Náttúrufræðingurinn - 1985, Side 51

SUMMARY
The Advances and Retreats
of the Skeidarárjökull Glacier
in Southeast Iceland
in the last 250 Years.
by
Haukur Jóhannesson
Icelandic Museum of Natural History
P. O. Box 5320,
125 Reykjavík.
The Vatnajökull glacier is by far the largest
glacier in Iceland. It is located in the southeast-
ern part of the island. It has many outlets and
the Skeidarárjökull glacier is one of its biggest
outlet glaciers. In this paper I present the results
of a survey of the available literary sources on
the position of its western margin during the last
250 years.
It is generally believed that the main glaciers
of Iceland started to advance around 1700.
Since then the history of the Skeidarárjökull
glacier has been one of successive advances and
retreats. The results are summarized in Figs. 2
and 3. Nothing is known about the location of
thc margin until in the year 1784, apart from
very inaccurate information which, non the less
puts limits on its maximum extent at each time.
In historical times, the glacier has advanced
to its maximum extent, four times in the last 250
years, i. e. in the years 1784, 1857, 1871 and
1890-95. It retreated about 1000 m between the
advances prior to the last advance. Since 1900
the glacier has retreated more or less con-
tinuously, and in the year 1961 the margin was
about 2500 m short of its maximum extent.
During each advance the glacier added new
material to the system of terminal moraines
encircling the snout of the glacier.
The advances are caused by a surge, which is
a common phenomena in Iceland. Then the
outlet glaciers advance a few hundred to a few
thousand metres in a few months or years, in
contrast to the usual steady creep.
The surge of the Skeidarárjökull glacier
seems to take several years, thus differing from
the other outlets of the Vatnajökull glacier,
which complete their surge in a few months.
It has been suggested that the Skeidarárjökull
glacier had advanced into Mt. Lómagnúpur in
the latter half of the 18th century, damming a
lake between Mt. Eystrafjall and Mt. Lóma-
gnúpur.
Floodings or glacial bursts in the river Súla
(Núpsvötn) are caused, either by draining of
lake Grímsvötn, a caldera lake in the central
part of the Vatnajökull glacier which simul-
taneously causes bursts in the river Skeidará, or
by draining of lake Grænalón which is an ice-
dammed lake by the side of Skeidarárjökull
glacier.
It has also been suggested that the draining of
a lake, supposed to have formed behind the
advancing Skeidarárjökull glacier, caused bursts
in Súla (Núpsvötn) in the latter half of the 18th
and the early part of the 19th centuries. The
study presented here does not confirm this
suggestion and it is thought very unlikely that
any lake was formed which had the capability to
cause such floodings. It is thus believed that the
glacial bursts were caused by draining of the
lake Grænalón as at present.
A still older moraine, Sandgígjur, lies 2 km
south of the main moraines formed in the last
couple of centuries. It is prehistoric and presum-
ably of Younger Dryas age.
45