Jökull - 01.12.1981, Page 30

less farthest away from the volcano. 'Fhe
tephra is usually well sorted Itut there are a
few exceptions which complicate the dis-
tribution pattern described. In a few of the
profiles on the northern side of the Snaefells-
nes peninsula the tephra is not as well sorted.
In profiles 49, 51 and 69 the lower part of
the layer consists of coarse-grained tephra
which grades into a finer upper part. In profile
62, on the other hand, the lower part is fine-
grained grading into coarse-grained upper
part. Profile 45 displays the following
sequence: a fine-grained base, grading into
coarse middle part which in turn grades into
fine-grained tephra towards the top of the
layer.
The above-mentioned variations can either
be attributed to changes in the phreatic
activity of the volcano or a change in the wind
strength.
Sn-3
Sn-3 is the lowermost layer yet found. It is
usually close to the base of the soil cover which
is underlain by gravel, morainic material or
basement rocks. The layer has been found in
Breidavík on the south side of the peninsula
and in the area from Ólafsvík to Hrauns-
fjördur on the north side (Fig. 5). West of the
volcano it is only found in profile 43, where it
has a considerable thickness but consists of
rounded grains and the tephra is without
doubt water-transported in similar way as
Sn-2 in the same profile. The layer is discon-
tinuous and seems to have been greatly dis-
turbed by wind. The maximum thickness runs
towards northeast as far as can be gathered
from the limited data. The distribution of the
tephra in western part of the area is obscure.
The 2.5 cm isopach shows a similar devi-
ation from the main trend as do the thinner
isopachs of Sn-1 and Sn-2 and can be ex-
plained in a similar manner. The layer consists
of light-coloured frothy pumice and ash. The
grain size is about 1—2 cm east of Ólafsvík,
but drops to less than 0.2 cm farthest away.
The maximum thickness observed is 30.5 cm
in profile 48.
The tephra layer in profiles 48 and 55 con-
sists of two separate parts, a coarse-grained
lower part and a finer grained upper part.
These two parts are separated by a 2 cm tliick
black basaltic or andesitic tephra layer, whose
origin is unknown but presumably comes frorn
the Snaefellsjökull volcano. It can bc con-
cluded that Sn-3 was produced in two
phreatic phases interrupted by a basaltic or
andesitic eruption.
IRREGULARITIES IN
THICKNESS AND DISTRIBUTION
As mentioned above some consistent irre-
gularities were observed in the distribution
pattern of all three layers. They are of three
types:
1. Uneven thickness from one profile to
another.
2. Unusual thinning or abrupt disappearance
of tephra west of the axis of maximum
thickness.
3. The eastward twist of the isopachs of the
smaller thicknesses.
The first type is demonstrated in unusual
differences in thickness from one profile to
another or even phasing out of the layer in
some restricted areas and abnormally great
thicknesses in others. The most likely explan-
ation is that all these layers were deposited
during wintertime, preferably on frozen or
snowcovered ground. Winds may sub-
sequently have swept the tephra off the
ground in some places and piled it into drifts
elsewhere. Water has also modified the layers,
especially on mountain slopes.
The apparent abrupt thinning out of the
tephra west of the axis of maximum thickness
is observed to some extent for all the layers.
Sn-1 is about 20 cm thick at Ólafsvík but
has not been found west of there. Sn-2 has
rather abnormal distribution in the same area
and is not found west or south of Hellissandur
where it is 12 cm thick. Sn-3 has an unilateral
distribution. It is 30.5 cm thick east of Ólafsvík
28 JÖKULL 31. ÁR