Jökull - 01.12.1982, Síða 69

Fig. 5. Block of soil > 90 m
long and > 2 m thick at
Höfðabrekkujökull. Trans-
ported by the Katla flow of
1721 or 1755.
Mynd 5. Jarðvegstorfa meira en
90 m á lengd og meira en 2 m á
þykkt, sem hlaupið 1721 eða 1755
hefur borið fram.
probably too high, 950-1000 tons is perhaps more
realistic. The density of the boulder, which is com-
posed of hyaloclastite is 2. 64 g/cm3. It is supposed
to have been carried passively along in a laminar
flow. The density of the debris flow I have earlier
estimated to be 2.6-2.7 g/cm3. This has been
criticized as being far too high a value. In this
respect it is worth pointing out that a density figure
rneasured by Rist (1955) on silt from Skeiðará flood
was 2.2 g/cm3. A density value of 2.5 g/ cm3 for the
Katla flow is hardly too high.
The water content of the Kada flows has long
been a riddle just because of the prevailing mis-
understanding of the true nature of the flows. Al-
though the total amount of water certainly is high, it
does not need to be so percentually. It is considered
that the first — and main phase of the Katla flows
consists, by at least 80%, of solid material and accordingly
these are true debris flows. They are besl described as
volcanoglacial debrisflows. (Jónsson op. cit.). Nothing is
known of the submarine continuation of the flows
but they are supposed to give rise to turbidity cur-
rents.
Submarine sedimentary ridges, for which Malm-
berg (1978) suggests the name Katla ridges, are
between canyons cutting the Icelandic continental
slope to the south of Iceland at 18°-19° west, i. e.
south of Mýrdalssandur. Perhaps their origin is the
Katla flows? Walker (1972) describes material from
a deep sea core in the Iceland Basin just south of
59°N which he states to be “compatible with an
Icelandic origin and carried in by turbidity flows.“
His opinion is that intraglacial eruptions in Iceland
constitute the most plausiblesourceofthematerial.
The origin of this material might be the Katla de-
bris flows. An eye-witness, HaraldurEinarsson, stated
that the iceblocks seemed to sink down when the
flow reached thesea. Similarflowshaveoccurred on
— and partly formed — the Sólheimasandur (Ein-
arsson et al. 1980) and a birch forest on the upper-
most part of Skógasandur seems to have been de-
Fig 6. The Kötluklettur (Katla rock). Approx.
950-1000 metric tons. Transported by the Katla
flow 1918.
Mynd 6. Kötluklettur um 950-1000 tonn, sem hlaupið
1918 barfram.
JÖKULL 32. ÁR 65