Náttúrufræðingurinn

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Náttúrufræðingurinn - 1970, Side 37

Náttúrufræðingurinn - 1970, Side 37
NÁTTÚRUFRÆÐ ÍNGURINN 155 Wnlker, G. P. L. 1962. Tertiary welded tuffs in Eastern Iceland. Quart. Journ. Geol. Soc. London 118: 273—293. — 1965. Some relationships resulting front the intimate association of acid and basic magmas. Quart. Jour. Geol. Soc. London 121: 31—49. — 1966. Acid volcanic rocks in Iceland. Bull. Volc. Napoli. 29: 375—406. S U M M A R Y A Pleistocene ignimbrite in Thórsmörk by Sigurdur Thorarinsson, Science Institute, Division of Geosciences, University of Iceland, Reykjavik. The writer gives a preliminary description of a tephra flow deposit in Thórs- mörk, South Iceland, and adjacent areas, discovered by him in 1961 but so far not studied in any detail. The best outcrops of the deposit in Thórsmörk are on a 4 to 5 km long stretch along the northern border of the sandur of the Krossá river, from Tröllabúdir (colour photo) eastwards. On this stretch the visible thickness of the deposit is up to about 30 nr (Fig. II a). The base of the deposit is nowhere visible. The upper part of the deposit is a nonwelded mixture of grayish ash and light puntice, refraction index 1.509 and 1.505 respectively, and black pumice, refraction index 1.557 (Fig. IV a). The hybrid character of the layer is the result of a simultaneous eruption íronr two types of magma. The main part is, however, rh.yolitic. The nonwelded layer gradually enrerges downwards into a typically welded layer (Fig. II a, IV b, V a) whiclr is also mixed. As to nomenclature the present writer finds it logical and fitting witlr the literal meaning of Marshall’s term ignimbrite to use it, like Walker (1966) has done, as a collective term for all tephra ílow deposits whetlrer welded or not, and as teplrra is a collective ternr for pyroclastic ejecta, tephra-flow deposit is preferable to pyroclastic-flow deposit or ash-flow deposit. The Thórsmörk ignimbrite originates in Tindfjallajökull, an Upper Plei- stocene, highly differentiated central volcano, rich in rhyolite and now partly ice covered. The volcano has a snmmit caldera which probably is a collapse caldera connected with the formation of the Thórsmörk ignimbrite. Eruptions along the margin of the caldera have continued into postglacial time. The area within which outcrops of the ignimbrite ltave already been foúnd (Fig. 6) is about 80 km2. The average thickness within this area is estimated at 20 to 25 m and the corresponding volume thus at 1.5 to 2.0 km3. No doubt the ignimbrite has spread over wider area and its total volume as fresh-fallen has probably been 2 to 3 km3. It is the same order of si/e as that of the Tertiary ignimbrites in East Iceland and the biggest rhyolitic tephra layers formed by eruptions in Iceland in postglacial and historical timés (H4, H:1, Ö 1362). The eruption which formed the Thórsmörk ignimbrite probably took place during the Mindel-Riss Interglacial.
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Náttúrufræðingurinn

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3.-4. Tölublað (1970)

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