Syneruptive sequential fragmentation of pyroclasts from fractal modeling of grain size distributions of fall deposits: The Cretaio Tephra eruption (Ischia Island, Italy)

In this work we used fractal statistics in order to decipher the mechanisms acting during explosive volcanic eruptions by studying the grain size distribution (GSD) of natural pyroclastic-fall deposits. The method was applied to lithic-rich proximal deposits from a stratigraphic section of the Cretaio Tephra eruption (Ischia Island, Italy). Analyses were performed separately on bulk material, juvenile, and lithic fraction from each pyroclastic layer. Results highlight that the bulk material is characterized by a single scaling regime whereas two scaling regimes, with contrasting power-law exponents, are observed for the juvenile and the lithic fractions. On the basis of these results, we infer that the bulk material cannot be considered as a good proxy for deducing eruption dynamics because it is the result of mixing of fragments belonging to the lithic and juvenile fraction, both of which underwent different events of fragmentation governed by different mechanisms. In addition, results from fractal analyses of the lithic fraction suggest that it likely experienced a fragmentation eventin which the efficiency of fragmentation was larger for the coarser fragments relative to the finer ones. On the contrary, we interpret the different scaling regimes observed for the juvenile fraction as due to sequential events of fragmentation in the conduit, possibly enhanced by the presence of lithic fragments in the eruptive mixture. In particular, collisional events generated increasing amounts of finer particles modifying the original juvenile GSDs and determining the development of two scaling regimes in which the finer fragments record a higher efficiency of fragmentation relative to the coarser ones. We further suggest that in lithic-rich proximal fall deposits possible indications about the original GSDs of the juvenile fraction might still reside in the coarser particles fraction.

💡 Research Summary

The paper presents a novel application of fractal statistics to decipher the fragmentation mechanisms operating during explosive volcanic eruptions, focusing on the grain‑size distribution (GSD) of pyroclastic‑fall deposits from the Cretaio Tephra eruption on Ischia Island, Italy. The authors collected a stratigraphic section of proximal, lithic‑rich deposits and separated each layer into three distinct components: bulk material (the whole sample), juvenile fragments (fresh magma‑derived particles), and lithic fragments (pre‑existing rock clasts). Grain‑size analyses were performed using sieving and laser diffraction, and the cumulative size distributions were plotted on log‑log axes to identify scaling regimes.

In fractal analysis, the cumulative number of particles larger than a given diameter d follows N(>d) ∝ d⁻ᴰ, where D is the fractal dimension. The slope of a linear segment on the log‑log plot yields a power‑law exponent β, which reflects the efficiency of fragmentation within that size range. The bulk material displayed a single, uninterrupted scaling regime, indicating a uniform β value across the entire size spectrum. By contrast, both the juvenile and lithic fractions exhibited two distinct scaling regimes, each characterized by a different β exponent.

For the lithic fraction, the coarse‑size regime showed a positive β (β₁ > 0), implying that larger clasts were preferentially broken, while the fine‑size regime exhibited a negative β (β₂ < 0), indicating a relative preservation of finer particles. This pattern suggests a size‑dependent fragmentation efficiency: mechanical processes such as collisions and shear preferentially reduce the volume of larger lithic fragments, whereas smaller clasts experience less efficient breakage.

The juvenile fraction revealed a different behavior. The coarse‑size regime possessed a relatively low or near‑zero β, whereas the fine‑size regime displayed a markedly higher positive β. The authors interpret this as evidence for sequential fragmentation within the conduit. Initially, juvenile magma fragments undergo a primary breakup that generates a relatively coarse distribution with modest fragmentation efficiency. As the eruption proceeds, repeated collisional interactions—enhanced by the presence of lithic debris—produce a secondary fragmentation wave that preferentially creates finer particles, thereby raising the fragmentation efficiency for the fine fraction. This dual‑regime signature is taken as a fingerprint of ongoing, conduit‑scale re‑fragmentation rather than a single, instantaneous breakage event.

The key implication is that bulk GSDs, being mixtures of juvenile and lithic fragments that have experienced different fragmentation histories, cannot reliably serve as proxies for eruption dynamics. Instead, separating the components allows the distinct physical processes to be resolved. Moreover, the authors argue that in lithic‑rich proximal deposits, the original juvenile GSD may still be preserved in the coarser portion of the juvenile fraction, offering a window into the primary fragmentation conditions.

Overall, the study demonstrates that fractal analysis provides a quantitative framework for distinguishing multiple fragmentation episodes and for assessing the relative contribution of mechanical versus magmatic processes in shaping pyroclastic deposits. It underscores the necessity of component‑specific grain‑size investigations when reconstructing eruption dynamics and suggests that similar approaches could be applied to a broad range of volcanic settings to improve hazard assessments and eruption modeling.