Greinar (Vísindafélag Íslendinga) - 01.01.1977, Síða 64

62
of lava and climatic periods, but we are confident that such
cannot be serious from the present point of view. In the Swed-
ish source the Younger Dryas ends at 10,000 B.P., and I have
put the corresponding Búðaskeið at the same age, although
usually, the Younger Dryas is taken to end 11,000 B.P.
Looking at the table, it is most striking that the lava flows
crowd in the two Moor Stages, the explanation of which, we
have indicated earlier. The Birch Stages are by comparison
nearly devoid of volcanism, as far as the evidence goes with
the surprising exception of the shieldvolcano Ketildyngja.
In addition to Table I, we reproduce in Fig. 9 the table on
the Búrfell- and Þjórsár-lavas, which originated in the Eastern
volcanic zone, and reached Búrfell or farther.
The two Þjórsárlavas reached the shore between Þjórsá and
ölfusá. Each is about 12 m thick, and on the second half of
its course, the younger one drops from a height of 60 m to
zero per 45 km. This gives a dip of 4.5' of arc or 1/13 of a
degree. No human eye will distinguish this from horizontality,
which gives us the answer to the planparallelism of plateau
basalts, which so long baffled geologists. It is the quantity of
flowing basaltic lava which matters, if it is to spread widely
and with a flat surface.
The above lavas have been studied by scientists of the Na-
tional Energy Authority in connection with the building of
the Búrfell Power Plant. The table, to which I have added the
climatic periods, was published by the geologist Haukur Tómas-
son: Hrauneyjafoss, Geological Report, March 1971 (Mimeo-
graph Reports of the National Energy Authority). We see that
six out of the ten lava flows were erupted during the Older
Moor Stage.
We have earlier mentioned global stress fields. Such would
be best expressed in the Older and Younger Dryas stages, which
we can take to have been global as to character of the atmo-
spheric circulation. We must clearly also refer to at least very
extensive stress fields when seeking the causes of seismicity
and volcanism on the submarine ridge system.