Estimating the frequency of volcanic ash clouds over northern Europe

- We compare Icelandic and European records of volcanic ash fallout over last 1000 years.
- We estimate the average return interval for ash fallout over northern Europe to be 44±7 years.
- Ash fallout over Europe is more common than previously proposed (average of 56±9 years).
- We estimate a 20% chance of an ash cloud over northern Europe in any 10 year period.
- Icelandic eruptions with a VEI ≥ 4 pose the greatest threat of ash fallout over northern Europe.

Fine ash produced during explosive volcanic eruptions can be dispersed over a vast area, where it poses a threat to aviation, human health and infrastructure. Here, we focus on northern Europe, which lies in the principal transport direction for volcanic ash from Iceland, one of the most active volcanic regions in the world. We interrogate existing and newly produced geological and written records of past ash fallout over northern Europe in the last 1000 years and estimate the mean return (repose) interval of a volcanic ash cloud over the region to be 44±7 years. We compare tephra records from mainland northern Europe, Great Britain, Ireland and the Faroe Islands, with records of proximal Icelandic volcanism and suggest that an Icelandic eruption with a Volcanic Explosivity Index rating (VEI) ≥ 4 and a silicic magma composition presents the greatest risk of producing volcanic ash that can reach northern Europe. None of the ash clouds in the European record which have a known source eruption are linked to a source eruption with VEI < 4. Our results suggest that ash clouds are more common over northern Europe than previously proposed and indicate the continued threat of ash deposition across northern Europe from eruptions of both Icelandic and North American volcanoes.