Jökull - 01.12.1979, Side 37

loessial soil is very rapid, which means that the soil
cover thickens so quickly that tephra layers of small
difference in age are separated in the soil sections.
Iceland is also a country where relatively reliable
written records of volcanic eruptions are more
abundant and go farther back than in most vol-
canic areas.
In Iceland tephrochronology has been applied
in:
1. Studies of eruption-history and eruption-
mechanism of the volcanoes.
2. Studies of fluvial erosion and wind erosion.
3. Datings of glacier oscillations.
4. Studies of periglacial phenomena, especially
frost crack polygons.
5. Pollenanalytical studies of vegetation changes.
6. Archaeological studies, especially datings of
farm ruins.
7. Dating of ice cores from glaciers.
8. Establishing a tephrochronological connection
between Iceland and other countries.
Three examples of the results of these appli-
cations will be mentioned here:
Eruption history
Tephrochronological studies of the eruption his-
Fig. 3. Outlines of the tephra sectors of some his-
torical eruptions.
tory of Hekla have revealed that this most famous
of Iceland’s active volcanoes is of Postglacial age,
that its activity is cyclic, each cycle beginning with
a purely explosive acid eruption preceded by a long
repose. Other Hekla eruptions are mixed ones. The
oldest acid Hekla tephra, designated H5, is about
7000 years old, the youngest, H1; was produced by
the first Hekla eruption in historical time in 1104
A. D. Since then Hekla has erupted 14 times (Fig.
2). The Si02 content of the initial phase of each
eruption is roughly proportional to the length of
the preceding repose. Fig. 3 shows the extension of
some historical tephra layers.
Fig. 4. Soil profile and diagram showing tephra layers, rate of soil thickening and variation in the
coarseness of loessial soil 15 km W of Hekla. The profile to the right is'the same as the left one except
that the tephra layers have been excluded. 1: black tephra; 2: light tephra; 3: sandy soil; 4: fine-sandy
soil; 5: silty soil.
JÖKULL 29. ÁR 35