Jökull - 01.12.1966, Page 4

fí — ——- —-
/// S / / ® / / /.
/ x / ' / / / ' /--~~-----------------------
. / / / / / / / // /////«,
Fig. 1. Explanation of isostatic and eustatic
changes. See text.
1. mynd. Til skýringar á lyftingu sjávarbotns
á isöld.
coastal areas. A and B in Fig. 1 are a section
of a continental and oceanic block, respective-
ly. By isostatic equilibrium before glaciation,
1—2 is a horizontal line of equal pressure.
During glaciation an oceanic layer of thickness
d, assumed here to be 100 m, is removed and
deposited as ice in special areas outside the
block A. As a result, pressure becomes inequal
along 1—2 and we may consider four types of
reaction to this. 1) Sub-crustal material is so
fluid that it yields immediately and fully to
the change in pressure. The ocean floor is
then generally raised by 100/3.4 = 29.4 30 m,
line 1—2 changes to 1—21. The coastal strip is
raised by half this value if the sea was at least
100 m deep at the original coast; if the shelf
clips 1/2 degree, a correction of only a fraction
of a metre must be applied, and thus can be
dropped.
The real vertical movements, relative to the
pre-glacial state, depend on the relative area of
oceans and continents ancl on the downwarping
in glaciated areas but this is of no avail here;
only the relative movements of ocean floor and
of continental margin, as given above, is of
interest. We thus find that the removal of an
ocean laver of 100 m thickness gives a measur-
able drop of sea-level by 85 m, relative to the
continental coast, or a corresponding relative
uplift of the ocean floor by 15 m.
2) Sub-crustal material vields with a time lag.
3) Sub-crustal material is plastic but the yield
value is larger than about 10 kg/cm2 (the
pressure difference corresponding to 100 m of
water). The ocean bottom will then remain
stationary when layer d is removed, and the
measurable drop of sea-level is 100 m. 4 )The
yield value is lower than in case 3, ancl there
is only partial response to the pressure dif-
ference.
Our rnain concern is the rise of sea-level in
late- and post-glacial time. This depends on
the two factors: sinking of the ocean floor
relative to the continent A, and the water
mass coming back to the ocean from the shrink-
ing glaciers. As a measure of the latter let us
tentatively take the shrinkage of the Scandinav-
ian glacier.
The size of the latter is inferred for three
epochs: 8800 B.P. (0.21 10« km2), 10.000 B.P.
(1.06 10« km2), and 18.000 B.P. (2.5 10« krn2),
from Sveriges geologi (Magnusson et. al. 1963,
p. 462), and a smooth curve, Ga in Fig. 2, con-
structed. If the volume of the glacier were pro-
portional to the area, the curve A in the same
figure shows the expected rise of sea-level for
the case of a fix ocean floor (case 3). If the
more realistic assumption is made that the thick-
ness of the glacier was at each time proportional
to the diameter, the curve for the volume is
given by Gv (Gv(xGa%).
The corresponcling rise of sea-level for case 3
is shown by the curve V3. For case 1 the ocean
floor sinks relative to the coastal area along
the curve Bi, ancl relative sea-level then rises
along the curve Vi. B2 and B4 are examples
of cases 2 and 4, respectively. V4 woulcl lie
between V4 and V3, and V2 would lie close to
V1 except at the right end.
The curve S4 (the ordinate of the minimum
near 18.000 B.P. is taken as unity) lies mostly
between V4 and V3 and it seerns on the whole
hardly possible to distinguish between the cases
1—4 by use of Si on one side and such a
general picture of glacier shrinkage as we have
used on the other. Sub-crustal material is prob-
ably plastic and a yield value of about 10
kg/cm2 is mostly assumed (for the Icelandic
area I have found that this is an upper limit).
This would very likely be equivalent to case 3,
1. e. a fix ocean floor, and this case will be
assumed in the following. In the interval 11.000
—9.000 B P. this appears to be a good approx-
imation. We shall also take Gv as a fair ap-
proximation to the relativ decrease of glaciers
in Iceland.
2. THE GLACIATED
SCANDINAVIAN AREA.
When the glaciers of the Ice Age shrank, the
glaciated areas began to rise isostatically. Con-
cerning the mode of rise, it has been suggested
in connection with the North American region
that the marinal areas began to rise earlier
158 JÖKULL