Náttúrufræðingurinn

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Náttúrufræðingurinn - 1988, Síða 22

Náttúrufræðingurinn - 1988, Síða 22
svenska Vetenskaps-Akademie handl- ingar, band 7 no. 1. Sigurður Þórarinsson. 1951. Laxárgljúfur and Laxárhraun. A Tephrocronological Stu- dy. - Museum of Natural History, Mis- cellaneous papers 2. reprint from Geogra- fiska Annaler H. 1-2 1951. Simkin, T., L. Siebert, L. McCelland, D. Bridge, C. Newhall & J. H. Latter. 1981. Volcanoes of the World. - Hutchinson Ross Publishing Company, Stroudsburg, Pennsylvania. 233 s. Sveinn P. Jakobsson. 1979. Petrology of Recent basalts of the Eastern Volcanic Zone, Iceland. - Acta Naturalia Islandica 26,103 s. Þorleifur Einarsson. 1968. Jarðfræði, saga bergs og lands. - Mál og menning, Reykjavfk, 335 s. Þorleifur Einarsson & Edda Lilja Sveinsdótt- ir. 1984. Nýtt kort af Skaftáreldahrauni. í „Skaftáreldar 1784-1784. Ritgerðir og heimildir". - Mál og menning, Reykjavík 1984, s. 37-48. Þorvaldur Thoroddsen 1911. Lýsing íslands, 2. bindi. Khöfn. SUMMARY The Þjórsá lava - the largest Holocene lava flow on Earth by Árni Hjartarson Orkustofnun (National Energy Authority) Grensásvegur 9 Reykjavík, Iceland The Þjórsá lava is the most voluminous lava in Iceland and the largest Holocene lava flow on Earth, both with respect to volume and area. It was erupted from a crater row in the Veiði- vötn volcanic area in Central Iceland. The craters and the uppermost parts of the lava seem to be mostly buried in younger erup- tives, lavas and tephra. Some 75 km south- west of the volcanic site the lava crops out and covers an extensive area between the glacial rivers Þjórsá and Hvítá/Ölfusá. These two, the largest rivers of Iceland, have found their way along the eastern and western edges of the lava all the way to the sea. The lava forms the reef-bound coast between the river mouths and stretches several hundred meters out from the shore below sea level. The sea level seems to have been 5-10 m lower at the time of the eruption than it is at present. The total length of the lava is at least 140 km, making it the longest flow in Iceland. Its thickness in the farming areas of South Ice- land is fairly well known, being 15-20 m thick in its distal parts in the Flói district. In Skeið further north it is 20-25 m thick and in Land the thickness is about 30 m. The average thickness calculated from 40 boreholes along most of its length is 22 m. The lava is estimat- ed to be about 950 km2 in area. The total volume is therefore 21 km3. All are minimal numbers. The Þjórsá lava is plagioclase porphyritic tholeiite with 2-20% by volume of plagio- clase macrophenocrysts. Phenocrysts of oli- vine are also found and, more sparsely, au- gites. The distribution of the phenocrysts is rather even, but in a few places plagioclase- rich bands are found. This phenomenon can be explained by assuming that the amount of phenocrysts in the lava did differ from one crater to another along the fissure. No ash layer is known to have accompanied this eruption, but in deep ice cores from Greenland there are indications of a violent eruption somewhere in the northern he- misphere around the date 6675±150 BC. A new radiocarbon dating of the lava gives the age of 7800±60 conventional radiocarbon years. These two individual age measure- ments fit fairly well and when they are con- verted into calendar years the outcome from both of them is close to 8600 B. P. 16

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