Jökull - 01.12.1960, Blaðsíða 22

Bedrock
Meltwater and
volcanic products
Glacier ice
Fig. 1. Evolution of an explosive subglacial
eruption. I: Beginning of the eruption. II: The
meltwater begins to flow below the glacier. III:
The final stage of the subglacial phase, just
before the eruption breaks through the glacier.
— Hér eru sýnd þrjií stig sþrengigoss undir
jökli.
Mýrdalsjökull, June 1955:
Here the jökulhlaup was of a very short
duration, it lasted only a few hours. It com-
menced at about 20*1 (GMT) and the maxi-
mum water discharge at the glacier front
was reached very quickly. The first earth-
quake however was recorded some five hours
before the jökulhlaup reaclied the glacier front.
The subglacial route of the jökulhlaup is about
14 km, with some 900 m height difference
(Thorarinsson 1957). The short duration of the
jökulhlaup and the steepness of its subglacial
route indicate great flow velocity of the hlaup-
water. This velocity is assumed to be of the
order of 10 m/sec, wich means, that the water
ílowed from its source to the glacier front in
about one hour. This means further that the
first earthquakes occurred before the commence-
ment of the subglacial water flow. If these argu-
ments are valid, the pressure change on the
earth’s surface cannot have caused the earth-
quakes, as this occurred after the occurrence
of the first earthquakes. Explanation 4 is also
impossible, at least for the first earthquakes,
as no or very small movement had then oc-
currecl. The earthquakes were so small, that
there is no reason to assume, that they have
weakened an ice dam, damming up a hypothe-
tical subgiácial lake.
The only explanation of a physical relation
between the recorded earthquakes and the jök-
ulhlaup, seems to be, that both were caused by
a sudden and intensive heat transport from the
earth to the glacier ice, i.e. by a volcanic
eruption. Tliis eruption may have commenced
at the time of the first recorded earthquake,
at 14h 32m (GMT) and continued for some four
hours, or as long as the seismic activity. The
heat energy of the eruption has completely
gone to melt the glacier ice. The water mass
of the jökulhlaup, some 2.8 • 107 m3 (Thorarins-
son 1957), gives a measure of the heat energy
of the eruption. By assuming that this water
was melted during four hours, the rate of melt-
ing was about 200 m3/sec. Assuming further,
that the heat energy of one m3 of the volcanic
products is sufficient to mlt 5 m3 of ice, then
the production rate of tlie eruption has been
some 200 m2 sec, which is only a few percent
of that of the first phase of the Hekla erup-
tion 1947 (Thorarinsson 1954b). The total pro-
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