Jökull - 01.12.1977, Síða 11

pp. 314—319) suggested that an eruption took
place in Hveradalur (Solfatara Valley) in Kverk-
fjöll in 1938 whereas Thorarinsson (1950) claims
that untenable on tephrochronological grounds,
and suggests an eruption there in 1929. How-
ever, the area is clearly very active still, as
abundantly evident from the liydrothermal
activity in Hveradalur, which in terms of energy
output is amongst the strongest in Iceland.
LAKAGÍGAR. The eruption of 1783 is the
largest known basaltic eruption in recorded
history, and the most catastrophic one in Ice-
land (Table II, nr. 10). The fissure extends
beneath the Vatnajökull ice slieet, which must
liave resulted in tephra-formation.
In the light of tlie present events at Krafla
in northern Iceland (Björnsson et al. 1977) the
possibility lias been considerecl by rnany Ice-
landers that the Lakagígar Fires were produced
by the lateral drainage of the Grímsvötn magma
chamber, in consequence of an episode of
spreading on the eastern volcanic zone. This
hypothesis is supported by a number of petro-
chemical arguments: The chemical composition
of the Lakagígar lava and the Grímsvötn pro-
ducts is similar; such “developed” chemistry is
out of equilibrium with a supposed source in
the mantle; the oxygen-isotopic composition of
the Lakagígar lava (Muehlenhachs et al. 1974)
is indicative of a long magmatic development
in the crust; and, finally, the parallel Eldgjá
fissure, about 5 km distant from the Lakagígar
one, has totally different chemical characterist-
ics, that of Katla. It is almost inconceivable
that these two fissures, parallel but 5 km apart,
that erupted within 900 years of each other
producing totally different magmas, could be
derivetl straight from a heterogeneous mantle.
There are many problems, however, to con-
sider, not least the volume of the Lakagígar
flow, which is about 13 km3. This is equivalent
to a sphere of 1.45 km radius, or a cylinder of
35 km2 cross section and 370 m depth (the area
of Grímsvötn is 35 km2). The average depth of
the Grímsvötn caldera is about 300 m — the
bottom is at about 1000 m altitude above sea
level, whereas the altitude of the rim ranges
from 1100 m in the east to about 1700 m in
the south, or about 1300 m on the average.
There is no question, however, of the caldera
having formed in 1783 — it was described long
before that. Furthermore, the 10-year period of
Grímsvötn jökulhlaups liad been established
long before the 1783 eruption, and in general
the behaviour of both the caldera and the Gríms-
vötn volcano seems to have been unaffected by
the Lakagígar eruption. On the other hand, the
shape of the Grímsvötn caldera suggests that it
may be composite (Thorarinsson 1978, pers.
connn.), and the northern part could liave forrn-
ed in 1783 without altering the pattern of either
jökulhlaups from Grímsvötn or eruptions there.
Another fissure eruption in the area west of
Vatnajökull is that of TRÖLLAHRAUN 1862-
64 (Thorarinsson ir Sigualdason 1972, see Table
II, no. 7). This eruption was formerly thought
to have taken place within the Vatnajökull ice
sheet (Thoroddsen 1924), or on Dyngjuháls
(Jónsson 1945). Little tephra was produced in
the eruption, which lasted intermittently for 27
months — only two eruptions in Iceland are
known to have lastecl longer during the 1100
years of settlement (Tliorarinsson ir Sigvalda-
son, op. cit.).
ASKfA or DYNGJUFJÖLL, some 60 km to
the north-east of Bárdarbunga, erupted in 1961
(Thorarinsson ir Sigualdason 1962, Table II, no.
1), giving rise to a relatively small basaltic lava
flow. The tephra from this eruption forms the
uppermost layer in the Bárdarbunga core. This
area was very active in the nineteen twenties,
but by no means all the eruptions taking place
then were noticed until “after the fact”, by the
presence of the new lava flows where none
existed before (Jónsson 1945; Thorarinsson
1963). This underscores the fact tlrat during
the centuries represented in the Bárdarbunga
core, countless eruptions could liave taken place
in Dyngjufjöll without theirs being recordecl
at all.
The Askja eruption of 1875 struck a calamity
in large areas of eastern Iceland, and drove the
population frorn those parts to America by the
hundreds (Thorarinsson 1944). However, as a
precursor to that great explosive silicic volcan-
ism, wliich started on March 28, 1875, had been
more than three months of earthquakes, and
over a year of increased hydrothermal activity
in Askja (Thorarinsson 1963). Tliere is evidence,
JÖKULL 27. ÁR 9