Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 51

Chapter 3
THE TIME CORRELATION BETWEEN
CLIMATIC PERIODS AND THE VOLCANISM AND
SEISMICITY OF ICEIAND
CONTENTS
Time correlation between climatic periods of certain characteristics, of
late-glacial, postglacial, and historic time, with volcanic and seismic
activity in Iceland is very clear. The seasonally variable rotation of the
earth is only one effect of the atmospheric circulation, wind stress has
also effects, volcanic and seismic, due to plastic state below a thin brittle
crustal layer. The predominant shallowness of foci in the volcanic zones
<about 3 km), indicates that deeper the wind-induced strain energy is
lost because of plasticity. This state allows the thin overlying layer to be
so twisted during such times as decades or more, under certain types of
climate, as to accumulate stresses up to the breaking point, which causes
shallow earthquakes. How this state also allows the formation of magma
and its extrusion, is discussed in Chapter 6.
Seasonal changes of the rotation of the solid earth, have been
known since atomic clocks came into use. In the present con-
text, we must write the angular momentum of the earth in
the following way: D ~ ISQ)3+I0öj0+Ia<5a, where I means moment
of inertia, for the solid earth (index s), the oceanic mass
(index o) and the air mass (index a). The <S-s are vectors,
I0 and Ia must in exact calculations be considered as matrices.
Apart from tidal friction, which need not be taken into account
here, D is constant.
What we know to happen seasonally, is a very small change
of I0. In winter in either hemisphere, a layer is taken from the
oceans and deposited as snow or lake water in high latitudes.
But this seasonal change can be negleeted here. The over-
whelming seasonal change is in the last member on the right
side, the change being due to the seasonal change in the atmos-
pheric circulation. Although for many people it seems sur-
prising, the winds do indeed have such a frictional effect as
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