrusive and extrusive rocks are exposed. Judging
from what is left of them today, the eruption sites
probably reached a few hundred metres above the
gently inclined northem slopes of the rising volcano.
Downwarping of the bedrock around these eruption
sites could have been caused by foundering of a part
of the volcanic edifice after a major explosive erup-
tion in one of the volcanic cones. The result was an
elongated depression, WNW-ESE striking and
roughly perpendicular to the general dyke direction,
which is N 20° E.
2. The Breiðavík central volcano comprises basal-
tic and a large volume of silicic rocks in the hills
and mountains east of Borgarfjörður, including an
ignimbrite sheet more than 300 m thick . The basal-
tic lava formation dips inward to the Breiðavík bay
east of Borgarfjörður, which suggests an area of
downsagging of a volcano floor or even the forma-
tion of a caldera. Downsagging of the volcano floor
is indeed evident in the horizontal banking of the
Hvítserkur and Leirufjall ignimbrite against north-
ward dipping basaltic lava flows, thus implying the
settling of the ignimbrite in a subsidence area (Fig.
3). The formation of a caldera lake above the ignim-
brite is indicated by the existence of pillow breccia
and palagonite tuffs on top of Hvítserkur. It is
unlikely that these rocks erupted under a glacier, as
in the present active volcanic zones of Iceland,
because no signs of Tertiary glaciation have been
found in this area.
3. A third eruption site in the mountain range
between Borgarfjörður and Loðmundarfjörður has
been termed Kækjuskörð rhyolitic volcano. Deam-
ley (1954) suggested an intrusive origin of most of
the silicic rocks occuring here. However, after
remapping the area that Dearnley covered in 1953, it
JÖKULL, No. 39, 1989 77