Jökull - 01.12.1970, Blaðsíða 76

Fig. 7. Lava flow near the top of Galtafell
(no. 4 in Fig. 4) showing the sharp boundary
between a colonnade (base) and an entabla-
ture (top).
Mynd 7. Skörp skil milli stuðlabergs og kubba-
bergs í ungu hraunlagi i Gaítafelli.
paper). Joints progressively developing towards
the interior of a cooling flow from top and
bottom tend to form an irregular jointing
pattern where they meet. This may also result
in a sort of entablature. A true kubbaberg-
type entablature has however also been report-
ed from the Tertiary Volcanic Districts of Scot-
land and Ireland, the Island of Staffa being a
famous example. The same applies to the
Columbia River Plateau (Mackin 1961).
The rapid cooling indicated by the glassier
texture and great thickness of the entablature
of the Hreppar lavas strongly suggests aqueous
chilling. More conclusive evidence is provided
by entablatures observed in the slopes of Galta-
fell grading into pillow lava and hyaloclastites
(Fig. 4). As shown before the young lavas in
the Hreppar all solidified within valleys with
rivers flowing along them as indicated by the
gravel beds that are often found below the
lavas. These rivers may have been dammed up
temporarily by tlie lavas and then flooded the
valleys with the still partly fluid lavas on bott-
om. The lavas which possess an upper chillecl
entablature probably mark the channels of such
íloods. The rubble of the aa lava surface was
already solidified at the time of flooding. The
water flooding the lava therefore seems mainly
to have acclerated the cooling process and caus-
ed extreme thermal stresses within the uncon-
solidated portion of the lava, responsible for
the extremely hackly jointing.
The writer would like to mention two ex-
arnples of the formation of entablatures in
postglacial lava flows obviously caused by flood-
ing shortly after their emplacement both from
Jökulsá á Fjöllum in Northern Iceland (Fig. 8).
The eruptive fissures of Hljódaklettar and
Sveinar (Thorarinsson 1960) cut across the river
and have poured lavas into the river bed. Later
erosion of these lavas, first recognized as post-
glacial by H. Tómasson in 1967 (oral commun-
ication), revealed the typical twofold division
also present in the Hreppar lavas.
It is interesting in connection with this
discussion to point out similar processes actu-
ally observed during the Lakagígar eruption in
1783. Lava flows of this eruption dammed up
the rivers Skaftá and Hverfisfljót. Both were
drained out over the lava several months later
when a great volume of water had accumulated
behind the lava darns. (Eyewitness account of
the spectacular phenomena by Jón Steingrims-
son publ. 1907). It is uncertain whether text-
ures and structures similar to those of the
Hreppar lavas resulted there. However accord-
ing to Jónsson (oral communication) an en-
tablature was formed when some smaller rivers
after torrential rain for several days flooded
the still advancing lava just west of Kirkju-
baejarklaustur. As a result the progress of the
lava was stopped. The entablature is revealed
in sections along the Skaftá at Eldmessutangi.
The theory of aqueous chilling causing the
formation of entablatures is by no means a
new one. Waters (1960), in cliscussing entabla-
74 JÖKULL 20. ÁR