Náttúrufræðingurinn - 1968, Side 70

56
NÁTTÚRUFRÆÐINGURINN
The average production of lava from Lakagígar during the first 50 days of
the eruption (June 8 to July 29), when only the fissure SW of Laki was active,
was about 2200 m3/sec of solidified lava, bulkweight about 2.4, corresponding
to a ílow of about 5000 m3/sec from the fissure. The maximum rate of ílow
was in all probability many times greater. Compared with such enormous lava
beds as the biggest Tertiary ones in the Washington plateau the Lakagígar
lava flow is not very big, yet the productivity of the Lakagígar eruption during
its first days was probably large enough to form a lava bed the size og the Rosa
flow in two to three years.
The thermal energy released by the Lakagígar eruption is about 3xl0-7 erg.
The chemical composition of the Lakagígar tephra and lava is shown in
table 2. The lava is tholeiitic, poor in olivine, witli approximately equal amounts
of plagioclase (Anoo—Anor in groundmass) and clinopyroxene.
I he length of the Lakagígar crater row proper, viz. the crater row built up
by the 1783 eruption, is about 25 km. Further northeast, nearly on the same
Iine, is an older crater row, and it has not been decided with certainty where
the Lakagígar proper ends and the older crater row begins. There is some
support for the assumption that the older crater row erupted in the last half of
the 13th century, thereby changing the water divide bctween the source rivers
of the Skaftá and Flverfisfljót rivers and increasing greatly the discharge of
Hverfisfljót.
The mountain Laki divides the Lakagigar crater row in two parts. The SW
on is 0.4 km shorter.
The number of individual craters in Lakagígar cannot be given with any
exactness as the craters merge and cut into each other in such a way that it is
really impossible to tell what is an individual crater. The vents visible on aerial
photos are about 115 and some of them are real miniature ones, whereas the
highest crater in the row (B on Fig. 5) is about 90 m high, and many craters
are 40 to 70 m high. The crater row is a mixed one and the diversity of the crat-
ers is striking. Some of the craters are built up entirely of lava lumps (Schweiss-
shlacken), otliers are fountain traters and at least two are built up of well
stratilied tephra (cf. crater G on Figs. 5 and V a ancl b), and probable the result
of a phreatic explosive activity.
The Lakagígar crater row is, at least partly, within a narrow graben, formcd
in all probability during the eruption. The fault-scarps show a maximum down-
throw of 6 to 8 metres. They cut through the móberg mountain Laki, but that
mountain was not quite split by the eruption fissure itself. The fissure cuts into
its SW and NE flanks (cf. III a and III b), but only a small amount of lava
has been extruded from small vents on its NW slope. Gonsequently the name
Laki eruption, often used for the 1783 eruption, is somewliat misleading and
Lakagígar eruption is a more adequate name.
RITSKRÁ (REFERENCES)
Anderson, T. 1903. Volcanic Studies in Many Lands. Lonclon.
Briern, 0. 1959. Utilegumenn og auðar tóftir. Reykjavík.