|dc.description.abstract||At present there is no consistent method for the identification of source volcanoes
for a tephra layer found in a stratigraphy. This has led to several studies
misidentifying source volcanoes. Geochemical analysis of the tephra layer can
identify the source volcano. A methodology for identification of most Icelandic
volcanoes is presented here following from the work of Jóhannsdóttir (2007). It has
been constructed using an existing major element geochemical dataset from
Icelandic tephra layers of known origin. In particular, it improves the difficult task of
distinguishing between Hekla and Katla basalts. It is hoped it is simple and quick
enough to use to encourage other workers to use it, but to still retain sufficient
accuracy that a statistics based computer program could not produce. It was found
that Cr, Ni, Sr, F, Cl and S are of no additional help in identifying the source volcano
of the tephra layer.
In order to aid production of this method and to evaluate its success, three lacustrine
cores from Iceland are analysed. 135 primary Holocene tephra layers were identified
in the combined stratigraphy, which includes 42 layers from Hestvàtn, 49 layers from
Vestra Gìsholtsvàtnand 69 layers from Hvítarvàtn. 106 of these tephra layers were
analysed for major element geochemistry. Together with the work of Jóhannsdóttir
(2007), this provides a unique tephrochronological record for Central South Iceland,
because it constitutes a continuous chronology spanning the last 12kyrs at the
resolution of decades to centuries.
These results enabled the examination of geochemistry, soil accumulation rate,
identification of historic layers, frequency of tephra fall and associated hazard risks
and preservation of tephra within lakes. 47.5% of the tephra layers from this study
are basaltic, and another 41% contain a basaltic component, highlighting the
importance of basaltic tephra layers in the tephrostratigraphy. Few tephra layers
from the Western Volcanic Zone are present in the record despite its proximity to the
study area. This emphasises the importance of ice caps on volcanoes for producing
sufficiently explosive basaltic eruptions to generate widespread tephra fallout.
Although not highly accurate the Sediment Accumulation Rate method gives a
reasonable age for eruptions that have not been dated by another method.
Frequency of tephra fall is highly variable in the area, it averages 8.4layers/500yrs.
Katla is the most hazardous volcano for south-western Iceland, even though only
5% of its eruptions have produced tephra that reaches this location.||en