Náttúrufræðingurinn - 1962, Blaðsíða 28

18
NÁTTÚRUFRÆÐINGURINN
The Askja-Eruption 1961.
On Oct. 6th after tveak earthquakes a N-S-striking hyclrothermal íissure was
opened in Askja south of Öskjuop. The solfatara-activity was observed on the
10th. After the middle of Oct a mud volcano became active. On 19th a great
steam eruption was observed in this mud volcano. Thc eruption threw out
mud and stones over 100 m in the air.
After earthquakes 10h 56m on 26th Oct. a lava eruption started on a 750 m
long fissure in Askja at Öskjuop. The fissure lias an E-W-strike and presuma-
bly follows the northern ring-faults of the Askja-caldera. This is the only
exception from the dominating N-S volcanic lines in Northern-Iceland. Only
in the peninsula Snæfellsnes in Western-Iceland are volcanic-fissures common
with this direction. It is very remarkable too, that the hydrothermal and
volcanic fissures lie at right angles.
The volcanic cloud rose to ca. 10.000 m in the first day and on the second
day to ca. 2000 m.
The eruption showed a typical lava fountain-activity. After tlie first hours
only the easternmost half of thc fissure was active. On the first day the lava
fountains reahed over 300 m height, on the third day 150—200 m and the fourth
75—100 m. After a week they rose only 50 m higli. The lava fountains did
not always rise straight into tlie air, but often they shot in various directions
over the crater rims. The rhythm of the fountainplaying was about 30 every
minute. Tlie lava fountains were affected very much by wind, and according
to the northernly wind in the first days of the eruption, the southernly crater
rims were built up much higher than the northernly ones. The crater rims
whicli consisted of „schweiss-schlacken“ glided slowly away from the fissure.
The first two weeks, two lava rivers poured out from the fissure, and after
Nov. 6th only one was observed.
In the first 24 hours the lava reached a length of 7—8 km, which gives
an average speed of 5 m/min. The mean value for the lava production during
the same time was in the order of 700—800 m3/sec. On the second day of the
eruption the lava front at 8 km distance from the fissure moved 1 m/min.
A week later the lava front did not move at all.
After Nov. Gth the viscosity of the lava decreased considerably and it consoli-
dated as pahoehoe lava, in contrast to the previous aa type. The eruption
finished before Dec. 15th.
A remarkable phenomenon was that the glowing lava did not melt the
snow in front of it. Hours or days later one saw steam rising from the lava-
field.
The new Askja-lava, Vikrahraun, has a length of 9 km and covers 11 km2.
The volume is about 100 mill. m3 (0.1 km3).
The lava is a basalt with glassy groundmass and small plienocrysts of basic
plagioclase and pyroxen. The SiÖ2-content is close to 51%.
The investigations liave mainly been carried out by the geologists Trausti
Einarsson, Jón Jónsson, Guðmundur E. Sigvaldason, Sigurður Thórarinsson,
Tómas Tryggvason and the author and will be continued in next summer.