The phreatomagmatic eruption at Aso volcano on October 8, 2016, released PDC and fallout up to 320 km from the vent, providing an opportunity to study the dispersal and sedimentation of tephra bearing wet and dry aggregates. We investigated fresh air-fall tephra and conducted both dry- and wet-sieving to assess the size distribution of both aggregate-bearing deposits on the ground and particles in the plume prior to aggregation. The tephra was asymmetrically distributed in the ENE direction, with a relatively wide distribution and small maximum grain sizes north of the depositional axis, reflecting different wind directions at each altitude. The secondary accumulation maxima at 4–8 km NE from the vent was characterized by the segregation of lapilli followed by accretionary pellets that were rapidly created by aggregation in the wet plume, thereby enhancing the removal of airborne ash. Coalesced aggregates possibly arose from salt precipitation processes. For dry aggregates fallen in medial-to-distal regions, observed deposition duration and fall velocity calculations concluded that fine ash was deposited not as individual particles but as aggregates. Most grain-size distributions along the dispersal axis based on the two sieving methods used were multimodal, with the coarser mode fining according to distance from the vent and the finer mode stabilizing at around 4 ϕ ; this is interpreted as resulting from the mixture of grain-size distributions with different origins, i.e., the normal settling of individual clasts and particle aggregation. The wet-sieved total grain-size distribution included 20.4 wt% fine ash compared with only 2.4 wt% fine ash in the dry-sieved distribution, suggesting that aggregated particles accounted for 18 wt% of the deposit. Total grain-size distributions based on wet-sieving showed extremely poor sorting that could be explained by the following causes: the release of coarse-grained ejecta originating from the host rock of the crater, fine particles due to fragmentation from a phreatomagmatic eruption, the supply of water and salt from the crater to the plume, and aggregation enhancement of the fine ash on the ground, reflecting the effect of the crater lake as external water source.

中文翻译：

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2016年10月8日阿苏火山喷发以来颗粒聚集对火山灰扩散和沉积的影响

2016 年 10 月 8 日阿苏火山的潜水岩浆喷发，在距喷口 320 公里处释放了 PDC 和沉降物，为研究含有干湿聚集体的火山灰的扩散和沉积提供了机会。我们调查了新鲜空气坠落的 tephra 并进行了干筛和湿筛，以评估聚集前地面上含有聚集体的沉积物和羽流中颗粒的尺寸分布。火山灰在 ENE 方向不对称分布，在沉积轴以北分布较广，最大粒度较小，反映了不同海拔高度的风向。距喷口东北 4-8 公里处的次生积累最大值的特征是 lapilli 的分离，然后是由湿羽流中的聚集迅速产生的增生颗粒，从而加强去除空气中的灰分。聚结的聚集体可能来自盐析过程。对于落在中间到远端区域的干骨料，观察到的沉积持续时间和下落速度计算得出结论，细灰不是作为单个颗粒而是作为聚集体沉积的。基于所使用的两种筛分方法，沿着分散轴的大多数粒度分布是多峰的，根据与通风口的距离细化较粗的模式，而较细的模式稳定在 4 φ 左右；这被解释为由不同来源的粒度分布混合引起的，即单个碎屑的正常沉降和颗粒聚集。湿筛分的总粒度分布包括 20.4 wt% 的细灰，而干筛分布中只有 2.4 wt% 的细灰，表明聚集颗粒占沉积物的 18 wt%。基于湿筛分的总粒度分布显示出极差的分选性，这可以用以下原因来解释：源自火山口主岩的粗粒喷射物的释放，由于水岩浆喷发破碎而产生的细颗粒，火山口向羽流供应水盐，地面细灰的聚集增强，体现了火山口湖作为外部水源的作用。