Acta naturalia Islandica - 01.02.1946, Síða 76

70
TRAUSTI EINARSSON
very swollen during the summer ancl he had not the impression in
the summer 1934 that the river was so unusually swollen as to
warrant the assumption of an eruption in the glacier at its source.
I believe that so long as no attempt has been made to parallelize the
flood of the river with a cold weather period it has little bearing
upon a supposed sub-glacial eruption.
Another fact that should be taken into account is, that if at the
source of the river the extrusion of the loose masses of a palagonite
breccia took place accompanied by the production of a great body
of melt water, we might expect that the river carried away and
deposited along its banks a perceptible amount of these eruptives.
But nothing of the kind has been observed.
As to the subsidence itself, it extends, according to Dr. Nielsen, in
his book Vatnajökull, over a zone 10 km long and a couple of km
broad. It lies along the very active volcanic line of Grímsvötn and it
occurred about 2 months after the great eruption of Grímsvötn in
1934. It might therefore be supposed that this disturbance of the
glacier was in some way connected with the great eruption, without
necessarily being caused by a sub-glacial extrusion, and when we
learn from Nielsen’s book that the disturbance extended right down
to the margin of the glacier where nevertheless no signs of an
eruption were seen, it would seem that the sub-glacial extrusion
hypothesis is inadequate. On the other hand it seems a more simple
explanation that earthquakes caused a slight run-off of this part of
the glacier which lies on a sloping ground. By thorough studies it
might have been possible to decide upon the cause of the disturbance,
but unfortunately observations are scant and we are left to guess
the best hypothesis.
This disturbance of the glacier is at any rate no proof of a “slow”
subglacial extrusion and it can still less be used as a basis for a
theory according to which the basic tuffs of Iceland are largely
formed by “slow” sub-glacial extrusion.
Noe-Nygaard reports that he found the products of recent sub-
glacial eruptions, i. e. globular basalt. But his deduction of this
result is not convincing. First he shows clearly that in earlier post-
glacial times the Vatnajökull was smaller than to-day. Thereupon he
states that a globular basalt, which he assumes to be of recent age
and lies outside the present margin of the glacier is of sub-glacial
origin. Then we learn that “following the globular basalt in a