Náttúrufræðingurinn - 1994, Síða 52

■ SUMMARY
On the Laki eruptions
Preliminary outline
The biggest lava-eruption known in history oc-
curred the year 1783 in the district of Skafta-
fellssýsla southem Iceland. The volcano is a 27
km long fissure running along a 200-260 m
wide graben divided in two approximately
equally long segments to the SW and NE of the
Laki mountain.
HlSTORY OF INVESTIGATIONS
The first person to visit the craters, to make a
preliminary description and a map of the vol-
cano was the Icelandic naturalist and physician
Sveinn Pálsson in the year 1793. However, his
work was not published until 1945.
The fírst geologically educated man to visit
the crater-row was Amund Helland (1886). He
stayed there for 3 days in 1881, made a map of
the craters and mesured the altitude of several
of them. He is the first to use the name Laki
craters and also to realize the existence of
some old craters in the row of 1783.
Thoroddsen (1894) investigated the whole area
during 10 days in 1893, made a geological map
and visited part of the crater-row and con-
firmed the observations of Helland concerning
volcanic activity before the last emption.
Sapper (1908) is without doubt the most ex-
perienced volcanologist ever to visit the Laki
crater. He stayed there for 3 days in 1906 and
has the honor of having made, up to now, the
most detailed map of approx. 12 km long cen-
tral part of the crater-row and the Laki moun-
tain. Reck (1910) also stayed at the craters for
3 days and later published some of his observa-
tions in his doctoral thesis 1910. He is in fact
the last one to deal seriously with investiga-
tions of the craters especially.
It is clear from the review of the research
history up to 1985, a total of less than 20 days
have been devoted to special investigations of
the Laki craters.
New investigations
During parts of the summer months 1990-1993
the author of this paper has been working at
geological investigations and mapping along
the whole crater-row as well as the surrounding
lava flows. It is now possible to prove that in
Postglacial times, 3 big eruptions have oc-
curred on the same eruptive fissure, each of
them showing a caracteristic form of eruptive
material. The first eruption has produced a row
of relatively low craters built up of exclusively
black, mixed, frequently lapilli-sized pyro-
clastic material. In places, debris flows have
been noted supposedly derived from gravita-
tional collapse of an eruption column. Pisolites
are not infrequent and a great number of
xenoliths is characteristic for these craters as
well as the fact that they are practically with-
out vegetation. The gray moss (Racumitrium)
so typical for the lava fields and most of the
craters is absent from the black craters.
The first eruption is presumed to have oc-
curred in lakes and shortly after the
deglaciation of the area. Only one big crater of
this type, provisionally named Stóri-Svartur
(Big-Black) is in the westem part of the erup-
tive físsure about 3 km west of Laki.
The second eruption, which is perhaps the
biggest of them all, is characterized by huge
craters built up of mostly red scoriae of all
sizes, and sinder. They frequently form
roundish hills but in between there are high (60
m or more) craters of thinly layered coarse
scoria, evidently deriving from fountain activ-
ity, and a lot of big bombs. Some typical lava
craters also belong to this emption. Lava flows
appear here and there at the side of these cra-
ters and are covered by thick strata of pumice
from the last emption. They are much more
marked as big kipukas frequently more or less
covered by pseudocraters. This fact is thought
to indicate an unusually high content of gas in
the magma. The lava from this emption most
probably occurred up to 8000 years ago. The
craters are cut by the narrow graben which ac-
cordingly was formed after the second eruption
but before the last one.
The third and last emption occurred in 1783
and lasted 8 months. It is characterized by
enormous lava production but relatively insig-
nificant quantity of pumice and ash. The lava
craters of this eruption penetrate many of the
big older scoria craters or intersect them.
As to the Laki itself it should be mentioned
that it is not a volcano sensu strictu, but a
hyaloclastite mountain, cut at the north side by
the graben and emptive fissure. So far, no
traces of volcanic activity have been found in
Laki itself nor the hill to west of it before 1783.
PÓSTFANG HÖFUNDAR/AuTHOr’s ADDRESS
Jón Jónsson
Smáraflöt 42
IS-210 GARÐABÆR
130