Náttúrufræðingurinn

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Náttúrufræðingurinn - 1968, Qupperneq 70

Náttúrufræðingurinn - 1968, Qupperneq 70
56 NÁTTÚRUFRÆÐINGURINN The average production of lava from Lakagígar during the first 50 days of the eruption (June 8 to July 29), when only the fissure SW of Laki was active, was about 2200 m3/sec of solidified lava, bulkweight about 2.4, corresponding to a ílow of about 5000 m3/sec from the fissure. The maximum rate of ílow was in all probability many times greater. Compared with such enormous lava beds as the biggest Tertiary ones in the Washington plateau the Lakagígar lava flow is not very big, yet the productivity of the Lakagígar eruption during its first days was probably large enough to form a lava bed the size og the Rosa flow in two to three years. The thermal energy released by the Lakagígar eruption is about 3xl0-7 erg. The chemical composition of the Lakagígar tephra and lava is shown in table 2. The lava is tholeiitic, poor in olivine, witli approximately equal amounts of plagioclase (Anoo—Anor in groundmass) and clinopyroxene. I he length of the Lakagígar crater row proper, viz. the crater row built up by the 1783 eruption, is about 25 km. Further northeast, nearly on the same Iine, is an older crater row, and it has not been decided with certainty where the Lakagígar proper ends and the older crater row begins. There is some support for the assumption that the older crater row erupted in the last half of the 13th century, thereby changing the water divide bctween the source rivers of the Skaftá and Flverfisfljót rivers and increasing greatly the discharge of Hverfisfljót. The mountain Laki divides the Lakagigar crater row in two parts. The SW on is 0.4 km shorter. The number of individual craters in Lakagígar cannot be given with any exactness as the craters merge and cut into each other in such a way that it is really impossible to tell what is an individual crater. The vents visible on aerial photos are about 115 and some of them are real miniature ones, whereas the highest crater in the row (B on Fig. 5) is about 90 m high, and many craters are 40 to 70 m high. The crater row is a mixed one and the diversity of the crat- ers is striking. Some of the craters are built up entirely of lava lumps (Schweiss- shlacken), otliers are fountain traters and at least two are built up of well stratilied tephra (cf. crater G on Figs. 5 and V a ancl b), and probable the result of a phreatic explosive activity. The Lakagígar crater row is, at least partly, within a narrow graben, formcd in all probability during the eruption. The fault-scarps show a maximum down- throw of 6 to 8 metres. They cut through the móberg mountain Laki, but that mountain was not quite split by the eruption fissure itself. The fissure cuts into its SW and NE flanks (cf. III a and III b), but only a small amount of lava has been extruded from small vents on its NW slope. Gonsequently the name Laki eruption, often used for the 1783 eruption, is somewliat misleading and Lakagígar eruption is a more adequate name. RITSKRÁ (REFERENCES) Anderson, T. 1903. Volcanic Studies in Many Lands. Lonclon. Briern, 0. 1959. Utilegumenn og auðar tóftir. Reykjavík.
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