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<p>The Ito pyroclastic flow deposits produced by the ~ 25&#xa0;ka Aira Caldera eruption are characterised by broad multimodal particle size distributions reflecting the combined effects of fragmentation, transport, and deposition. Although the Ito deposits have been used to infer pyroclastic flow emplacement mechanisms, the conventional grain size characterisation methods applied in previous studies rely mainly on bulk measurements obtained using sieve-sedimentation or laser diffraction techniques, which typically assume a unimodal log-normal distribution. Therefore, such methods cannot adequately resolve heterogeneous particle populations. In particular, the fine fraction may preserve important but under-recognised selective transport and sedimentation signatures during emplacement. To address the limitations of conventional methods, we applied dry static mode automated particle image analysis (DSM-APIA) to the fine fraction (≤ 100&#xa0;μm) of the Ito pyroclastic flow deposits to obtain integrated size and shape characterisations. This analysis revealed a statistically distinct size-based subpopulation that was not resolved by conventional methods, thereby indicating that the fine fraction is not a single hydraulically equivalent population. Size-dependent analyses of the circularity and aspect ratio exhibited nonlinear trends, highlighting previously unrecognised morphological heterogeneity. These variations likely reflect transport-induced modifications and intrinsic compositional differences (e.g. glass shards versus lithic fragments). This study establishes a robust high-resolution analytical framework that complements conventional bulk analyses, providing a reproducible tool for evaluating complex morphological heterogeneities in pyroclastic materials.</p>

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Grain size and shape characterisation of the fine-grained Ito pyroclastic flow deposits using particle image analysis: a methodological framework for heterogeneous populations

  • 期刊论文

  • 发表日期：

    2026年7月6日

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  • Daisuke Sasakura,
  • Aiko Hayauchi,
  • Sho Kimura,
  • Shinya Nakamura

摘要

The Ito pyroclastic flow deposits produced by the ~ 25 ka Aira Caldera eruption are characterised by broad multimodal particle size distributions reflecting the combined effects of fragmentation, transport, and deposition. Although the Ito deposits have been used to infer pyroclastic flow emplacement mechanisms, the conventional grain size characterisation methods applied in previous studies rely mainly on bulk measurements obtained using sieve-sedimentation or laser diffraction techniques, which typically assume a unimodal log-normal distribution. Therefore, such methods cannot adequately resolve heterogeneous particle populations. In particular, the fine fraction may preserve important but under-recognised selective transport and sedimentation signatures during emplacement. To address the limitations of conventional methods, we applied dry static mode automated particle image analysis (DSM-APIA) to the fine fraction (≤ 100 μm) of the Ito pyroclastic flow deposits to obtain integrated size and shape characterisations. This analysis revealed a statistically distinct size-based subpopulation that was not resolved by conventional methods, thereby indicating that the fine fraction is not a single hydraulically equivalent population. Size-dependent analyses of the circularity and aspect ratio exhibited nonlinear trends, highlighting previously unrecognised morphological heterogeneity. These variations likely reflect transport-induced modifications and intrinsic compositional differences (e.g. glass shards versus lithic fragments). This study establishes a robust high-resolution analytical framework that complements conventional bulk analyses, providing a reproducible tool for evaluating complex morphological heterogeneities in pyroclastic materials.