Rit (Vísindafélag Íslendinga) - 01.07.1962, Qupperneq 195

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detail, but the main conclusion seems clear that the material
was originally produced as fragmental material and mainly
hy explosive activity in basins.
The youngest Fragmental Rocks comprise such bodies as
the relatively fine-grained tuff of Sveifluháls formed by ex-
plosive activity in the Kleifarvatn graben; the fine-grained
tuff heaps Hrúðurkarlar formed by sub-aerial explosive ac-
tivity; and the flat tuff-breccia cones Hvalfell and Ok form-
ed in a similar way. These young masses demonstrate especial-
ly well the great explosivity of the eruptions, both in basins
and in such plases (Ok, Hvalfell) where sub-aquatic condi-
tions seem to be ruled out.
In addition we may mention breccia flows running down
the Sveifluháls ridge and down the slope of Bjarnarfell. Here
the explosivity was probably much less and instead a coarse
breccia may have been more or less directly extruded.
An example of post-glacial production of Fragmental Rocks,
Sandey, was mentioned above. Finally, the active volcanoes
Hekla, Katla, and Grímsvötn produce sideromelane tuff
(Hekla of an andesitic composition) and here it seems quite
clear that water entering the eruptive process at depth is the
fundamental cause.
Such masses that might have been formed sub-glacially
seem to be rather insignificant. The main possible cases are:
several small heaps in Hítardalur, Jarlhettur, Brekknafjöll,
and Bræðravirki. But in most cases a proof of sub-glacial ori-
gin is not evident.
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