Jökull - 01.12.1964, Blaðsíða 18

SIGURDUR THORARI NSSON :
Sudden Advance of Vatnajökull Outlet Glaciers 1930-1964
INTRODUCTION
While waiting for the preparation of maps,
based on aerial photos, to illustrate the cata-
strophic advance of Brúarjökull 1963/64, the
present writer decided to compile the available
information of sudden advances of other outlet
glaciers of Vatnajökull during the last three
and a half decades in the hope that it may
throw some light on this interesting pheno-
menon. As to such advances before 1930 the
best known are the catastrophic advances of
Brúarjökull and Eyjabakkajökull 1890 (cf. Jök-
ull 1962, pp. 47—48), when Brúarjökull ad-
vanced about 10 km. A similar advance of Brú-
arjökull is known to have occurred in 1810,
and possibly also 1625 (Ólafsson 1772, p. 792).
Breidamerkurjökull is said to have advanced
1 km in 1820 and up to 8 m per day during
that summer (Thienemann 1824, pp. 311—313).
Systematic observations of the fronts of Vatna-
jökull outlet glaciers began in 1930 when H. H.
Eiríksson mapped the fronts of Hoffellsjökull,
Fláajökull and Heinabergsjökull, and built
cairns for continued observations (Eiríksson,
1931). Since 1932 Eythórsson and his collabora-
tors in Skaftafellssýslur have measured annuallv
the variations of most of the southern outlet
glaciers of Vatnajökull as well as the Öræfa-
jökull outlet glaciers (Eythórsson, 1963, pp. 31—
33). The big, lobe-shaped outlets of SW, W and
N Vatnajökull have been observed occasionally,
but only one of these glaciers, Tungnaárjökull,
is systematically observed, and these observa-
tions began only 10 years ago. Yet it is possible,
thanks to fortunate circumstances, to date with
tolerable exactness the sudden advances of these
outlets that have occurred since 1930. These
advances will now be shortly dealt with.
SÍDUJÖKULL 1934
In connection with the Grímsvötn-eruption
in April 1934 two expeditions went from Fljóts-
hverfi to the volcano via Sídujökull short W
of Hágöngur, returning the same way. The
76
second of these expeditions, led by N. Nielsen,
left the ice margin about May 15th on its way
back to Fljótshverfi. The surface of the glacier
was then very even except just W of Hágöng-
ur, where there are usually some crevasses, and
the glacier was a typical recession margin, thin
and gently sloping (Nielsen, 1937a, pp. 19—20,
1937b, pp. 95-98).
In June that year the river Djúpá, which
drains the E part of Sídujökull, began to carry
unusually much water and kept on doing so
until early August.
Towards the end of July this summer a Ger-
man-Austrian expedition, led by E. Herrmann,
intended to reach Grímsvötn by the same route
as Nielsen. This expedition arrived at the mar-
gin of Sídujökull near the outlet of Djúpá July
28, but to the great astonishment of the parti-
cipants, who had learned about the conditions
from Nielsen, they now founcl a glacier front,
which was high and steep and frightfully cre-
vassed. They tried to force the glacier, but
after five days they had to give up, not having
covered more than about two kilometers be-
cause of the crevassing. Returning to their first
camping place at he glacier margin they founcl
that during these five days it had advanced
nearly 30 m, or 6 m per day (Jonas 1948, pp.
48-53).
The following summer one of the members
of this expedition, R. Jonas, participated in an
Austrian expedition, led by Fr. Stefan. This
expedition traversed Vatnajökull from N to S
along the route Dyngjufjöll—Grímsvötn—Páls-
fjall—Sídujökull. Sídujökull was still badly cre-
vassed, but the glacier E of Hágöngur had a
smooth surface and its front was a thin reces-
sion front (cf. Jonas, op. cit., fig. 69). The
expedition reached the margin of Sídujökull
June 24, and Jonas estimated its advance W
of the outflow of Djúpá about 500 m since
lie was there the previous year. Further east
the glacier had receded somewhat. The steep
front of 1934 had thinned out greatly, and
obviously its advance had come to an end
JOKULL 1964