Náttúrufræðingurinn - 1970, Page 37

NÁTTÚRUFRÆÐ ÍNGURINN
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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.