Jökull


Jökull - 01.12.1982, Side 68

Jökull - 01.12.1982, Side 68
Fig. 4. Material deposited by the Katla flow of 1918, resting on undisturbed soil at Núpar. MyndL 4. Setlögfrá Kótluhlaupinu 1918 ofan á óhreyfbum jarðvegi við Núpa. The material. The material transported by the Katla flows is enormous. During less than 20 hours in the 1918 flow the ocean coast was in parts extended some 4 km offshore and out to a depth of approx. 40 m. Similarly an enormous amount of volcanic debris mainly pumice was deposited on the sandur plain. In the area between Kaplagarðar and Hjörleifshöíði some 8 m thick pumice layer was added on the sandur plain, (Einarsson 1975). A good exposure of the material is found in Sandgil (Fig. 3). The coarser material was mainly deposited on the sandur at the margins of main flow channels parallel to ílow direction. Plastic clay has also been observed in the deposits presumably derived from high-temperature geothermal areas beneath the glacier. Material from the previous Katla flows is still retained in Austurjökull and Höfðabrekkujökull (Fig. 1) which seems to derive mainly from the flow of 1721 but partly from 1755. At the western margin of the “jökull“ there is ablockofalmost undisturbed soil more than 2 m thick and more than 90 m iong (Fig. 5). Other dimensions are not known. Pres- umably this was deposited in the sea which up to 1721 was only a short distance from Skiphellir, a cave where the fishing boats were kept up to that date. Skiphellir is approx. 2. 2 km from the present beach. It is interesting to note that the coarser material is conspicuously more abundant in the upper partofa section through this material and big boulders have only been found at the top of Höfðabrekkujökull. Most of the flow material, however, consists of vol- canic pumice, ca. 95%. Same is the case in the above mentioned Sandgil. The grain size of the pumice is dominantly, or 60-70%, 0.5-4.0 mm and material <0.074 mm is only 4-6% (Fig. 8). Judged from a great many analyses kindly put at my dis- posal by Mr. Gylfi Einarsson, geologist of the Tech- nological Institute of Iceland, similar material cov- ers wide areas of Mýrdalssandur. The pumice is highly porous and non-porous grains are almost lacking. Petrographically the material from the Katla area is transitional alkali basalt Jakobsson 1979). Fig. 4 shows a section through a part of the mat- erial deposited at the margin of the flow of 12th of October 1918 at Núpar. The flow material rests on undisturbed soil. Next to the soil, i. e. at the base of the flow is a pumice layer as was mentioned above. Coarser material, clasts, mostly consisting of bas- altic rock fragments are common in the upper part of the profile and several boulders up to 1 m in diameter are found at the top. This seems to indi- cate that the coarser material was mainly transported in the upper part of the flow, on top of finer grained material mainly consisting of pumice (Fig. 7). The stratification of the pumice, both in this profile and in Sandgil, seems to indicate a laminar movement dominated at the flow base. A hugeboulder, Kötluklettur, (Fig. 6) was carried approx. 15 km by the 1918 flow. I have earlier estimated it to be 1400 metric tons Jónsson 1980), but upon reestimation the value is 64 JÖKULL 32. ÁR
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