Náttúrufræðingurinn - 1987, Side 26

In 1982 I succeeded in finding a 6-8 m
thick soil underneath the Eldgjá lava flow
at Nyðri Ófæra, and in this soil section
several of the same tephra layers which are
found at Dalbær in Landbrot on top of the
lava. This shows firstly that the Landbrot
lava can not be the same as that one at
Eldgjá and secondly that the Landbrot
lava must be very old and does not derive
from the Eldgjá proper, as originally
stated by Thoroddsen (1884). Further,
Thoroddsen states that the Eldgjá lava
disappears under the Skaftá (Laki) lava of
1783 at Nyrðri Ófæra. This statement has
been repeated by Sapper (1908) as well as
Robson (1952), but neither of them seems
to have visited the area.
The lava flow from Eldgjá at Nyrðri
Ófæra seems to end more or less abruptly
a few kilometres east of the volcano (Fig. 1
and 3) and almost certainly did not reach
the Skaftá valley. The volume of the
Eldgjá lava is accordingly highly overesti-
mated. The above-mentioned soil section
at Nyrðri Ófæra indicates in fact that the
Eldgjá eruption must have occurred rather
late and perhaps early in historical times.
However the uncertainty as to the time is
far too great to account for the printing of
a date (934) on the geological map of the
area. Tephrochronology is a very useful
method, but should not be overestimated.
Neither 14C datings nor tephrochronology
can be considered as exact, but if sensibly
combined their usefulness is unques-
tioned.
A recent 14C dating of vegetative
remains on top of the Landbrot lava gave
6200 ± 100 Y. B. P. This indicates a still
older age of the lava than earlier esti-
mated, perhaps up to 7000 years (Fig. 11).
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