Jökull


Jökull - 01.12.1952, Blaðsíða 17

Jökull - 01.12.1952, Blaðsíða 17
layers were separated from the first ash by clay bands. Identification of the volcanic ashes was made by determination of colour, refractive index (in sodium light), and by measurement of grain size. Thorarinsson also used silica content as a feature in identification, but this was not possible here, owing to the prohibitive cost of analysis. However, Thorarinsson stated that identification was usually possible without the use of silica content. The results obtained were sufficient to iden- tify the three ashes with three of the Hagavatn soil profile. (The data obtained concerning the various ash analyses are present in detail in Thorarinsson’s „Tefrokronologiska studier pá Island”, 1944). The data obtained were as follows: — Ash No. Particle diameter. T >2.0 mm. 2 Very fine. < 0.2 m 3 > 2.0 mm. The ashes 1, 2 and 3, ashes Nos. III, IV and V of the soil profile measured by Thorarinsson below Leynifoss (cf. Fig. 1). These ashes were from the erupt- ions of Hekla 1693, Katla 1721, and Hekla 1766, respectively. An examination of the glacial materials along tlie moraines forming the southern shore of the present lake, showed that there was much var- ved sediment present. This had been thrust up from the bed of the lake during an advance of the ice. Among these disturbed sediments was a section which contained a rhyolitic and a basic ash, both within the thickness of a single varve. The sediments in which this ash was found approached closer to the true varve, and showed a gradation between winter and summer clays, with no minor laminations as were found in the clays of the western basin. The complete section containing these ashes was as follow: Refractive Index. 1.606 1.608 1.618 1.619 1.555 1.557 Colour. Black Black Black are identified with Fig.5. Part of the gully section near the southern margin of the lake bed. Note thicker sand intercalating between winter layers, showing slumped bedding effects. Hvörf í botnseti nœrri suðurströnd Hagavatns (þeim hluta, sem nú er þurr.) One varve 2i/2" (6.2 crn) Grey (laminated) clays. ' Yellow-fawn clay. Dark grey ash, slightly decomposed. c. Grey clay. Dark brown ash b. Grey clay. Grey laminated clay. White volcanic ash. a. Fawn clay. No. Particle Refractive Ash diameter Index. Colour. c. 0.2 mm. diameter 1.598 Dark grey (originally b. 0.2 mm. diameter f 1.589 black). Dark J 1.499 brown. a. 0.2—2.0 mm. diam. 1.498 White. The layer „c“ can be identified with layer Vlla, and the layer „a“ with Vllb. These are two ashes of a profile characteristic over much of South Iceland, which was pre-colonization in age. It can also be noted that both eruptions occurred in the same year, the basic layer being

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Jökull

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