Jökull - 01.12.1964, Blaðsíða 28

Fig. 12. The areas of Vatnajökull that are known to have been affected by sudden advances
during the last 75 years: 1: Sídujökull, 2: Skaftárjökull, 3a: Southern Tungnárjökull, 3b: Nortli-
ern Tungnárjökull, 4: Dyngjujökull, 5: Brúarjökull, 6: EyjabakkajÖkull.
pointing to volcanological causes. Consequently
the explanation has to be sought in the shape of
the glaciers, in their regime, and in the mechan-
ism of glacier movement.
Fig. 13 shows the sudden advances in rela-
tion to the variation of the Vatnajökull outlet
glaciers, expressed by the average variation for
all the outlets measurecf by Eythórsson, usually
21 to 24 glaciers (Eythórsson 1963). Tungnár-
jökull is excluded, as it has only been measnr-
ed the last 10 years. Thus none of the glaciers
affected by sudden advances since 1930 is in-
cluded in the average figures. We find that the
advances in 1934 and 1951 were preceded by
a year or years during which the general re-
treat of the glaciers was slowed up consider-
ably. As a matter of fact some ot' the outlets
advanced during these years. Of 17 outlets mea-
sured 1932/33, 5 were advancing and 1 station-
ary. 1933/34 only one was advancing. 1949/50
4 of the 23 measured showed advance and 2
were stationary, ancl during the three preceding
years the retreat was on the whole slow and
3 to 4 glaciers slightly advancing.
The 1945-advance was preceded by three
years of relatively slow retreat, whereas the two
last years preceding 1963/64 advance of Brúar-
jökull and Sídujökull clid not show any slowing
up of the retreat. Yet it should be kept in
rnind that (a) the first signs of the advance
of Sídujökull were observed already in 1962,
(b) that in 1960/61 there was a slowing up of
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JOKULL 1964