Jökull - 01.12.1982, Síða 65

Notes on the Katla volcanoglacial Debris Flows
JÓN JÓNSSON
National Energy Authority, Grensásvegur 9, 108 Reykjavík, Iceland
ABSTRACT
The nature ojKatla Jlows is discussed. It is demonstrated
that they are different Jrom other features which in Iceland
are termedjökulhlaup and shouldpreferably be described as
volcanoglacial debris Jlows. The total volume of the peak of
the flows is supposed to amount to even more than 2 million
md/sec. The flows seem to contain 80% or more solid
material, to a great extentpumice, derivingfrom the volcanic
centre. The density of the Jlow is supposed to be 2.5 g/crrd.
The flows are supposed to continue as submarine turbidity
currents.
INTRODUCTION
The subglacial volcano Katla situated in the east-
ern part of Mýrdalsjökull in South Iceland is one of
the most active in Iceland and known for its high
production of tephra but still more feared for the
tremendous and dangerous flows, Kötluhlaup,
caused by the eruptions. As far as is known to the
author of this note they are the most voluminous
and catastrophic known to occur on earth in our
times.
HISTORY
The first mentioned eruption in the Mýrdalsjök-
ull (jökull = glacier) seems to have occurred about
930 A. D. From that time and up to the year 1625 A.
D. information about volcanic activity in this area is
rather uncertain but there seem to have been 17
eruptions (Thorarinsson 1975). Some of these erup-
tions, however, did occur in the western part of the
glacier, and it is therefore questionable if they
should be termed Kötluhlaup (Katla flows). From
the year 1625 and onwards we have quite reliable
information written by eye-witnesses. Information
on Katla eruptions in historical time has been coll-
ected and printed in Safn til Sögu Islands IV. The
most detailed information we have on the Katla
flows is found in the records of the last eruption of
Katla which occurreed on the 12th of October 1918
(Jóhannsson 1919, Sveinsson 1919, Sveinsson 1930). In
this paper I will not deal with the Katla eruptions
sensu strictu but only with the above mentioned
flows caused by the eruptions.
The 1918flowwill bethemain topic of this paper.
The nature of this flow as well as the material
deposited by the flows, however, represents the
Katla flows in general.
Undoubtedly the flowsof 1660, 1721and 1755 are
the most voluminous and violent in history. Part of
the material left by the latter two is still retained in
Höfðabrekkujökull and possibly also in Austurjök-
ull (see the map). Through history the Katla flows
have in Iceland been referred to as “jökulhlaup",
glacier burst, mainly consisting of ice and water,
and even in our time compared to other floods
known to occur in Iceland, e. g. the well known
floods of Skeiðará. The Skeiðarárhlaup, however,
are normal glacial drainage floods, sometimes as-
sociated with volcanic activity in the Grímsvötn
caldera (Thorarinsson 1957). It is only in 1974 that
the author of this paper, in a Report to the Icelandic
Cival Defense Board, pointed out that the Katla
flows are of entirely different nature (Jónsson 1974)
and should preferably be indicated as volcano-glac-
ial debris flows (Jónsson 1980 a, b). They are identi-
cal with the debris flows described by Gascoyne
(1978) and by Johnson (1970) and Flink et al. (1981).
THE KATLA ERUPTION OF
12th OCTOBER 1918.
Course of events
'Fhe first warning of an eruption was a sudden,
sharp earthquake, felt in the village Víkjust about 1
p.m., followed by tremours for about halfan hour.
(Jóhannsson 1919). Near 3 p.m. an eruption cloud
was seen above the glacier, and at the same time or
slightly later a tremendous flood was seen rushing
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