Náttúrufræðingurinn - 1966, Side 64

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NÁTTÚ RU FRÆÐINGURl NN
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 tlie 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 of 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 if 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 sliowed large anoma-
lies caused by the lava which then covered the southern part of the islantl. 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 lielicopter 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 of December 1963. A steadily increasing magnetization of the
lava at the crater due to cooling is reflected in fig. 4, showing the difference in
magnetic field intensity at a place 200 m NE of the crater and at the magnetic
observatory near Reykjavik.
The temperature of volcanic material ejectecl during tlie 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 liow much water is contained in the magma. The deuterium
content of the magmatic water is 5—6% lower than in seawater.