Náttúrufræðingurinn - 1978, Page 29

S U M M A R Y
Volcanic eruption through a deep
drillhole in the Krafla area, north
Iceland
by Axel Björnsson and Oddur Sigurdsson
National Energy Authority,
Laugavegur 116, Reykjavik, Iceland
The rifting episode which started in
1975 on the constructive plate boundary
in north Iceland is characterized by short
volcanic and tectonic active pulses at
intervals of a few months. During the
active pulses volcanic tremor, earthquake
swarms, widening of fissures and forma-
tion of new geothermal fields have been
observed. Tlnee times an active pufse has
been accompained by a volcanic erup-
tion.
Around 4 p.m. on September 8, 1977
an intense active pulse started in the
Krafla area. The floor of the caldera sub-
sided about 30 cm and an earthquake
swarm started inside the caldera, but the
activity spread southwards to the Náma-
fjall area within few hours. An east-
west widening of the lissure swarm of 1 m
near Námafjall was observed. A volcanic
eruption started at about 6 p.m. near tlie
north rim of the caldera. The volume of
the lava is estimated to be 2 • 10(i m3. At
about 11.30 p.m. another volcanic erup-
tion occured through a 1138 m deep drill-
hole in the geothermal area at Námafjall.
About 2500 kg of basaltic pumice were
erupted. The drillhole is located some 8
km south of the center of the caldera and
12 km from the place of the lava erup-
tion.
The active pulses in the area are inter-
preted as caused by east-west widening of
the fissure swarm and horizontal flow of
magma from a magma chamber below the
caldera towards north or south into the
fissure swarm. Both eruptions on Septem-
ber 8 support strongly this model of flow
of magrna. The drillhole eruption ocurr-
ed about 7 liours after the land deflation
started in the center of the caldera in-
dicating a velocity in the order of 1 km/
hour for the flow of the magma.
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