School of Geosciences, University of Edinburgh, Edinburgh, UK.
Department of Earth Sciences, University of Oregon, Eugene, OR, USA.
Nat Commun. 2023 Apr 12;14(1):2079. doi: 10.1038/s41467-023-37867-1.
Pyroclastic density currents (PDCs) are the most lethal volcanic process on Earth. Forecasting their inundation area is essential to mitigate their risk, but existing models are limited by our poor understanding of their dynamics. Here, we explore the role of evolving grain-size distribution in controlling the runout of the most common PDCs, known as block-and-ash flows (BAFs). Through a combination of theory, analysis of deposits and experiments of natural mixtures, we show that rapid changes of the grain-size distribution transported in BAFs result in the reduction of pore volume (compaction) within the first kilometres of their runout. We then use a multiphase flow model to show how the compressibility of granular mixtures leads to fragmentation-induced fluidisation (FIF) and excess pore-fluid pressure in BAFs. This process dominates the first ~2 km of their runout, where the effective friction coefficient is progressively reduced. Beyond that distance, transport is modulated by diffusion of the excess pore pressure. Fragmentation-induced fluidisation provides a physical basis to explain the decades-long use of low effective friction coefficients used in depth-averaged simulations required to match observed flow inundation.
火山碎屑密度流（PDC）是地球上最致命的火山过程。预测其淹没范围对于减轻其风险至关重要，但现有的模型受到我们对其动力学理解不足的限制。在这里，我们通过理论研究、沉积物分析和天然混合物实验，探讨了不断变化的粒度分布在控制最常见的火山碎屑密度流（即块状和灰烬流（BAFs））的流动范围方面的作用。结果表明，BAFs 中搬运的粒度分布的快速变化会导致在其流动的最初几公里内孔隙体积减少（压实）。然后，我们使用多相流模型来展示颗粒混合物的可压缩性如何导致 BAF 中的破碎诱导流态化（FIF）和过孔隙流体压力。该过程在其流动的最初约 2 公里内占主导地位，在此期间，有效摩擦系数逐渐降低。超过该距离后，扩散过孔隙压力会调节运输。破碎诱导流态化提供了一种物理基础，可以解释数十年来在深度平均模拟中使用的低有效摩擦系数，以匹配观察到的流动淹没。
