Rit (Vísindafélag Íslendinga) - 01.07.1962, Blaðsíða 191

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verse to the sub-glacial extrusion hypothesis and in general
1 have not found any direct evidence of a suh-glacial origin
of the breccias in the Young Plateau Basalts. On the other
hand it may be argued that perhaps swampy ground or lakes,
so typical for a newly deglaciated area, may sometimes have
been the extemal conditions under which the fragmental
rocks formed here.
But it was just by studying these rocks in my early work
that I was in many cases led to the conclusion that the glass
masses were not the immediate result of chilling by contact
with water. Very thick flows of extremely fine-grained basalt
with internal parts of sideromelane glass, often more or less
massive, are better understood as the result of high viscosity
of the original magma than as a result of rapid cooling. Also
the horizontal variations within such flows suggest breccia-
tion in a flowing mass; and in some cases it seemed likely
that brecciation had taken place during extrusion. Finally,
there is the rapid vertical altemation of massive lavas and
fragmental layers and the considerable amounts of tuffs of
fine grain. AU this leads me to think that external chilling
was not the only or even the main factor. The character of
the eruptions, sometimes high explosivity, sometimes viscous
magma must, it seems, be taken into account.
In the Young rocks in the Middle Northern highlands we
have found a rather gradual increase in the fragmental com-
ponent with increasing stratigraphic height, until predomi-
nance of the latter, seemingly mainly the result of explosive
emptions, is reached. Two interpretations seemed possible:
first, that the topmost layer was a final part of the Plateau
and that then the conditions were the end-stage of a gradual
development. Second, that the topmost layer was much
younger than the Plateau. Mainly the conditions farther
southwest, where the Plateau Basalts dip considerably (c.f.
Súlur area), support the latter alternative. We are in any
case confronted with a main and rather general layer of
tuff-breccias, several hundred metres thick and very exten-