Timing and nature of volcanic particle clusters based on field and numerical investigations

Abstract

Aggregation processes are known to play an important role in volcanic particle dispersal and sedimentation. They are also a primary source of uncertainty in ash dispersal forecasting since fundamental questions, such as the timing and deposition dynamics of volcanic aggregates, still remain unanswered. Here, we applied a state-of-the-art combination of field and numerical strategies to characterize volcanic aggregates. We introduce a new category of aggregates observed with high-speed-high-resolution videos, namely cored clusters. Cored clusters are mostly sub-spherical fragile aggregates that have never been observed in the deposits nor on adhesive tape as they typically break at impact with the ground. They consist of a core particle (200–500μm) fully covered by a thick shell of particles < 90μm. The low preservation potential of cored clusters in ash deposits explains the poor documentation in the literature and the low consideration attributed so far. Cored clusters can also better explain the deposition of fine ash in proximal and medial regions and the polymodality observed in many ash deposits. In addition, numerical inversions show how cored clusters can rapidly form within 175s from eruption onset. Finally, our observations represent the first field-based evidence of the so-called rafting effect, in which the sedimentation of coarse ash in cored clusters is delayed due to aggregation

The importance of grain size and shape in controlling the dispersion of the Vedde cryptotephra

“…The influence of atmospheric velocity partly explains why grains of a single size fraction () can deposit over a range of distances (Fig. ); this is also due to the vertical spread of ash in the plume and spatial variation in depositional processes such as removal by precipitation (Webster and Thomson, ), aggregation (Bagheri et al ., ), topographic effects (Watt et al ., ) and gravitational instabilities in the proximal ash cloud (Manzella et al ., ). Aggregation causes both early fallout of fine particles and delayed sedimentation of larger particles due to coating with finer particles, forming low‐density composites in a process known as ‘rafting’ (Bagheri et al ., ).…”

“…); this is also due to the vertical spread of ash in the plume and spatial variation in depositional processes such as removal by precipitation (Webster and Thomson, ), aggregation (Bagheri et al ., ), topographic effects (Watt et al ., ) and gravitational instabilities in the proximal ash cloud (Manzella et al ., ). Aggregation causes both early fallout of fine particles and delayed sedimentation of larger particles due to coating with finer particles, forming low‐density composites in a process known as ‘rafting’ (Bagheri et al ., ). All these factors may explain the poor correlation between modal cryptotephra shard size and distance (Watson et al ., ), and explain why a range of ash particle sizes are found in any given sample location.…”

The Effect of Wind and Atmospheric Stability on the Morphology of Volcanic Plumes From Vulcanian Eruptions

“…All available options were used aside from particle aggregation. Although aggregation is known to be an important process in Sakurajima (Bagheri et al, ; Gilbert & Lane, ) and the total grain size distribution (TGSD) of each eruption is known to impact plume height (Macedonio et al, ; Michaud‐Dubuy et al, ), owing to the lack of concrete data and the large span of the study period, the TGSD used is the one assigned to Sakurajima by Mastin et al (), that is, 0.24, 0.25, 0.2, 0.12, 0.09, 0.0425, 0.0325, 0.0125, 0.0075, and 0.005 for ϕ ranging between −1 and 8 ( in millimeters).…”

Statistical analysis of dispersal and deposition patterns of volcanic emissions from Mt. Sakurajima, Japan

“…Proximal to the vent, ash fallout is also heavily affected by volcanological effects such as changes in the eruption dynamics and sedimentation regime (Bonadonna and Costa, 2013), ash aggregation (i.e. the joining of airborne ash particles) within the volcanic plume that affects the total GSD of the ash (Cornell et al, 1983;Bonadonna and Houghton, 2005;Sulpizio et al, 2012) and can both enhance the sedimentation of light ash (Brown et al, 2012) and impede the sedimentation of heavier ash via rafting (Bagheri et al, 2016), as well as downwards propagating instabilities that arise from local differences in ash concentration (Manzella et al, 2015;Scollo et al, 2017). These effects act in tandem with the meteorological fields and topography, with the final ash fallout patterns decided by the complex interplay of all these factors.…”