Jökull - 01.12.1970, Blaðsíða 77

tures caused by late movement in a cooling
lava flow, points out that they may also form
in other ways such as when “water from an
adjacent stream pours over the surface of a
lava flow before the lava completes its solidi-
fication”. Saemundsson (1967) adopting this
view explained small patches of irregular col-
umns within the top basalts of table mountains
as due to water ingressions creating irregular
cooling faces. Direct subaqueous origin of the
kubbaberg-type entablatures is favoured by Sig-
valdason (1968) who observed a sucession of
flows consisting of three divisions in the socle
of Herdubreid, Iceland’s largest table mountain.
The lowest division is composed of pillows,
the upper two of columnar basalt and an irre-
gularly jointed uppermost division. He inter-
prets these lava flows as subaquatic since they
occur at the base of a volcanic structure (i.e.
a table mountain) that is in general interpreted
as subaquatic. Similarly Walker and Blake
(1966) report a subglacially emplaced lava flow
from Eastern Iceland where the same type of
jointing occurs in a basalt layer forming the
basal portion of a pillow lava — hyaloclastite
mass.
The question whether the kubbaberg-type en-
tablatures were formed under water after flow-
ing into an aqueous environment or by water
from streams that poured on to a still hot lava
must be considered individually. Probably gen-
eralizations as to one or the other cause of
formation are not justified.
ORIGIN AND AGE
OF THE INTERGLACIAL
LAVA FLOWS
The eruptive source of the young lavas is
unknown and it may be different in case of
the olivine basalts and the tholeiites. The
source of the olivine basalts probably lies to
the east within the eastern limb of the active
volcanic zone. The olivine basalts may have
followed a similar path as did later the post-
glacial Thjórsá lavas (Kjartansson 1958). This
is indicated by the altitude of their base, which
diminishes towards the south and southeast.
Foreset bedded hyaloclastite breccias, occasion-
ally found at the base, dip toward the west or
southwest, indicating flow from the east or
northeast. The provenance of the tholeiites is
more problematic. Their source almost certain-
ly lies outside the boundaries of the map (Fig.
1) somewhere to the north or northeast. This
is judged again from the increase in altitude
of the base of the lavas towards the north.
The center of eruption may lie as far as the
active volcanic zone, but since a few volcanic
eruptions are known to have occurred within
the boundaries of the Flreppar Series as late
as the end of the last glaciation, they may have
a closer origin.
The younger age of the olivine basalts is
indicated by their less advanced erosion as well
as the generally much lower altitude of their
base and top levels. This implies more ad-
vanced denudation of the Hreppar Series in
the area covered by the olivine basalts and may
in part be due to a longer period of erosion
in this part of the area.
Fig. 8. Colonnade-entablature structure expos-
ed in a postglacial lava flow at Vígabergsfoss
in the river Jökulsá á Fjöllum. Photo: Bjarni
Sigurdsson.
Mynd 8. Kubbabergsmyndun i hrauni runnu
eftir isöld. Forvöð við Jökulsá á Fjöllum.
JÖKULL 20. ÁR 75