Náttúrufræðingurinn

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Náttúrufræðingurinn - 1977, Qupperneq 13

Náttúrufræðingurinn - 1977, Qupperneq 13
rétt ummæli eins sjónarvottar nærri upphafi greinarinnar eru úr bók Guð- jóns Ármanns Eyjólfssonar: Vest- mannaeyjar, byggð og eldgos (ísafold- arprentsmiðja 1973). S U M M A R Y Tlie eruption fissures of the 1973 Heimaey eruption by Sigurdur Thorarinsson, Science Institute, University of Iceland. On January 23, 1973, a fissure opened up on Heimaey. This occurred between die tinies of 0140, when, locally, a rather strong earthquake shock was felt, and about 0155, when an eruption started. About 20 minutes later an almost con- tinuous wall of lava fountains had been formed on a 1.5 km long fissure running N20°E througli eastern Heimaey. Eight hours later the erupting fissure stretched some 120 m northwards into the sea. By 1400 hrs on January 24, narrow non- erupting fissures liad opened up between the southern end of tlie eruption fissure and the shore in Bót. That same day a short ca. 12 m high ridge was built up on the sea floor 0.9 km NNE of the continuous Heimaey fissure (cf. Eig. 1) and on February 0—7 submarine activity in the same area built up another small ridge and damaged an electric cable and one of two pipelines carrying fresh water from the mainland. Surprisingly, the second pipeline remained intact although situated between the two. At noon on January 26, a small eruption was observ- ed in the sea at Bót, some 300 m from the southern end of the fissure. Tlie Heimaey lissure system liad by then reach- etl a length of 3.5 km and the submarine activity of February 6 aclded very little to that length. On May 26, 1973, a month before the end of the Heimaey eruption, signs of submarine volcánic activity were observ- ed between Heimaey and the mainland only 4 km from the mainland. At 1400 hrs that day captain Bogi Finnbogason of the fishing vessel Ver VE 200, observed dead fish floating in a dark patch of sim- mering sea. His echo-sounder registcred wliat looked like a narrow column rising from the sea floor at 38—40 fathoms (ca. 70 m) depth to about 18 fathoms below sea level (Fig. 2). This column appeared to consist of some scattered material penetrable to a considerable deptli below the sea floor. It is suggested that the echogram re- gistered lumps of lava and/or rock frag- ments thrown upwards into the sea from an eruption fissure and escaping gases caused the surface to boil. This eruption was probably of very short duration and a Coast Guard vessel sounding the area on June 17, 1977, did not find any ridge on the sea floor in the area. The overall length of the fissure system formed during the Heimaey eruption thus appears to be about 10 km, and to- gether with the fissures tliat opened up cluring the Surtsey eruption 1963—1967 it forms a 35 km long lefl lateral en echélon fissure system. 7
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