Jökull - 01.12.1989, Page 80

Fig. 3. Southem margin
of the Breiðavík caldera.
Ignimbrite on the left side
of the picture banks
against northward dipping
basaltic lava fiows.
Mynd 3. Basaltísk hraun-
lög halla til norðurs undir
flikruberg í Leirufjall
(fyrir ofan vinstri vörð-
una) og Hvítserk.
is the opinion of the present authors that the silicic
rocks are of extrusive origin. The Kækjuskörð vol-
cano is no typical central volcano composed of
basaltic, silicic and a few intermediate rocks. It must
have formed a roughly circular elevation built
almost exclusively of silicic rocks. A more detailed
description of the volcano will be given below.
4. An extensive ignimbrite sheet occurs in the
mountains to the north and northwest of Loðmund-
arfjörður, with a maximum thickness of about
120 m. Deamley (1954), who recognized the ignim-
brite, made no attempt to locate its origin. In the
process of remapping the Loðmundarfjörður area it
became clear that the ignimbrite must have ori-
ginated from an area close to the mountain Herfell.
Other silicic rocks from this area are known, but no
mapping has been carried out yet to reveal their
structure. The name ’Herfell central volcano’ for
this eruption site is proposed.
5. Silicic rocks, which occur on the coastline at
Álftavík north of the mouth of Loðmundarfjörður
and in the cliffs on the southem side of Seyðisfjörð-
ur, are probably not related to any of the volcanic
centres named above. The outcrops could be the
remnants of the fifth major eruption site in the Borg-
arfjörður/Seyðisfjörður area, which is, however,
almost completely eroded away and concealed
below sea level.
Since only the Dyrfjöll and Kækjuskörð vol-
canoes have been thoroughly mapped, it cannot be
excluded that the above eruption sites represent
vents of a large volcano. The frequent occurence of
ignimbrites with considerable volume (5-10 km3)
and thickness (at least 300 m at Hvítserkur) suggest
unusually violent eruptions compared with other
Tertiary volcanic centres in Iceland. Large silicic
volcanoes with many smaller eruption sites instead
of a prominent cone are common in many volcanic
terrains, but the only known example in Iceland is
the Torfajökull volcano. However, these volcanoes
have large calderas, from several tens (e.g. Taupo
volcano in New Zealand) up to 100 km in diameter
(Lake Toba volcano in Indonesia) and are covered
and surrounded by much more voluminous ignim-
brite sheets than those encountered in the Borgar-
fjörður/Loðmundarfjörður area (Ninkovich and oth-
ers, 1978; Walker, 1980; Wilson, 1985). Basaltic
rocks occur only in minor fractions in such vol-
canoes, whereas they form the major part of the
country rock in the Borgarfjörður/Loðmundarfjörður
area.
Basaltic lavas, which are a part of the volcanoes,
or which have formed in their vicinity, often show
irregular dips due to downwarping of the bottom of
a volcano or due to the formation of calderas. Most
of the plateau basalts in this area, however, are flat-
lying or have shallow dips (3°-6°), even at sea level.
The strike of the basalts also varies from place to
place. This is different from the situation farther
south, where the basaltic lava flows generally dip to
the west or northwest, most clearly at sea level (8°-
10°).
78 JÖKULL, No. 39, 1989