Jökull - 01.12.1953, Blaðsíða 4

TRAUSTI E I NARSSON :
Depression of the Earth’s Crust
under Glacier Load. Various Aspects
The rise of Scandinavia in late- and post-
Glacial times has been used by various writers
as a basis for calculating the value of viscositv
of subcrustal matter. There is general agreement
on a value of about 1022 poises.
It is tempting to assume that in such a vol-
canic country as Iceland a much more fluid
substratum would be found. However, the slow-
ness of post-Glacial rise in this country seeins
definitely to disprove that supposition. The
rise of Iceland was a faster process than that of
Scandinavia, but not very much so. From the
work of G. G. Bárðarson (1,2) and J. Áskelsson
(3) it is clear that the depressed land had risen
but little at a time when sea temperatures had
reached the present values, and when glacier
load must have remained negligible for one or
two millennia. The shore line was then about
90 m higher than at present, in an area where
the highest shoreline found is about 120 m
above sea level.
But a few thousand years later the shore íine
had receded to the present position where it
has remained practically stationary since then.
This is proved by the general occurrence of
thick soil and relatively old peat close to the
present coast.
Judging by these facts it appears certain that
the rise of leeland was not 100 times faster
than that of Scandinavia, but may have been of
the order of 10 times faster.
The latter value leads to a viscosity of the
substratum of the order of 1021 poises.
The stiffness of the plastic substratum is then
comparable to that given for Solenhofen lime-
stone. For further comparison we remark that
the viscosity thus found is 100 million times
that of ice, 100 thousand times that of Iceland
spar, and 1000 times that of rock salt. In or-
dinary sence the substratum is then absolutely
solid. There is not even a thin general layer of
notable fluidity.
In consequence the reaction of the crust to
changes in ice load must be very slow and for
full adjustment a lapse of several thousand
years would seem to be demanded.
More detailed studies of the post-Glacial' rise
of Iceland than have been undertaken so-far
will probably not affect this conclusion notably.
But such studies are most essential to enable
the approach to another geophysical problem.
In the case of a very extensive ice-sheet the
fully developed depression of the land depends
on the thickness of ice and on the ratio of the
densities of ice and plastic substratum. Elastic
properties of the crust have no influence except
at the rand of the depression.
In the case of a small ice-cap, however, the
outcome is different, for with a strong crust
we get a wide and shallow depression, while
with a weak crust the depression will be deeper
and narrower.
The Quaternary ice-cap of Iceland and all
smaller ones produce depressions whose depth
depends markedly on the strength of the crust,
and strength means mainly thickness of the
non-plastic elastic crust. Thus glacial geology
may here enable an important approach to the
study of properties of the earth’s crust.
From this point of view I have considered
relevant facts of Icelandic Glacial geologý (1,
2, 3, 4, 5), but with negative results. Our know-
ledge is still far too limited. Not only is the
thickness of the maximum ice-sheet unsatis-
factorily known, but it is not yet feasible to
distinguish between the signs of the last glacia-
tion and those of the maximum one.
Secondly the depth of the depression caused
by the last glaciation is not known with suffici-
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