The Hoftorfa tephra: a 6th Century tephra layer from Eyjafjallajökull

Abstract SÞ3-5 from Nordic Geological Winter Meeting 2012.

Kate Smith, Andrew Dugmore, Kerry-Anne Mairs, Thorvaldur
Thordarson, Costanza Bonadonna, Guðrún Larsen, Anthony

Eyjafjallajökull tephra was brought to worldwide attention and put under detailed scientific scrutiny with the eruptions of 2010. Previous historical eruptions have been studied in some detail also, but until recently few details have been known about prehistoric explosive activity. Tephrochronological investigations have shown that explosive activity from the central crater of Eyjafjallajökull in the mid-6th century AD produced a distinctive tephra deposit, the Hoftorfa tephra (also previously known as Layer H or E500). This paper describes this tephra layer, it’s distribution and volume, based on over 100 profiles from south

The Eyjafjallajökull Hoftorfa eruption produced a pale white to yellow, silicic tephra that is thickest on the northern flanks of the volcano. The tephra deposit generally has a matrix of medium to fine grained ash supporting coarse grained ash to lapilli and in some locations bedding and grading are apparent. The extent of the tephra is similar to the Eyjafjallajökull 1821-3 eruption though it is somewhat greater in volume and is considerably thicker on the northern flanks. It is significantly smaller in volume and area than the 2010 Eyjafjallajökull tephra. Hoftorfa (the bedrock outcrop anchoring the north east margin of the great terminal moraine of Gígjökull) is proposed as the type site for this tephra layer being close to its thickest point and easily accessible for further study, following Dugmore (1987; 45).

Deposits have only been found within 20 km of the crater, but being highly visually distinctive, the Eyjafjallajökull Hoftorfa tephra layer still forms a very useful key marker horizon in the late prehistoric geological record, covering at least 600 km2 including the forelands of 8 major outlet glaciers from two separate icecaps.

Despite being of a limited volume and visible extent this tephra is important because its identification extends the record of known activity in the central crater of Eyjafjallajökull from the 17th century AD to the 6th century AD. It shows that the style of activity observed in 1821-3 also occurred some 1300 years previously; moreover, the same stratigraphic sections that have been used to identify this tephra also show that no similar events have taken place in the last c.7000 years.