Jökull - 01.12.1981, Page 26

The zone consists of three volcanic systems or
central volcanoes which are arranged en
echelon (Jóhannesson 1982a). These volcanic
systems are fundamentally different from the
volcanic systems of the axial rift zones of Ice-
land. The latter are characterized by a ten-
sional stress field resulting in open fissures and
normal faults (Saemundsson 1978). On the other
hand, the volcanic systems of Snaefellsnes do
not exhibit such features. The volcanism is re-
stricted to a narrow zone, often only a few
hundred metres across and no rifting is
observed.
The Snaefellsjökull central volcano is the
westernmost volcanic system of the Snaefells-
nes volcanic zone. It is about 30 km long and
stretches nearly E-W from Maelifell in
Stadarsveit to the tip of the peninsula. The
volcanism in the eastern part is restricted to a
narrow zone but in the western part it is more
widespread especially around Mt. Snaefells-
jökull. The activity seems to have been more
vigorous in the western part where a 1446 m
high stratovolcano, Snaefellsjökull, has deve-
loped.
The oldest rocks exposed at the base of the
volcano are reversely magnetized lava flows,
supposed to be just over 700.000 years old.
They rest unconformably on an eroded Ter-
tiary basement.
The cone itself consists of hyaloclastites
formed during glacials, and of interglacial
lava flows. The postglacial activity seems to
have been vigorous. Over 20 individual lava
flows have been identified so far and they are
all prehistoric. The eruption sites are either
close to the top of the mountain or in groups in
the lowlands around the volcano. The craters
close to the top usually erupt intermediate or
acid magmas in contrast to the basaltic
magma erupted in the lowlands. (Jóhannesson
1982a).
THE ACID TEPHRA LAYERS
OF THE SNAEFELLSJÖKULL
CENTRAL VOLCANO
In August of 1981 a total of 95 soil profiles
were studied in the Snaefellsnes peninsula (see
Flores 1981). The aim of this study was to trace
tlie acid tephra layers of the Snaefellsjökull
central volcano and to map their distribution
in the Snaefellsnes peninsula. The location of
the soil profiles is shown in Fig. 1. The thick-
nesses of tephra and soil layers were measured
and characteristics, such as the angularity of
glass fragments, colour etc., of the tephra were
studied, but grain-size analyses were not
undertaken. Representative soil profiles are
shown in Fig. 2. In this survey the existence of
the three acid tephra layers was confirmed.
They are in this paper termed Sn-1, Sn-2, and
Sn-3, Sn-1 being the youngest and Sn-3 the
oldest. A few basaltic tephra layers were en-
countered but their distribution seems to be
restricted to the neighbourhood of cinder
cones, most of which belong to the Snaefells-
jökull volcanic system.
The thickness of each acid tephra layer
varies considerably from one profile to
another, even across short distances. In many
cases this difference may be due to the ash
being piled into drifts by wind; especially if
the eruption took place in wintertime and the
tephra was deposited on frozen or snow cov-
ered ground, resulting in an uneven thickness.
In a few places the tephra layers have ob-
viously been modified by water (redeposited).
Such instances are easily identified because
the ash and pumice fragments are more or less
rounded.
Due to the above-mentioned irregularities
and the similarity of the tephra, it was
necessary to trace each layer from one area to
another by studying soil profiles every 2 — 3
km. In spite of the number of profiles studied,
the correlation between some of the sections is
difficult.
Sn-1
Sn-1 is the youngest acid tephra layer. It is
generally at less than 40 cm depth below the
present surface. The maximum thickness runs
towards the northeast as demonstrated by the
isopach map (Fig. 3). The layer is found on the
northern side of the Snaefellsnes peninsula but
has not been identified on the south side. For
24 JÖKULL 31.ÁR