Jökull - 01.12.1964, Qupperneq 30

tions on central Vatnajökull, but annual ac-
cumulation-measurements carried out in the
Grímsvötn area since 1953 are in good agree-
ment with the precipitation-measurements at
Fagurhólsmýri, high precipitation tliere coin-
ciding with great accumulation within the
Grímsvötn area. It can therefore be regarded
as likely that the 1934-advance of Sídujökull
was influenced by a heavy accumulation on the
intake area of that glacier 1932/33, and the
last advance of Sídujökull and Brúarjökull may
have been initiated by the high accumulation
1960/61.
As pointed out above all the Vatnajökull
outlets, that have advanced suddenly, are of a
similar flat shape, and this is a shape, which
makes them more sensitive to variations in
temperature and precipitation than more steep
glaciers. And, somehow, the shape of these gla-
ciers has the effect, that changes within their
accumulation areas are not spread slowly along
the glacier, instead stress accumulates until it
has reached a certain limit, when it is suddenly
released. This limit is determined by the bal-
ance between the ablation and accumulation
areas, and may probably be reached by a thin-
ning of the ablation area as well as by a thick-
ening of the accumulation area. It should also
be taken into consideration that the highest
cupolas of Vatnajökull, although at present
of the temperate type, are very near to the arctic
type, and slight temperature-changes may play
a role in their movement mechanism. Changes
in water-lubrication are likely to play a role as
well; yet the known sudden advances of Vatna-
jökull outlets do not seem to be restricted to
any special season of the year.
In his important papers on the mechanism.
of glacier-movement and oscillations Nye has
repeatedly stated that his kinematic wave theory
does not cover catastrophic advances of glaciers
(cf. Nye 1965 and previous papers, and Chum-
skiy 1965). A student of the oscillations of the
Vatnajökull outlet glaciers must find this a
serious defect, as catastrophic advances seem to
be “normal” behaviour of so many of its out-
lets. But few, if any, glaciated areas seem more
likely than Vatnajökull to furnish us with suf-
ficient data for the solution of this problem.
In order to get these data we need, however,
a more detailed mapping of the subglacial topo-
graphy of the areas where these advances occur,
as well as a systematic study of one or more
of these glaciers during many consecutive years.
This includes the measuring of movement and
regime, and repeated levellings of profiles both
within the ablation and accumulation areas,
and frequent aerial reconnoitring in order to
time the beginning of crevassing. A meteoro-
logical station situated in the highland interior
N or W of Vatnajökull and run the whole year
round, is also highly desireable.
REFERENCES
Ahlmann, H. W:son, 1939. The Regime of Hof-
fellsjökull. Geogr. Ann., Stockh., 21: 171—
188.
Bárðarson, G. G., 1934. Islands Gletcher. Soc.
Sci. Isl. XVI. Reykjavík.
Chumskiy, P. A., 1965. Theory of Glacier Varia-
tions. J. Glaciol. 5: 515—517.
Eiríksson, H. H., 1931. Observations and Mea-
surements of some Glaciers in Austur-
Skaftafellssýsla. Soc. Sci. Isl. XII. Reykja-
vik.
Eythórsson, J., 1951. Fransk-íslenzki Vatnajökuls-
leiðangurinn, marz—apríl 1951. Jökull, 1:
10-14.
— 1952. Landið undir Vatnajökli. Jökull, 2:
1-4.
— 1963. Variation of Iceland Glaciers 1931 —
1960. Jökull, 13: 31-33.
Hannesson, P., 1958. Landið okkar. Bókaútgáfa
Menningarsjóðs. Rykjavík.
Jonas, R., 1948. Fahrten in Island. Verlag L.
W. Seidel & Sohn. Wien.
Nielsen, N., 1937a. A Volcano under an Ice-
Cap. Vatnajökull, Iceland, 1934—36. Geo-
gr. J., 20: 6-23.
— 1937b. Vatnajökull. Kampen mellem Ild
og Is. Köbenhavn.
Nusser, Fr., 1936. Bericht iiber die österreich-
ische Island Vatnajökull Expedition. Mitt.
geogr. Ges. Wien. Bd. 78.
— 1943. Der Dyngjujökull auf Island in den
Jahren 1935 und 1939. III. Jahresbericht
des Archivs fur Polarforschung im Natur-
hist. Museum in Wien: 4—20.
Nye, J. F., 1960. The Response of Glaciers and
Ice-sheets in Seasonal and Climatic Chang-
es. Proc. Roy. Soc. Ser. A. Vol. 256, No.
1287: 559-584.
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