Jökull - 01.12.1968, Page 53

An evaluation of the measurements from this
standpoint is given in Table 4 and Fig. 5. A
comparison of ablation and degree days at 1000
m altitude (according to soundings) at Keflavík
Airport about 300 km distant is given in Fig. 4.
A closer study of the relations between the
budget and meteorological factors is hindered
l)y the lack of a permanent meteorological sta-
tion in the vicinity of Tungnárjökull.
Figure 12 shows the variation of the glacier
front in Tungnárbotnar near Nýjafell since
1889, compiled from the available observations,
maps and air photographs. (It should be noted
that the best known and only commercially
available maps of Vatnajökull, those of The
Danish Geodetic Institue, are very inaccurate
and not fit for glaciological studies, except
perhaps in limited areas.) The maximum acl-
vance of Tungnárjökull in postglacial times was
about 1880—90. A sudden advance occurred in
1945 and most likely also between 1915 and
1920. A simplified model to explain the sudden
advances of Tungnárjökull (and other flat out-
let glaciers of Vatnajökull) is obtained by as-
suming that the glacier is practically stagnant
between advances. This will cause a thinning
of the glacier below and a thickening above
the equilibrium line, and the slope of the
surface gradually gets steeper. Sooner or later
the stresses in the glacier will reach the limit
and the glacier will advance, which in turn
relieves the stresses. According to changes in the
Nýjafell profile from 1946 to 1967, Fig. 13,
this model seems to be plausible. The shear
stress at the bottom of the glacier in the upper
part of the Nýjafell profile has been increasing
since 1946, mostly because of thickening of the
glacier, and is possibly not far from the critical
value now. The approximate thickness of the
glacier in the Nýjafell profile is known from
a gravity survey in 1967.
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JÖKULL 18. ÁR 387