Jökull - 01.12.1957, Síða 33

Fig. 7. A diagram illu-
strating the possible
formation of a single
wave and its associated
ogive. On Svínafells-
jökull several such
waves are formed each
year.
Tilraun til skýringar
á myndan einfaldrar
svigðu á Svínafells-
jökli.
cess that may be happening on Svínafells-
jökull. First the increased weight on the ice-
fall in winter causes a thrust plane to develop
under the steepest and fastest part of the ice-
fall in a nearly horizontal position. This is
deduced from the position of the highest single
ogive referred to above and the estimates of
the glacier velocity at that point. During the
movement of the thrust plane dirt from the
bed of the glacier is carried up with the up-
thrust ice of the thrust plane, this movement
may be of the order of 60—70 m. Later the
upthrust ice assumes the forrn of a ridge about
100 m across, and by mid summer the ridge
will have reached the bottom of the ice-
fall. If the movement along the thrust plane
was great enough the dirt line will appear on
the forward slope of the ridge as it is reduced
in size by ablation — as is in fact the case with
the small ridges on Svínafellsjökull. If the
movement has not been great the ridges will
initially be smaller in size, and the ogives will
not appear until lower down the glacier when
ablation exposes them.
This theory requires that the ridges found
below icefalls are the remains of the over-
thrust ice. However, the work of the Cam-
bridge Austerdalsbreen Expedition in Norway
(13) has resulted in another explanation for
the waves found on that glacier. The waves
were smoother than those found on Svínafells-
jökull, and the wavelength was everywhere
close to the annual movement at that point.
There were no smaller ridges superimposed.
Surveys of a stake system showed that the
waves were not apparently the result of stresses
operating in the summer, in fact the descrip-
tion of “pressure waves” seemed to be in-
accurate. The conclusion reached was that the
waves were the result of the effect of ablation
on the icefall, different amounts of ice reach-
ing the bottom of the icefall in different sea-
sons. The initial amplitude of the wave system
was found to be equal to the net annual abla-
tion on the icefall.
In connection with this theory it is notice-
able that Svínafellsjökull appears to have two
sets of waves superimposed, and the long, low
undulations previously referred to may have a
similar origin to the waves on Austerdalsbreen.
The thrust plane theory accounts for the small
irregular ridges with their associated dirt lines.
On the other hand the long, low undulations
of Svínafellsjökull may be the result of a
number of thrust planes forming very close
together.
It must be emphasized that these conclusions
have been reached by observation and circum-
stantial evidence only. A satisfactory explana-
tion for the formation of ogives has not yet
been found, and meanwhile the consideration
of every possible alternative is worth while. The
main object of the Cambridge South-East Ice-
land Expedition was to study the wave system
on Svínafellsjökull, and the measurements taken
on Austerdalsbreen were repeated on Svína-
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