Tephrochronology is a dating technique based on the identification and correlation of deposits of volcanic ejecta, (volcanic ash or pumice, all known as tephra) (Thórarinsson 1944, 1981). In practice in Scotland this means either atmospheric fallout of particles <200 microns in size, or cobble-grade piece of pumice dispersed over the ocean by wind and waves; virtually all Scottish tephra deposits originate in Iceland (Dugmore 1991; Dugmore et al. 1995b). Dating does not take place on the tephra itself, but on the eruption that produced it. As a result it is the correlation of a tephra to its source eruption enables the dating of that event to be applied to tephra wherever it is found (Dugmore and Newton 2009). Dating may be derived from a number of sources such as written records, ice core dates, sediment accumulation rates and radiocarbon. Correlation of tephra deposits generally relies on accurate grain specific chemical analysis of major and minor elements.
Firmly identified tephra deposits have the potential to define an isochron, or horizon of equal age. This may be very precisely defined even if the absolute age of the tephra is unknown. The best isochrons in Scotland are formed by in situ deposits from atmospheric fallout that were originally deposited within a matter of few hours or days (Dugmore et al. 1995a). Pumice in Scotland is rarely found in situ in natural contexts partly due to sea level changes, but it does occur in a range of archaeological contexts that are not contemporaneous with the original, pumice-forming eruptions. Since the first discovery and identification of Icelandic volcanic ash in microscopic amounts (cryptotephras) in Scotland (Dugmore 1989), tephrochronology (Lowe 2010) has become a standard palaeoenvironmental tool in Scotland. Tephra layers have been identified in Scotland’s peat deposits and lake sediments dating from the late glacial until the eruption of Hekla in 1947 (Dugmore et al. 1995b; Turney et al. 1997; Lowe et al. 2008; Housely et al. 2010). Recent events, such as the 2010 eruption of Eyjafjallajökull, have highlighted the importance of understanding the distribution of volcanic ash deposits, but the tephra preserved in Scotland’s peat and lake deposits contributes directly to science-based archaeology. The presence of unambiguously identified and dated tephra layers can provide a crucial test of other chronological methods (e.g. Dugmore et al. 1995 a and b), as well as providing otherwise unobtainable precise dating of palaeoenvironmental and proxy climate records (e.g. Langdon and Barker 2004). This allows precise correlations to be made between high resolution palaeoenvironmental records within Scotland, across Greenland, the North Atlantic and north-west Europe.
Scotland also has excellent facilities to geochemically analyse volcanic ash. The Natural Environmental Research Council Tephra Analytical Unit (http://www.geos.ed.ac.uk/facilities/tephra/) (electron and ion microprobes) is based in the School of GeoSciences at the University of Edinburgh (School of GeoSciences 2010).
It is through the definition of isochrons and the precise correlation of environmental records and chronology that tephra allows, that the greatest potential contribution to Scottish archaeology by this means can be expected. While the presence of more than a dozen tephras can add spot dates of great utility to long-term records (such as 1510 AD), it is the use of precisely-defined isochrons that lie across most of Scotland that holds the greatest future potential. Spatial patterns and their changes through time are crucially important to our understanding of the past, particular during periods of rapid cultural or environmental change and tephrochronology can enable correlations to be made to within a year. This can add a spatial dimension of great utility to other very precise records such as dendrochronology.