Ash Fingers Modify Tephra Sedimentation During a Coarse-Ash-Dominated Eruption: Evidence from the 23 July 2024 Etna Paroxysm

Abstract. Ash fingers are descending particle-laden plumes that develop beneath volcanic clouds and have been observed during several explosive eruptions worldwide. By promoting collective settling, they modify the atmospheric transport and deposition of volcanic ash and remain a significant source of uncertainty in tephra-dispersal forecasting. However, direct measurements of ash-finger properties are rare, and their depositional signature remains poorly constrained. Here, we investigate ash fingers generated during the 23 July 2024 paroxysmal eruption of Mt. Etna (Italy) using high-resolution visible-wavelength video observations, detailed deposit characterization from ground-based and drone-collected samples, thin-section analyses, disdrometer observations and numerical simulations. The resulting deposit is dominated by coarse ash and shows no evidence of aggregation, providing a unique opportunity to isolate the influence of ash fingers on sedimentation processes. Our observations demonstrate that ash fingers can develop in the absence of abundant fine ash (<63 µm), challenging the commonly held assumption that fine particles are required for their formation. Comparison between observed ash-finger descent velocities and settling velocities calculated for irregular particles indicates that ash fingers were capable of entraining particles up to ~950 µm in diameter. This threshold is highly sensitive to particle shape, highlighting the importance of non-spherical settling behaviour in controlling ash-finger transport. Particle concentration also appears to be a primary control on finger formation, as ash fingers were observed only during phases characterized by high mass eruption rates (MER > 2.5 × 10⁵ kg s⁻¹). Deposit analyses show that ash fingers locally modified proximal sedimentation by increasing the proportion of coarse ash and shifting grain-size distributions toward finer values than expected from individual particle settling. In contrast, simulations of TEPHRA 2, a tephra dispersion model, predict only minor differences between scenarios with and without ash-finger settling. This limited sensitivity could arise because the model assumes spherical particles, leading to an underestimation of the grain-size fraction susceptible to ash-finger entrainment. The discrepancy highlights a fundamental limitation of standard dispersal models based on the assumption of individual settling of spherical particles and suggests that particle morphology must be explicitly considered to realistically represent ash-finger transport and deposition. These findings provide new constraints on ash-finger dynamics and demonstrate that particle shape exerts a first-order control on the interaction between collective sedimentation processes and tephra deposition, with important implications for volcanic ash forecasting and hazard assessment.