Náttúrufræðingurinn - 1968, Síða 60

168
NÁTTÚ RUFRÆÐINGURINN
SUMMARY
The Steinsholt Hlaup, Central-South Iceland, on January 15, 19fi7
by Gudmundur Kjartansson
Museum of Natural History, Dept. of Geology and Geography, Reykjavík.
The Icelandic term hlaup denotes a rapid mass movement suddenly set ofl
and accelerated by gravity. The term comprises all types of rock-falls, landslides,
avalanches, and floods that are not causecl directly by rain or thaw. As the
present author knows no other siiort term that comprises all the three successive
phases of the event to be described, this Icelandic term will be used liere.
It is known to glaciologists in the seconcl half of the term jökulhlaup.
The Steinsholt hlaup started as rockslide. Some 15.000.000 m3 of rock fell
or slid from the mountain Innstihaus down onto the glacier Steinsholtsjökull.
The two most different profiles of the rockslide are about 300 m liigh and
500 m long, and 470 m liigh and 900 m long respectively. Most of the rock-
slide debris came to rest on the glacier margin where it formed a large rugged
heap of rocks. But a considerable portion of tlie rock debris travelled farther.
Tliis part, mixed witlr a similar volume of ice ploughcd up frorn tlic glacier
surface, spread at great speed over the glacier tongue and farther down. This
second phase of the hlaup swept witli it and incorporated water from the
Steinsholtslón lake, which lies at the snout of the glacier, and proceeded
down the valley of the Steinsholtsá. In this section, about 5 km long, the hlaup
mass consisted of a mixture of fragments of rock and ice with water and
compressed air. The last-mentioned constituent, originally trapped by the
rockslide, was the greatest in volume.
The debris of rock and ice deposited on the glacier liad a comparatively
uniform thickness of 2—5 m over most ol the area covered by it. But in front
of the glacier and the lake it banked up to a thickness of 33 m. Still farther
down the valley tlie cover of debris shrank to a scattering of rocks.
The hlaup left its marks on the steep side of the valley up to a conspicuous
undulating line, consisting of lumps of ice and showing height levels up to
75 m above the valley floor. These levels indicate decrease in the cross sections
of the rushing mass from 22.000 m2 near the lakc to 4000 m2 in the mouth
of the valley. This decrease represents shrinkage in volume caused by the rapid
escape of trapped air from the hlaup into the atmosphere.
On each of the two bends ol' the valley (see map) the level of the hlaup
was much higher and also more distinct on the concave than on thc convex
side. For instance:
Upper bend: right side 75 m, lel't side about 20 m.
Lower bend: right side 15—20 m, left side 40 m.
The escape of trapped air from the hlaup mass was completed a short
distance (some 500 m) in front of tlie the mouth ol' the vallcy. From there
on the hlaup continued in its third and final phase as a flood of water, following