Jökull - 01.12.1979, Síða 36

are excellent time-stratigraphic markers for
geology, geomorphology and archaeology.
The identification of tephra layers in the field is
achieved in many ways, such as by their strati-
graphic relation, their colour, which is an indicator
of their chemical composition, their thickness and
grain sizes, which are, within certain limits, an ex-
ponential function of their distance from their
source.
In the laboratory the tephra Iayers are identified
by determination of their mineralogical, chemical
and physical characteristics, microprobe analyses
of their glasses being most useful. Various granu-
lometric parameters reveal the type of explosive
activity.
As for age determination of tephra layers, those
deposited in historical time can often be identified
as resulting from eruptions mentioned in written
records and can thus be dated more or less accu-
rately according to the accuracy of the records
concerned. In some cases they are exact even to the
day of the tephra fa.ll. In Iceland eruptions dated by
written records go back to about 900 A. D.
Tephra layers from eruptions not mentioned in
written records can often be more or less exactly
dated by various means, such as by C14-dating of
peat soil immediately above and beneath them or
trees or shrubs embedded in them. They can also be
dated by extrapolation from other exactly dated
tephra layers which show the rate of sedimentation,
by dendrochronology, pollenanalysis, and last but
not least with the aid of archaeology.
The conditions prerequisite for the establish-
ment of a tephrochronological time scale in Iceland
are in fact very good. With the reservations
outlined in the previous chapter one can say that
about 200 volcanoes have been active in the coun-
try in Postglacial time, between 30 and 40 of which
since the Nordic settlement began 11 centuries ago.
The tephra layers are more numerous than one
would expect in a volcanic area producing mainly
basalt. This is due mainly to the fact that the ice
covered basalt volcanoes, such as Grímsvötn and
Katla, produce tephra instead of the lava they
would produce under subaerial conditions. Up to
about 150 Postglacial tephra layers have been
detected in the field in a single profile. On the
whole the tephra layers are more numerous and
thicker E of the active volcanic zones than W of
them, as the air currents in the higher strata of the
troposphere are predominantly westerly.
The tephra layers mainly used hitherato as key
horizons in the Postglacial soils of Iceland are the
acid ones, as they are light coloured and therefore
easily traced in soil profiles. There are 12 widespread
acid layers in the Postglacial soils.
In Iceland the establishment of tephrochrono-
logy by field studies is facilitated by the country
being one of the few highly active volcanic areas in
the world where peat formation and formation of
Fig. 2. Map showing in
which direction tephra was
dispersed during the initial
phase of each of Hekla’s 15
eruptions in historical
time. The width of each
arrow indicates roughly
the relative size of the
estimated volume of the
layers.
34 JÖKULL 29. ÁR