Náttúrufræðingurinn - 1969, Blaðsíða 94

198
NÁTTÚRU F RÆÐINGURINN
Mér virðist allt mæla með því, að vikur þessi sé kominn frá
neðansjávargosi, sem orðið hefur á þessum tíma, sennilega á Yest-
mannaeyjasvæðinu og líklega utarlega á því. Vel gæti verið um að
ræða sprengigos, sem varað hefði aðeins stuttan tíma.
Ekki virðist líklegt, að vikur geti myndast við gos, sem á sér stað
á miklu dýpi. Þar með eru gosstöðvunum sett nokkur takmörk hvað
snertir fjarlægð frá landi.
HEIMILDARIT - REFERENCES
Lirtdal, J. (1964): Með huga og hamri. Reykjavik.
Moorhouse, W. W. (1959): Tlie Study of Rocks in Thin Section. New York.
Tröger, W. E. (1959): Optische Bestimmung der gesteinsbildenden Minende.
Stuttgart.
SUMMARY
Drift of pumice at Grindavík
by Jón Jónsson, National Energy Authority, Reykjavík
Late in the winter 1940 large quantities of floating pumice were found on
the sea, south of Grindavik, a fishing village on the south coast of the Reykja-
nes peninsula, Iceland.
It appears that this lasted only for a few days. About a year later, the pumice
was flung upon the shore in a south easterly storm and heavy sea. The pumice
is still found there. Some samples of it were taken for examination. The pieces
of pumice are of two kinds, black and brown. Both are of similar composition.
Most of the pieces are rather big (fig. 1) and extremely porous.
Under microscope the pumice is lound to consist of brown glass with a
refractive index of n 1,522—1,525 and a SÍO2 content of 64% according to
chemical determination, i.e. the pumice is of andesitic composition. Some
very small crystals of felspar (andesine?) are found in the glass, but they
are to small for optical determination. Quite a lot of small spherulites are in the
glass (fig. 2 and 3). They must have formed simultaneously with the pumice
itself, because they are often cut by vesicles lormed by gases in the liquid
glass. So far the pumice has been found only on the south coast of the Reykja-
nes peninsula.
Author’s conclusion is that the purnice derives from a submarine explosive
eruption, most likely somewhere in the area southwest of the Vestman Islands
(Vestmannaeyjar).