Abstract Dilute pyroclastic density currents (dilute PDCs) are frequent and highly dangerous flows of hot gas and particles occurring at explosive volcanoes. The study and interpretation of the sedimentary characteristics of their associated deposits is one of the most important approaches to better understand these violent phenomena and to characterise their dynamics, frequency and magnitude in the geological record of volcanoes. Current strategies are based on sediment transport principles developed for fluvial and aeolian systems. How well these analogies capture the sediment transport behaviour of dilute PDCs remains poorly understood, as the hot and hostile conditions of these fast-moving volcanic flows hamper direct measurements. Here we report the results of large-scale experiments that aim to synthesise the behaviour of dilute PDCs in order to investigate the transport and sedimentation processes inside the hot flows. These flows reproduce the spatiotemporal deposit facies variations seen in natural deposits of dilute PDCs. Through measurements of the evolving flow structure (velocity, particle concentration and flow grain size distribution), we show that the lower region of the density stratified current can be subdivided from the aggrading deposit upwards into a dynamic bedload region with particle concentrations of c. 0.5 to several vol%, a transient region with particle concentrations of c. 0.1–1.5 vol%, a dilute, fully turbulent region with formation of mesoscale clusters, and an upper dilute turbulent region absent of mesoscale clusters. We show that the particle feeding mechanisms of the transient region is related to the occurrence of mesoscale clusters. This process has a key role in modifying the sediment transport modes inside the bedload region. These modifications cause a variation of the dynamics of the lower flow boundary, including erosion/deposition events, and variations of the deposition rate over at least three orders of magnitude. We present images and video footage of these processes that include the formation of shifting sandwaves, rolling and saltation of particles inside the experimental dilute PDC, and analyse how these relate to the development of massive, stratified and laminated deposit structures. Through mapping the isochrones of deposition across the flow length and measuring the propagating flow above, we capture a spatiotemporal view of the aggrading deposit, which can guide the interpretation of natural deposits.

中文翻译：

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实验性稀火山碎屑密度流中沉积物的时空迁移和沉积过程

摘要 稀火山碎屑密度流（稀 PDC）是发生在爆炸性火山中的高温气体和颗粒的频繁且高度危险的流动。对其伴生矿床的沉积特征的研究和解释是更好地了解这些剧烈现象并在火山地质记录中表征其动力学、频率和幅度的最重要方法之一。当前的策略基于为河流和风成系统开发的沉积物输送原理。这些类比如何很好地捕捉稀释 PDC 的沉积物运输行为仍然知之甚少，因为这些快速移动的火山流的炎热和恶劣条件阻碍了直接测量。在这里，我们报告了大规模实验的结果，这些实验旨在综合稀释 PDC 的行为，以研究热流内部的传输和沉积过程。这些流动再现了在稀释的 PDC 的天然沉积物中看到的时空沉积相变化。通过对不断变化的流动结构（速度、颗粒浓度和流动粒度分布）的测量，我们表明密度分层流的下部区域可以从堆积的沉积物向上细分为颗粒浓度为 c 的动态床负载区域。0.5 到几个 vol%，粒子浓度为 c 的瞬态区域。0.1–1.5 vol%，一个稀释的、完全湍流的区域，形成了中尺度簇，以及一个没有中尺度簇的上层稀释的湍流区域。我们表明瞬态区域的粒子馈送机制与中尺度簇的发生有关。该过程在改变底床区域内的沉积物输送模式方面具有关键作用。这些修改导致较低流动边界的动态变化，包括侵蚀/沉积事件，以及至少三个数量级的沉积速率变化。我们展示了这些过程的图像和视频片段，包括在实验性稀释 PDC 内形成移动的沙波、粒子的滚动和跃迁，并分析这些过程与大规模、分层和层状沉积结构的发展之间的关系。通过绘制整个流长的沉积等时线并测量上面的传播流，