Grimsvötn in eruption
Tephra on Shetland
Definition: fragments of volcanic rock and lava of any size expelled from a volcano. The long distance of travel from Icelandic volcanoes means that only ash and pumice have reached Scotland in the past.
Shetland lies 700 kilometres south-east of the major volcanic centres of Iceland. There are historic records of ash falls in Shetland from Icelandic volcanic eruptions, for example, associated with the 1755 eruption of the volcano Katla. Over the last 15 years detailed analyses of lake sediments on Shetland have revealed thin bands of tephra. These ash bands are invisible to the naked eye but visible under the microscope and can be traced quickly using their magnetic signature. Using ion microprobe analysis the chemistry of individual shards of tephra can be established As many Icelandic eruptions have a characteristic geochemical signature it is thereby possible to link the geochemistry of the ash on Shetland to its source volcano on Iceland.
These tephra bands provide precise time markers within sequences of peat and organic lake sediments. The presence of tephra greatly improves correlations between sites and allows environmental changes to be pinpointed in time. The role of major ash falls on vegetation and human history can also be assessed.
A major ash fall around 10,600 years ago is named the Vedde Ash - but this ash has yet to be discovered on Shetland. Its presence in sediments sequences in western Norway and along the coast of western Scotland suggests that it must be present on Shetland. Analyses of lake sediments at Gunnister and Dallican Water on Shetland have revealed younger ash falls (Bennet et al, 1992). The most clearly defined is a large fall named after its type locality on Faeroe at Saksunarvatn which dates to around 9300 to 9000 BP. Younger acidic ash falls at Gunnister date from around 4000 BP and may be part of the Hekla 4 ash zone. This fall is recognised at Kebister, together with a younger tephra (Dugmore et al.,1995).