Náttúrufræðingurinn - 1968, Blaðsíða 69

NÁTTÚRUFRÆÐINGURINN
in 1908, P. Hannesson in 1937, G. Kjartansson in 1956 and Jón Jónsson several
times), it has not yet been studied in detail as no scientist has hithcrto spent
niore than a few days there.
A detailed and reliable eyewitness account of tlie Lakagígar eruption was
written by the Reverend Jón Steingrímsson of Prestbakki. The course of events
of the eruption outlined here on pp. 32—37 is an extract of that account.
The eruption, which was preceded by earthquakes lasting a week, began on
Wliit-Sunday, June 8, and lasted until early February the following year. Until
July 29 only the crater row SW of the móberg mountain Laki (cf. the map Fig. 2)
was active and the lava flowed down the riverbed and canyon of Skaftá. july 29
the fissure NE of Laki opened up and frorn tlien on the eruption was confined
mainly to that fissure ancl lava flowed for the main part along the riverbed of
Hverfisfljót.
The map of the lava flow here presented (Fig. 2) is based on G. Kjartansson’s
geological map of South Iceland (Sheet 6, scale 1:250 000), but somewhat cor-
rected with the aid of aerial photos. The lines showing the front of the lava at
different times are based on Rev. Jón Steingrhnsson’s account. l’he more de-
tailed map (Fig. 5) of Laki and adjacent parts of Lakagígar is based on aerial
])hotos and on the author’s observations during the two days’ stay at Laki in early
September 1967.
According to Þ. Thoroddsen the area of the Lakagígar lava flow is 565 km2
and tliis figure has proved correct. His estimate of the volume of lava, 12.3 km3
is also the best one yet available and a reasonable one. Approximately 9.5 km3
of this lava were exlrucled from the fissure SW of Laki. But Thoroddsen’s
estimate, 3 km3 , for the volume of the tephra expelled, is in the opinion of the
present writer far too high. The area within which the 1783 tephra can be
traced by field observations alone (cf. Fig. 2) is about 8000 km2 l’lie present
writer’s estimate of the volume of the 1783 tephra as fresh fallen is 0.85km3,
corresponding to about 0.25 km3 of dense rock. This tephra was mainly pro-
duced by extremely powerful lava fountains. Production of Pélé’s hair was very
great during this eruption.
Tlie 1783-lava covered completely 2 churches and 14 farmsteads, ancl 30
farms in addition were badly damaged. But far ntore disastrous was the
stunting of the grass growth all over Iceland during the summer of 1783. Tliis
was only to a small extent caused by the tephra fall. Its main cause was the
bluisli haze which lay all over the country and was felt over most part of
Europe and adjacent parts of Asia and Alrica. This haze was caused by the
enormous amount of volcanic gases released from the lava extruded. Thc gas
most effective in stunting thc grass growth was in all probability SCU. Given
the same SO2 percent in tlie Laki magma as in the basaltic Surtsey magma
the total amount of SO2 released from the Laki magma is about 107 tons.
The result of the stunting of the grass crop was a catastrophic decimation
of the livestock in the country (50% of the cattle, 79% of the sheep, and
76% of the horses). This in turn caused the worst famine in the history of Ice-
land, decimating the population from 48 884 in 1783 to 38 363 in 1786, a deci-
mation of 24%. The famine was adequately called the Haze Famine.