Náttúrufræðingurinn - 1983, Blaðsíða 115

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SUMMARY
The Graenalón region in
Vestur-Skaftafellssýsla,
South Iceland
by
Haukur Jóhannesson,
Icelandic Museum of Natural History,
P. O. Box 5320, Reykjavík, Iceland.
A historical review of Lake Grænalón and
Mt. Grænafjall in the pre-1900 literature is
presented. The Lake Grænalón is first
mentioned around 1700 and frequently
since, but usually under the name Gríms-
vötn, and later as Núpsárvatn, Græna-
fjallslón and Grænalón. Mt. Grænafjall is
first mentioned in the year 1793. Two Brit-
ish travellers were the first men to reach
Mt. Grænafjall and it occurred as late as
1904.
The Skeiðarárjökull glacier flows along
the Skeiðarárdalur valley and dams a
tributary valley on its western side forming
the Lake Grænalón. The Lake Grænalón
has been emptied repeatedly or its water-
level lowered drastically in the course of
the water forcing its way beneath the gla-
cier, causing floodings on the Skeiðarár-
sandur plain (glacial burst, jökulhlaups).
Prior to 1935 the lake had an outlet to the
Núpsá river through a mountain pass at
635 m a. s. 1. Pronounced shore-line indi-
cates the former high waterlevel of the
lake.
The geology of the region is outlined.
The rocks are all volcanic in origin and
supposed to be erupted during the early
Matuyama epoch, 1.6 to 2.0 million years
ago. The sequence is about 500—700 m
thick and is characterized by subglacial
basic hyaloclastites and interglacial basalt
lava flows. The section indicates three
glaciations and three interglacial periods,
respectively.
In the region are two recent basalt lava
flows, Bergvatnsár flows, which are about
4000 years old and believed to be formed
in the same eruption. The eruption sites
are at present covered by the Vatnajökull
glacier which is supposed to have been
considerably smaller in the early part of
the Holocene. The flows cover an area of
about 10.7 km2 off the glacier and their
volume is about 0.07 km'.
Newly discovered cold mineral springs
on Mt. Grænafjall are described and evi-
dence for high temperature geothermal
field in the Þórðarhyrna area in the Vatna-
jökull glacier is presented. Þórðarhyrna is
an active central volcano and the Berg-
vatnsár lava flows may be erupted from a
fissure swarm presently subglacial, stretch-
ing towards southwest from the main
volcano.
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