Náttúrufræðingurinn

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Náttúrufræðingurinn - 01.02.1966, Blaðsíða 32

Náttúrufræðingurinn - 01.02.1966, Blaðsíða 32
210 NATTÚRUFRÆÐINGURINN formed when the molten lava entered the sea. The results are given in the references (Anderson et al. 1965) and (Bjornsson et al. 1966). Earthquakes originating at Surtsey were detected at seismic stations 112 and 140 km from the volcano and by a field station which was operated for 3 months in Vestmannaeyjar 20 km from the volcano. The formation of a new crater seems to be accompanied by a series of earthquakes lasting a few weeks. However, hardly any earthquakes were detected at the beginning o£ the Surtsey eruption. Long-lasting earth tremors were also detected at these stations, especi- ally during the explosive phases of the eruption. It is not known i£ these tremors were caused by explosions in the crater or perhaps by the lavaflow under ground. The period is 1—2 seconds. An aeromagnetic survey of a large area surrounding Surtsey was carried out in February 1964 by the Office of Naval Research in Washington. At the same time an expedition from the University of London made a magnetic survey from a boat in a limited area southwest of Surtsey. All these measurements showed a very homogenous magnetic field in the Surtsey area and revealed no influence from the eruption. In the summer 1964 magnetic measurements in Surtsey showed large anoma- lies caused by the lava which then covered the southern part of the islanti. The pumice in the northern part only shows a very veak magnetziation. This is reflected in fig. 3 which shows one of many magnetic profiles measured with a helicopter in the summer 1965. Fig. 2 shows a magnetic profile across a sub- marine formation from an eruption which only barely reached the surface of the sea at the end o£ December 1963. A steadily increasing magnetization of the lava at the crater due to cooling is reflected in fig. 4, showing the difference in magnetíc field íntensity at a place 200 m NE of the crater and at the magnetic observatory near Reykjavik. The temperature of volcanic material ejected during the early stages of the eruption was, according to optical pyrometer measurements, 650—700°C. Later thermocouple measurements gave temperatures as high as 1190°C at the edge of the advancing lava. This high temperature has not been fully accepted as real as most of the time the temperature at the lavafront turned out to be about 1140°C. A single temperature measurement in the crater gave 1150—1160°C. Determinations of viscosity in the crater and in the advancing lava gave a co- efficient of viscosity of about 104 poise. The electric resistivity was found to by about 4 ohmmeters. Radon concentration in the volcanic gasses is about 100 pC per liter. When radium concentration of the lava has been measured this result will tell how much gas and how much water is contained in the magma. The deuterium content of the magmatic water is 5—6% lower than in seawater.

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