Kathy Cashman FRS (Bristol) / 2018 IGT Green Lecturer: Volcanic ash - how does it form, and how does it fall?

Professor Katherine Cashman FRS, University of Bristol

Of all rapid-onset natural hazards, only large volcanic eruptions have far-reaching and long-lasting impacts, as illustrated by the widespread disruption of European airspace by volcanic eruptions in Iceland because of concerns about ash damage to airplane engines. Given its importance for both hazard assessment and fundamental understanding of volcanic activity, however, there are still a number of things that we don’t know about volcanic ash. We do not know, in detail, how and where small ash particles form, how they interact with volcanic plumes, how quickly they are lost from volcanic clouds. These knowledge gaps have profound consequences for the accuracy of model forecasts of ash cloud concentrations in time and space; they also affect our very understanding of eruption processes. To address these gaps, we have been working on several aspects of volcanic ash studies, including (1) primary fragmentation during eruptions, (2) secondary fragmentation in pyroclastic density currents, (3) settling velocities of ash particles, (4) depositional patterns of far-travelled ash, and (5) resuspension of volcanic ash long after eruptions. A fundamental result of this work is documentation of the close connection between the conditions of magma ascent and eruption, the physical properties of resultant ash, and the timescales of ash transport, deposition, and remobilisation.