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.

tephra shard

Grímsvötn is Icelands most active volcano and lies under the ice-cap of Vatnajökull glacier.

Tephrabase
Ocean-transported pumice

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. It coincides with the arrival of Corylus avellana in the islands at ~ 9,200 BP  Bunting (1994). The Hekla 4 tephra has been identified on Hoy, together with other tephra layer on unknown age (Dugmore et al.,1995).

Orkney Landscapes

  • The rocks of Orkney are dominated by flagstones and sandstones deposited in a huge fresh water lake. They belong in time to the Devonian (Old Red Sandstone) period 416 – 359 million years ago). The sediments of Lake Orcadie are superbly exposed along the many cliffs and shore platforms and so Orkney gives us one of the best examples of a Devonian lake basin in the world. ...

    Read Article
  • The landforms of Orkney are special not only in terms of the magnificent diversity and quality of the island scenery. This is a sandstone landscape, with features similar to those found in many other parts of the world where sedimentary rocks lie just below the surface. And an Ice Age landscape, where glaciers cut corries and shelly boulder clay was dredged from the bed of the sea by ice sheets. Then there is the magnificent coast, retreating from Atlantic and North Sea storms but slowly building in sheltered firths. Orkney is all the while changing and fascinating.

    Read Article