tephrabase

ocean-transported pumice

tephra shard

Grimsvötn in eruption

Tephra in Caithness

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.

Caithness lies 900km down wind from the active Icelandic volcanic centres and evidence of ash falls was noted as early as 1896 by Geikie. Hekla, in particular, intermittently puts vast amounts of volcanic ash into the upper atmosphere. Some of these ash fragments (tephra) are distinguished in the peat deposits of Caithness (Hekla 5 about 4400 BC, Hekla 4 in 2340 BC and Hekla 3 in 1140 BC).

Over the last 20 years detailed analyses of peat bogs have revealed thin bands of tephra. The Flows of Caithness have provided several important sites - Slethill, Loch Leer, Altnabreac and Glen Na Beiste. The 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 possible to link the geochemistry of the ash on Orkney to its source volcano on Iceland. These signature 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 the North Atlantic 10,600 years ago is named the Vedde Ash - but this ash has yet to be discovered on Orkney. The most clearly defined tephra is a large fall named after its type locality on Faeroe at Saksunarvatn which dates to around 9300 to 9000 BP. The Glen Garry (~2100 yr BP) and Kebister (3600-3800 yr BP) are represented in several cores from Caithness (Dugmore et al., 1995). High-resolution, millimeter-scale pollen, charcoal and tephra data from Loch Leir in Caithness show a stratigraphic link between the decline of Pinus sylvestris pollen and the deposition of the Hekla 4 tephra (Blackford et al. (1992). The implication is that significant ecological changes occurred in this area of Scotland at about the time of the deposition of the tephra.