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.
Orkney lies 900km down wind from the active Icelandic volcanic centres. 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 Orkney (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 on Hoy 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 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.