Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 201

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Our conclusion is that through the entire geological history
of Iceland the share of acid and intermediate eruption pro-
ducts has been of the order of 10% of the total production,
whereof rhyolites 3 to 5%. This is certainly a higher per-
centage than found in most other, if not all the other plateau
basalt areas. It seems to me rather difficult to explain this
high percentage without assuming some assimilation of acid
material beneath the plateau basalts of Iceland.
The difference between Katla and Hekla would then be
that Katla is fed by a deep-seated layer of basalt magma like
our shield volcanoes and most of our fissures, whereas Hekla
is fed by a separate magma chamber at smaller depth, hav-
ing an acid roof which can be remelted. This might i.a. ex-
plain the sudden drop in the Si02 content of the Hekla
tephra that sometimes occurs during the initial phase of a
Hekla eruption.
REFERENCES
Van Remmelen, R. W. and M. G. Rutten, 1955: Tablemountains of north-
ern Iceland. E. J. Brill. Leiden.
Kjartansson, G., 1960: The Thjórsá lavas (Thjórsárhraun). In: On the
Geology and Geophysics of Iceland, 38—41. Int. Geol. Congr. Norden.
Thorarinsson, S., 1954: The tephra-fall from Hekla on March 29th 1947.
The Eruption of Hekla 1947—1948, 11,3: 1—68.
— 1967: The eruptions of Hekla in historical times. The Eruption of
Hekla 1947-1948, I: 1-170.
Tómasson, J., 1967: Mineralogical and petrographical characteristics of
Icelandic tephra. The Eruption of Hekla 1947—1948, I: 171—183.
Walker, G. P. L., 1959: Geology of the Reydarfjördur area, eastern Iceland.
Quart. J. Geol. Soc. London 114: 367—393.
— 1966: Acid volcanic rocks in Iceland. Bull. volc. Napoli XXIX: 375
-406.
DISCUSSION
Trausti Einarsson: Taking the Hekla reservoir 5 km long,
10 m wide and 25 km deep the volume is about 1 km3.
With the data given by Thorarinsson this means nine times
refilling of the reservoir. I doubt that assimilation would each
time repeat itself.