Náttúrufræðingurinn - 1964, Page 44

38
NÁTTÚ RU FRÆÐINGURINN
feíl. With the exception o£ Bláfell, which is considerably dissected, the table-
mountains are isolated plateaux, roughly circular in form, with flat or gently
convex tops and abrupt sides. They are mostly built up of móberg (or pala-
gonite, i. e. vitreous, clastic rocks of basaltic composition) and pillow-lavas. But
the tops of these mountains are capped by lavaflows of crystalline basalt.
According to a hypothesis on the origin of tablemountains first advanced by
the author in 1943, the socle of móberg and pillows was formed by subglacial
eruption piling up this material in water-filled vaults melted into the ice-slieet
from beneath. Later on, as the ice had melted through and the accumulated
material reached above the water, the eruption became subaerial and changed
its character, producing the normal lavaflows which are now seen to cap the
tablemountains and in some cases form vcritable shieldvolcanoes on the top
of them (e. g. Hrútfell). During the whole process tlie erupted material was
moulded within the walls of ice rnore or less into the present shape of the
mountains.
The volcano Leggjabrjótur is in every respect a typical shielclvolcano —
except where it meets low ground on the south and southeast sicle. There the
lava ends in an abrupt scarp, up to 300 m. high, facing the lake Hvítárvatn and
the valley Fródárdalur. This scarp was interpreted by Niels Nielsen (1927) as
a tectonic fault. And as the flat or gently sloping lava plateau above the scarp
was obviously never covered by ice, he assumecl a postglacial age for the vol-
cano as well as for the fault.
The author’s research led to a different interpretation: The irregular sinuous
form of the scarp of Leggjabrjótur is not indicative of a fault line. On the other
hand it coincides exactly with the border of the unglaciated lava. The scarp is
striped horizontally with a serics of shore-lines up to a lieight exceeding 100
m. above its base (Fig. 5). On the top of the scarp the lava plateau has
broken up, tilted, and subsided in many places near thc eclge (Fig. 6.) But below
the scarp there are no signs of fracturing ordislocationof any kindof thebedrock.
The edge of the scarp shows lava structure quite normal for shicldvolcanoes,
but down on the scree-covered slope only pillowlava and sonte móberg crop out.
In the authour’s interpretation the scarp of Leggjabrjótur is the front of a
lava flow, owing its extraordinary height and anomalous rock structures to
ponding of the lava against a thick ice-margin, possibly floating in deep water.
In other words, the origin ol' Leggjabrjótur is analogous to that of the table-
mountains as outlined above. Tlie only difference is tbat during their formation
the typical tablemountains were moulded by thicker ice on all sides, and most
of them were afterwards covered by the regenerating ice-sheet.
As the highest ice-marginal shore-line (630 m. a. s. I.) in Kjölur is not found
on Leggjabrjótur it ntay be assumed that this volcano first came into existence
after the ice-sheet to the south ol' Kjölur had thinned sufficiently to allow a
southward draining of the area. According to our present knowledge of stages
in tlie deglaciation of Iceland (cf. References) this development must have
taken place near the end of the Budi stage, whicli in all probability was syn-
chronous with the Salpausselká — Ra stage of Fennoscandia.