Jökull - 01.12.1981, Page 25

A short account of the Holocene tephrochronology
of the Snaefellsjökull central volcano, Western Iceland
HAUKUR JÓHANNESSON1
ROBERTO MACIEL FLORES2
JÖN JÓNSSON3
ABSTRACT
Three major acid teþhra layers occur in the þost-
glacial soil cover in the Snaefellsnes þeninsula. They
are lermed Sn-1, Sn-2 and Sn-3 with increased age.
They have a common origin in the Snaefellsjökull
central volcano at the tiþ of the þeninsula. In all three
eruþtions a southwesterly wind sþread the teþhra to
the northeast of the volcano. Reþorled C'4-ages of
Sn-I and Sn-2 are 1750 ± 150 and 3960 ± 100
years, resþectively. The exact age of Sti-3 is not
known, but is estimated to be about 7000—9000
years.
introduction
This work was carried out as a part of the
United Nations University Geothermal
Training Programme. Its aim was to demon-
strate how past activity of an active central
volcano can be established by studying the
tephra layers emitted from it. The tephra
usually forms distinct layers in the soil, and
numerous such layers have been identified in
Iceland. The number of layers in a single sec-
tion can reach 150, but there exist only 12
widespread acid tephra layers in the post-
glacial soils (Thorarinsson 1981).
1. Museum of Natural History, Laugavegur 105,
Reykjavík, Iceland.
2. United Nations University, Geothermal Training
Programme, National Energy Authority, Grens-
ásvegur 9, Reykjavík, Iceland.
3. National Energy Authority, Grensásvegur 9,
Reykjavík, Iceland.
Tephra layers can be used to establish the
history of volcanoes. Thorarinssoti (1967) traced
the historic eruptions of Mt. Hekla and by
comparing the tephrochronology with literary
sources obtained the exact age of each in-
dividual tephra layer. For layers older than
Settlement (about 874 AD) one has to rely on
Cl4-datings of vegetation remains close to the
layers.
The tephrochronology of the Snaefellsnes
peninsula is poorly known. Sigurdsson (1966)
reports three acid tephra layers from the Set-
berg area. These layers are identified as I, II,
and IV but III is basaltic. The layers are
numbered from the base upwards. The basal-
tic tephra layer III originates from the cinder
cones in the Berserkjahraun lava field in
Helgafellssveit. Sigurdsson (1966) concludes
that layers I and IV have their origin in the
Snaefellsjökull central volcano.
Steinthórsson (1967) presents C14-datings of
two acid tephra layers from a soil section at
Fagrahlid in Fródárhreppur. These datings
are most likely of layers IV and III mentioned
above. The age of layer IV is 1750 ± 150years
and of layer III 3960 ± 100 years. Jóhannesson
(1977) studied tephra layers in Hnappadalur,
but no trace of the three acid layers were
found there, only basaltic tephra from local
cinder cones.
GEOLOGICAL SETTING
The Snaefellsnes volcanic zone stretches
WNW-ESE from Nordurárdalur in Borgar-
fjördur west along the Snaefellsnes peninsula.
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