Jökull - 01.12.1960, Blaðsíða 24

found in the mountain Thverfell (Tryggvason
and Báth 1961). Eruptions of this kind will
melt the glacier very slowly, ancl no jökulhlaup
will occur. Fig. 2 shows three possible stages
of an effusive subglacial volcano, where the
last stage represents a “table mountain”.
The behaviour of the meltwater flow from
a subglacial explosive eruption to the glacier
front is not known in details, but a short discus-
sion of the problem will be given here.
If the glacier surface above the eruption
place is slooping, the pressure of the melt-
water will exceed that of the glacier (static
pressure) below the place where the glacier
surface is lowest over the meltwater area. The
order of magnitude of the time of relaxation
due to plastic flow in ice is given by Benioff
and Gutenberg (1951) as 108 sec. The rigidity
of glacier ice is about 2.5 • 1010 dyn/cm2 (com-
puted from wave velocities in Breiðamerkur-
jökull from data given by Joset 1954). This
indicates, that the movements in the glacier
ice, caused by the water flow, will be purely
elastic, if the area of water-ice contact with
pressure excess in the water exceeds some cri-
tical amount, which depends on ice thickness
and the pressure difference ice-water. In case
of Katla the glacier thickness is assumed to be
500 m and this critical area is about 101 m2,
if the pressure excess in water is 10° dyn/cml
ancl the area is approximately circular. This
means, that the water flow will be controlled
by the elastic movements of the ice almost from
the beginning, and the glacier will be of very
limited hindrance to the flow. The water will
flow in a thin film between the glacier and
the subglacial earth’s surface. The velocity of
this subglacial water flow depends mainly on
the mass of water and the steepness of the
glacier surface. The time lapse from the be-
ginning of melting, untill the stage of fast
flow, governed by the elastic behaviour of the
glacier, is reached, depends mainly on the
melting rate and on the slope of the glacier
surface at the place of melting. If the melting
rate is some 104 m2/sec, this time lapse is prob-
ably less than one hour in the case of Katla.
The fast flow of water from Katla to the glacier
front, will probably carry the water this distance
of 14 km in less than one hour.
REFERENCES:
Benioff H. ancl Gutenberg B. 1951. Strain Cha-
racteristics of the Earth’s Interior. In B.
Gutenberg (ed.) International Constitution
of the Earth, 384 (Dover, New York).
Joset A. 1954. Expedition Franco-Islandaise au
Vatnajökull, mars—avril 1951. Jökull 4: 1—
33 (Reykjavík).
Rist S. 1955. Skeiðarárhlaup 1954 (The Hlaup
of Skeiðará 1954). Jökull 5: 30—36 (Reykja-
vík).
Thorarinsson S. 1953. Some New Aspects of the
Grímsvötn Problem. Journ. of Glaciol. 2:
267-274 (Cambridge).
— 1954 a. Athuganir á Skeiðarárhlaupi og
Grimsvötnum 1954 (The Jökulhlaup [Gla-
cier Burst] from Grimsvötn in July 1954).
Jökull 4: 34-37 (Reykjavík).
— 1954 b. The Tephra-fall from Hekla on
March 29th 1947. Tlie Eruption of Hekla
1947-1948 II, 3: 1-68 (Reykjavík).
— 1957. The Jökulhlaup from the Kat.la Area
in 1955 compared with other Jökulhlaups
in Iceland. Jökull 7: 21—25 (Reykjavík).
Thorarinsson S. and Rist S. 1955. Rannsókn á
Kötlu og Kötluhlaupi sumarið 1955. Jökull
5: 43-46 (Reykjavík).
Tryggvason E. and Báth M. 1961. Upper Crustal
Structure of Iceland. Journ. of Geoph. Res.
(in press).
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