Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 52

50
to change seasonally the rotation of the solid earth by a mea-
sureable and well established amount. The seasonal fluctua-
tion in the length of the day amounts to about 0.02 seconds.
Although on the average the seasonal wind stresses are only
of the order of dynes/cm2, they do indeed suffice to explain
these variations in the length of the day. (37 and 38).
But when once we know that the seasonal changes in the
atmosphere are capable of changing the rotation of the solid
earth, it becomes less surprising that the atmosphere can do
more. What affects the rotation of the solid earth, is only
that component of Iaéa which is parallel to I0é0. But generally
there are also the two other components. All three components
of Iaáa must contribute to a global stress field which varies
seasonally, but especially with climate fluctuations of the
duration of decades, centuries, and thousands of years. Thus,
climatic fluctuations undoubtedly lead to a similary changing
global stress field. If these stress fields have visible effects,
they will be clearest during periods of severe climate with
strong winds.
Are there such effects? Yes, indeed. However strange it may
sound, the volcanism in Iceland shows correspondance to cli-
matic periods, and there is a way to understand this.
The empirical Relationship between Climate and
Volcanism in Iceland.
In Chapter 1 we pointed out that the greatest shieldvolcano
in the northermost part of the Eastern volcanic zone of Iceland,
the Þeistareykjabunga, had been formed during glacial climate,
just before the maximum of the severe Older Dryas Stage,
i.e. about 13,500 B.P., cf. Fig. 2.
On the basis of Chapter 1, Figs. 2 and 4, the area of the
giant Skjaldbreiður would almost certainly have been ice-free
during the Older Dryas stage. Thus Skjaldbreiður could have
formed during the Older Dryas, in which case the two giants in
the northeast and in the southwest would both correspond to
the very severe climate of the Older Dryas. The third giant,