Jökull - 01.12.1953, Blaðsíða 6

ion which generally will not have formed be-
fore a lapse of many thousand years after the
respective ice load was laid down.
With this method one finds easiiy and rapidly
the depression of a chosen point for various
assumptions of T. If a glacier and the depression
caused by it at a certain point be known, one
finds easily the value of T that best fits the
observations. It is clear from the definition of l
that this quantity, and with it the depression
calculated, is only slightly sensitive to uncertain-
des in E, m, and d, while it is especially sensitive
to changes in T. Therefore the method is well
adapted to the determination of T. —
In a note in Jökull 2. ár, 1952 (10) I discussed
the depression to be expected from a recent inc-
rease of Vatnajökull as suggested by J. Eythors-
son. I estimated that a load corresponding to an
ice sheet with the area of Vatnajökull and with a
thickness of 300 m would, after attainment of
equilibrium, cause a depression of the central
area by 10—20 m.
With the present method I have calculated
the equilibrium depression at various points
corresponding to the total load of the present
glacier, with a thickness of about 600 m inside
the marginal zone (8). The result (in metres) is
given in Table III. The value 10—20 m, given
in my former note corresponds roughly to the
assumption T = 30 km.
TABLE III.
T
10 km 30 km 50 km
Centre of glacier 33 m 19 m
Mouth of Jökulsá (Breiða-
merkursandur) 40 m 22
Höfn in Hornafjörður . . 16 18 11
Djúpivogur 0 12
Reyðarfjörður 7
Seyðisfjörður 3 5
Vík in Mýrdal . .. 4
Akureyri 4
I consider that the most probable value of
T is between 10 and 30 km, which, however, is
hardly more than a guess. A few further exam-
ples may be added. Depression at Vík in Mýrdal
under influence of Mýrdalsjökull, 2 m (Assumed
average thickness 300 m, T = 30 km). Depress-
ion at Akureyri caused by Hofsjökull and Lang-
jökull, 1 m (Assumed average thickness 200 m,
T = 30 km).
In the foregoing I have consiclered the equi-
librium depression which is reached only after
flow has gone on in the substratum for thous-
ands of years. It may now be considered that a
slight depression is formed immediately after
deposition of the glacier load, namely because
of the elastic (non-plastic) yield of the substrat-
um. This may be founcl in the following way.
Consider a large elastic body bounded by a
horizontal (infinite) plane surface. Pressure is
applied at a point of this surface. Then the
depression of the surface is a known function
of the distance from the pressed point (9). We
use this general solution and, as in the previous
case, integrate the depressions caused by the
vertical columns into which we may divide the
glacier. We then get the simple expression^w =
1,69 (ro — ri), where w is the depression in metr-
es at a point P, caused by a 1000 m thick glacier
which covers the complete circular zóne with
radii ro and n, with centre at P. The unit of
the radii is 10 km. Thus a glacier, 1000 m thick,
covering the zone between n = 10 km and r2
= 20 km gives a depression of 1,69 m at the
central point. The values of E and m used here
are those given earlier in this article.
The depression is proportionate to the thick-
ness of the glacier, and, if a zone is only partially
filled by the glacier, say an arc of 90°, then the
contribution to the depression is proportionate
to the arc occupied, here V4 of that of the
complete zone.
I have applied this method to the Vatna-
jökull. Its present quantity, as given earlier,
will cause the following values of elastic de-
pression:
At centre of glacier ................. 4,2 m
Mouth of Jökulsá ..................... 2,0 —
Djúpivogur ........................... 0,9 —
A change of 50 m in the thickness of Vatna-
jökull will cause a vertical elastic movement of
the land at Jökulsá by 17 cm. Thus there ap-
pears to be no chance of late climatic fluctua-
tions causing immediate detectable changes of
sea-level and it is even questionable whether
the movement of the land could be found by
precision nivellement.
With the progress of time the substratum,
4