Jökull - 01.12.1974, Side 37

Kornastœrð /Grainsize
0.02 -0.05mm
Skeiðaró
Gígjukvísl
Súla
Ósar og sjór / Mouth and the sea
Aur í grunnstingli / From anchor
Aur í jökulís /From glacier ice
Volcanic glass
Crystals
Rock
fragmem
ice
Fig. 8. Petrographical
composition o£ the coarse
silt material from Skeid-
ará shows a regular
change with time as the
glass content increases
with time, and on March
26th the material was al-
most 90% glass of the
same refractive index as
Grímsvötn volcanic ash.
The numbers at the
points indicate dates of
samling in March 1972.
Mynd 8. Bergfrœðileg
samsetning mósins í
Skeiðará sýnir reglulega
breytingu, þannig að gler-
innihald vex eftir því sem
á hlauþið liður, og 26.
mars er hér um bil 90%
efnisins gler með Ijósbrot
ösku úr Grímsvötnum.
Tölurnar við punktana
tákna dagsetningu í mars
1972.
tion of all grain size fractions except clay. The
addition of this material to the coast would
rnake a thickening of about 4 cm over the
coastal area down to the 50 m depth, which
corresponds to 2 m seaward extension of the
coast.
JÖKULHLAUP MECHANISM -
DISCUSSION IN LITERATURE
The earliest theory on the mechanism of
jiikulhlaup is the buoyancy theory first put
forth by Norwegian scientists (cf. Liestöl, 1956).
This theory states that when the water level of
an ice-dammed lake reaches 9/10 of the height
of the ice barrier the ice starts floating and a
jiikulhlaup is triggered.
An alternative theory was proposed by Glen
(1954). He maintains that water-filled holes in
the glaciers tend to enlarge below a certain
depth as the horizontal stress becomes larger
than the vertical because of the density differ-
ence of ice and water. At a depth of 150—200 m
this difference becomes large enough for a
water-filled hole to enlarge. In this manner a
deep lake (150—200 m) will have a tendency
to create and gradually enlarge lioles at the
bottom of the ice wall which will lead to a
tapping of the lake. However this theory can
not explain the tapping of shallow lakes.
Liestöl (1956) proposed an explanation of
the tapping mechanism of ice-dammed lakes.
He states that once a flow of water is started
the passage will be widened by the melting of
the ice due to the heat capacity of the water
and kinetic energy transformed into heat as
the water discharges from the ice-dammed lake
to the glacier snout. Liestöl’s theory explains
very well the usual jökulhlaup hydrograph; with
the exponential increase in flow until most of
the water has escaped and then a rapid de-
crease. The melting of the ice tunnel is the
JÖKULL 24. ÁR 35