Jökull - 01.12.1982, Page 68

Fig. 4. Material deposited by
the Katla flow of 1918, resting
on undisturbed soil at Núpar.
MyndL 4. Setlögfrá Kótluhlaupinu
1918 ofan á óhreyfbum jarðvegi við
Núpa.
The material.
The material transported by the Katla flows is
enormous. During less than 20 hours in the 1918
flow the ocean coast was in parts extended some 4
km offshore and out to a depth of approx. 40 m.
Similarly an enormous amount of volcanic debris
mainly pumice was deposited on the sandur plain.
In the area between Kaplagarðar and Hjörleifshöíði
some 8 m thick pumice layer was added on the
sandur plain, (Einarsson 1975). A good exposure of
the material is found in Sandgil (Fig. 3).
The coarser material was mainly deposited on
the sandur at the margins of main flow channels
parallel to ílow direction. Plastic clay has also been
observed in the deposits presumably derived from
high-temperature geothermal areas beneath the
glacier.
Material from the previous Katla flows is still
retained in Austurjökull and Höfðabrekkujökull
(Fig. 1) which seems to derive mainly from the flow
of 1721 but partly from 1755. At the western margin
of the “jökull“ there is ablockofalmost undisturbed
soil more than 2 m thick and more than 90 m iong
(Fig. 5). Other dimensions are not known. Pres-
umably this was deposited in the sea which up to
1721 was only a short distance from Skiphellir, a
cave where the fishing boats were kept up to that
date. Skiphellir is approx. 2. 2 km from the present
beach.
It is interesting to note that the coarser material is
conspicuously more abundant in the upper partofa
section through this material and big boulders have
only been found at the top of Höfðabrekkujökull.
Most of the flow material, however, consists of vol-
canic pumice, ca. 95%. Same is the case in the
above mentioned Sandgil. The grain size of the
pumice is dominantly, or 60-70%, 0.5-4.0 mm and
material <0.074 mm is only 4-6% (Fig. 8). Judged
from a great many analyses kindly put at my dis-
posal by Mr. Gylfi Einarsson, geologist of the Tech-
nological Institute of Iceland, similar material cov-
ers wide areas of Mýrdalssandur. The pumice is
highly porous and non-porous grains are almost
lacking. Petrographically the material from the
Katla area is transitional alkali basalt Jakobsson
1979).
Fig. 4 shows a section through a part of the mat-
erial deposited at the margin of the flow of 12th of
October 1918 at Núpar. The flow material rests on
undisturbed soil. Next to the soil, i. e. at the base of
the flow is a pumice layer as was mentioned above.
Coarser material, clasts, mostly consisting of bas-
altic rock fragments are common in the upper part
of the profile and several boulders up to 1 m in
diameter are found at the top. This seems to indi-
cate that the coarser material was mainly transported in the
upper part of the flow, on top of finer grained material
mainly consisting of pumice (Fig. 7). The stratification
of the pumice, both in this profile and in Sandgil,
seems to indicate a laminar movement dominated
at the flow base. A hugeboulder, Kötluklettur, (Fig.
6) was carried approx. 15 km by the 1918 flow. I
have earlier estimated it to be 1400 metric tons
Jónsson 1980), but upon reestimation the value is
64 JÖKULL 32. ÁR