20-EPN-078: Effect of disperse grain size distributions on the aeolian remobilisation of volcanic ash
June 3, 2023

20-EPN-078: Effect of disperse grain size distributions on the aeolian remobilisation of volcanic ash

Visit by Allen Fries, University of Geneva (Switzerland) to TA2.4 Planetary Environment Facilities (PEF), AU (Denmark)
Dates of visit: 13-17 June 2022

Report summary: Thanks to the collaboration between the University of Geneva and the University of Aarhus through the Europlanet Research program we performed a set of experiments on the remobilisation of volcanic particles. Since removal processes of volcanic particles are relatively poorly characterised, these experiments represent a unique opportunity to study the dynamics of aeolian processes. The obtained results provide valuable information on the threshold friction velocities (i.e. wind friction velocity above which particles start to detach from deposits) of different ash compositions, fundamental for modelling and forecasting remobilisation events.

A total of 32 experiments were performed by using a setup composed of a sample plate (i.e., bed of volcanic ash exposed to gradually increasing wind friction velocities) from which particles were removed by wind and imaged using various techniques simultaneously (i.e. microscopes, and webcam)). These direct observations were combined with multiple particle collection methods to study the characteristics of remobilised particles (i.e. sediment traps and adhesive papers). A complete set of half-phi grainsize classes (from 0 to 500 μm) from 4 volcanoes were analysed. 

Preliminary results show a relation between the threshold friction velocity and the grainsize, in agreement with erosion theories. In addition, these experiments illustrate variations in threshold friction velocities as a function of magma composition: lighter particles (i.e. rhyolite) are easier to remobilise than denser particles (i.e. basalt). These results are pioneering, systematically quantifying threshold friction velocities of volcanic ash for wide grainsize and composition ranges for the first time. 

Read the full scientific report, with kind permission from Allen Fries.