Ice-kept tephra from sub-glacial eruptions

The sub-arctic island Iceland is formed by volcanic eruptions, many of which take place beneath glaciers. The largest glacier in Europe covers four active volcanic systems, namely Öræfajökull, Kverkfjöll, Bárðarbunga and Grímsvötn and their eruptions have formed widespread tephra layers. Tephra, the term for air-borne volcanic material such as volcanic ash, is formed when hot magma is fragmented due to contact with ice-cold glacier melt-water. Tephra layers are thickest close to the subglacial eruption site and after each eruption buried in the glacier by snow precipitation in the following winter. The snow-accumulation area on the glacier, therefore, generates a record of the eruption history of the glacier-covered volcanoes. We propose to sample, from the ablation areas on Vatnajökull, newly exposed glacier-kept tephra layers and trace their origin by geochemical analysis to the different volcanoes, all of which produce magma of distinctive composition. The oldest known tephra dates back to the 12th century allowing time-series for the last eight centuries to be generated. The compositional variations with time should reveal not only eruption frequency of each subglacial volcano but also the behaviour of the magma systems at depth. Magmatic timescales at depth will be derived from diffusional chronometry and atmospheric loading of volatile elements such as S, Cl and F as a function of time will be estimated.