Sustained ash emission during lava effusion is a hidden volcanic hazard of silicic eruptions

Volcanic ash poses diverse and widespread hazards. Hybrid silicic volcanism - concurrent lava effusion and explosive ash venting - may constitute a significant but poorly quantified source of ash, so far only documented from a small number of eruptions, most notably the 2011-2012 eruption of Cordón Caulle (Chile). From that eruption, veneers of ash found welded on lava fractures have been attributed to the hybrid activity. Here, we report similar ash veneers from Holocene rhyolitic lavas at Torfajökull, Iceland, and Lipari, Italy. Focusing on the 877 CE Hrafntinnuhraun eruption of Torfajökull, we highlight that these veneers are widespread across its lavas, indicating that, similar to Cordón Caulle, Hrafntinnuhraun experienced long-lived ash venting alongside lava effusion. We develop a quantitative ash capture model based on textural complexities within the veneers, anda material and textural complexitiesrgue that hybrid activity is common to silicic volcanism and therefore represents a substantial and under-recognized hazard.