VSF 2010 Report - Nabo

CORRELATION OF TEPHRA LAYERS FROM VATNSFJÖRÐUR WITH THE
ERUPTION OF HEKLA 1693
Jamie K. Anderson
University of Oxford
Research aims
The Vatnsfjörður archaeological site has had limited chronological evidence available to it
throughout the project, and as a result it is important to correlate the site’s visible tephra
layers with known eruptions to provide stratigraphic chronological markers and link the site
to wider regional chronologies, both archaeologically and environmentally. The aim of the
research presented here was to identify the visible tephra layers at the site and correlate them
with known eruptions. Although a similar layer had previously been sampled and possibly
identified from the site (Sigurgeirsson, 2006), the research presented here describes the
geochemistry and correlations of layers from within the archaeological site itself.
Introduction to Tephrochronology
Tephrochronology is a chronological technique widely used in archaeological and
environmental studies which uses volcanic ash horizons to date and correlate stratigraphic
sequences. Tephra is the volcanic ash and larger fragments that have been ejected into the
atmosphere during an explosive eruption. Individual ash layers can be correlated to eruptions
using the chemistry of the vitreous glass component of the volcanic ash. Tephrochronology
relies upon the notion that the span of a tephra-producing volcanic event is short enough and
discrete enough that it appears to be instantaneous in the stratigraphic record (Thorarinsson,
1955b). Tephra is dispersed hundreds of miles from the volcano, depositing isochronous
stratigraphic markers in a variety of environments. Tephra, either visible or microscopic, can
be extracted from the sedimentary record with field and laboratory techniques. In theory,
each volcanic eruption produces a unique geochemical signature and distal layers can be
matched by their geochemistry to particular eruptions that are characterised at proximal
locations where the stratigraphy is well constrained. Often these proximal deposits have been
dated via radiocarbon methods or have been documented in historical records and the tephra
can then provide absolute chronology for the sequences in which they are preserved.
Holocene Icelandic volcanism is characterized by explosive basaltic eruptions, and
only a few silicic eruptions. These eruptions are explosive due to interaction between the hot
molten rock and water from the ice caps covering volcanic zones. 75% of known Icelandic
eruptions during the Holocene were explosive and generated tephra; of these, 80% are
basaltic (Larsen & Eiriksson, 2008). However, the frequent volcanic activity means that the
tephrostratigraphy for the region is complex, with over 900 eruptions in the past 9000 years
(ibid.: 110, 111). A detailed tephrochronology for the last c. 900 years on Iceland can be
constructed with historical documents that date Icelandic eruptions (e.g., Larsen, Dugmore, &
Newton, 1999; Larsen & Eiriksson, 2008; Wastegard, 2002).
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