Náttúrufræðingurinn - 1987, Page 25

Þorvaldur Thoroddsen. 1894b. Ferðir um Vest-
ur Skaftafellssýslu sumarið 1893. — Andvari
19.
Þorvaldur Thoroddsen. 1905—1906. Island.
Grundriss der Geographie und Geologie. -
SUMMARY
The Eldgjá eruption and the
age of the Landbrot lava.
by
Jón Jónsson
Orkustofnun
National Energy Authority
Reykjavík
In my paper “On lava flows in Skafta-
fellssýsla” (Jón Jónsson 1979) I tried to
show that the lava flow in Landbrot (Land-
brotshraun) could not be of historical age.
My conclusion was based on a series of
four 14C datings (Jón Jónsson 1979), two of
them made on remains of birch wood, and
two of a sample of peat. These samples
were collected at the farm Ytri Dalbær by
the author. All the samples were taken
from an approximately 10 m thick soil
section on top of the lava flow (Fig. 8).
The birch wood samples (U-2416 and U-
2417) were collected from within layers of
acidic tephra, which I at that time assumed
to have originated from Hekla volcano,
and one of them (U-2416) to have prob-
ably been identical with H-4. Later it
turned out that most probably neither of
these tephra layers derive from Hekla.
However, these age determinations
showed the approximate age of the two
tephra layers, which are widespread in
Skaftafellssýsla, and accordingly strati-
graphically important. Based on these four
age determinations I estimated the age of
the Landbrot lava to be at least 5200 years.
Gudrún Larsen (1979) rejected my conclu-
Pettermanns Mitteilungen Justus Pertes
Gotha.
Þorvaldur Thoroddsen. 1925. Geschichte der
Islándischen Vulkane. - D. Kgl. Danske
Vidensk. Selsk. Skrifter, Naturvidensk og
Mathem. Afd. 8, IX Köbenhavn.
sions but without substantial arguments.
For some unknown reason Gudrún Larsen
was later supported by Sigurdur Thorarins-
son (1981), although he had earlier stated
that the Landbrot lava was prehistoric
(Sigurður Thorarinsson 1955). Gudrún
Larsen put forward the idea that the lava
had crept in under the soil, which accor-
dingly could be older than the lava, and
the 14C dating therefore wrong as to the
age of the lava. In this connection the
following should be borne in mind:
1) The thickness of the lava flow is more
than 20 m (drillhole data).
2) The temperature of a flowing basaltic
lava is at least around 900—1000°C.
3) It is a well known fact that a lava flow
which has traveled long distances, in
this case at least 30-40 km, advances
with a rolling movement like an “end-
less track on a tractor”.
4) The soil section at Dalbær is 10 m or
less in depth.
5) The soil is partly peat, indicating a wet
area or bog (occurrence of diatom-
aceous earth).
6) The peat layers closest to the lava and
approx. 30—50 cm above it are undis-
turbed, as are the two tephra layers
which have been dated.
How could the last observation be possible
if a 20 m thick and up to 1000°C hot lava
had crept under the soil? And what about
the water in the soil? A considerably lower
temperature than 900—1000°C is needed
to bring it to boil, and what would become
of the soil in such a situation? Moreover,
the vegetation remains, birch (Betula) and
willow (Salix), do not show the slightest
degree of carbonizing.
19