Jökull - 01.12.1979, Blaðsíða 35

4 Tephrochronology and its application in Iceland
SIGURDUR THORARINSSON
Science Institute, University of Iceland, Reykjavík
In his treatise: De mirabilibus Islandiæ (On the
wonders of Iceland), written in April 1638, bishop
Gísli Oddson of Skálholt, South Iceland, mentions
volcanic ash layers in Icelandic soils and is the first
author to do so. The keenwitted bishop states that
in Icelandic humus soils there are thick ash layers
separated by humus layers, and that remnants of
trees are embedded in the lower layers. From this
he concludes that vólcanic ash layers have indis-
putably covered the main part of the country and
repeatedly caused destruction.
The facts mentioned by bishop Gísli are in
reality the conditions prerequisite for the esta-
blishment of the geological dating and working
method which in 1944 was named tephrochrono-
logy, the term tephra borrowed from Aristotle’s
account in Meteorologica of an eruption on the island
Hiera (now Vulcano), where he uses the word
tephra — one of the two words in classical Greek for
ash — for the volcanic ash that reached the Italian
mainland. The term tephra fits phonetically well
with the terms magma and lava and is used as a
collective term for all airborne pyroclasts, ir-
respective of their size, shape or composition.
Tephrochronology is thus a chronology that in-
volves the dating of tephra layers, and the use of
these dated layers for various geological,
geomorphological and also archaeological pur-
poses.
Every wholly or partly explosive volcanic erup-
tion leaves on the ground a more or less extensive
layer of tephra. In extreme cases these layers may
spread over enormous areas. On March 30, 1875
the light of the gas lamps in the streets of Stock-
holm was obscured by a light-gray ash from the
Askja eruption in Iceland of March 28 to 29, about
1900 km distant from Stockholm. The map of that
tephra sector (Fig. 1) is the first one ever made of a
big tephra layer.
Where the tephra layers have not been stripped
off by wind or water, but covered by subsequent
soil formation, they appear in more or less distinct
horizons in the soil profiles. Owing to their,
geologically speaking, instantaneous formation
and very wide dispersal, coupled with usually in-
considerable thickness and characteristic ap-
pearance, these layers satisfy every claim as good
geological guide horizons. When exactly dated they
Fig. 1. The tephra sector
of the Askja eruption 1875.
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JÖKULL 29. ÁR 33