Náttúrufræðingurinn

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Náttúrufræðingurinn - 1969, Side 94

Náttúrufræðingurinn - 1969, Side 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).
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