Grain characteristics of tephra from the subglacial SILK-LN Katla eruption ~3400 years ago and the subaerial Hekla eruption in 1947

Abstract

Chemical composition of the tephra from the Katla and Hekla volcanoes is similar but the eruption environment is quite different, the Katla volcano is ice-covered whereas the Hekla volcano is ice-free. The ∼3400 years old Katla SILK-LN and the Hekla 1947 tephra layers were studied with respect to spatial and temporal changes in grain size and their grain characteristics with main focus on the finer fraction of the tephra grains. The results show obvious difference both in mean grain size and the fraction of finest particles from the two volcanoes. The Hekla 1947 tephra has significantly higher mean grain size and much lower content of material finer than ≤4 Φ (0.063 mm). The grains from the Katla tephra are elongated and are even needle shaped, but the grains from the Hekla tephra are more equant. The silicic Katla eruptions produce greater quantities of ash ≤4 Φ than Hekla eruptions with similar magma composition and are therefore more likely to affect the human environment, including health of people and aviation.