Rit (Vísindafélag Íslendinga) - 01.07.1962, Side 191

Rit (Vísindafélag Íslendinga) - 01.07.1962, Side 191
189 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-
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Rit (Vísindafélag Íslendinga)

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