Collaborative Research: A Closer Look at the May 18th, 1980 Pumice Plain Deposits: Implications for Assessing Eruptive Conditions and Pyroclastic Density Current Dynamics

火山碎屑密度流，简称PDCs，是高温气体和火山碎屑物质的混合物，紧贴地面，沿着火山两侧高速传播。虽然PDCs是一些最危险的火山现象，但它们的研究具有挑战性，因为喷发期间产生的大量灰烬阻碍了对内部流动动力学的视觉观察。因此，对PDC动力学的理解大部分来自于对其产生的沉积物的分析。1980年5月18日圣海伦火山的喷发对世界对爆炸性喷发的认识和理解产生了重大影响，对这次喷发期间产生的喷发产物的研究有助于我们对许多其他喷发沉积物的解释。早期对这次喷发期间产生的PDCs的研究与喷发强度和行为的变化有关。然而，从那时起，深切口排水提供了新的、广泛的暴露，其中包含了产生它们的水流的重要信息，从而可以对这些沉积物进行更全面的研究。该项目代表了一种多学科方法，可以更好地了解有记录的火山喷发之一的PDC动力学。计划将质量通量的估计、最近暴露地层的详细测量（包括探地雷达研究）和多相数值模拟技术结合起来，以更好地约束5月18日产生的pdc的动力学。特别地，该提案侧重于关于PDCs运输的三个问题。1)流动强度和浓度与岩屑输运能力之间的关系，以及这如何导致离源距离的电流偏析和密度分层？许多PDC矿床的一个引人注目的观察结果是非常大（1米）的浮石和岩屑碎屑嵌入在细粒基质中。新的多相数值技术的出现可以更好地计算从喷口到最终沉积的粒子轨迹。这些信息将与对最近暴露的矿床的粒度分布和结构的仔细分析相结合，以更好地解释运输这些大型岩石所需的条件。2) PDC输运过程中有多少碎屑粉碎，这对后续动力学有何影响？当火山颗粒相互碰撞或滑动时，它们往往会破裂或粉碎。这一过程的累积效应是使大颗粒变圆，并在电流中产生更高比例的细灰。早期的模型表明，这种较高的细灰比例可以在这些流动中产生紧急动力学，从而导致更大的跳动距离。单位和源条件之间的详细相关性使圣海伦斯山成为详细圆度研究的绝佳候选者，这反过来将获得这些流动中小规模动量转移的信息。3)邮储物流的自通道化如何影响运输距离？据推测，在新近沉积的未固结火山碎屑容易侵蚀的帮助下，自河道化在圣海伦斯火山一些最大的pdc中发挥了作用。改进的新暴露的沟槽截面图、探地雷达和多相模拟将提供St. Helens PDC自通道化和流动反馈的进一步信息。综上所述，对圣海伦斯火山的模拟与野外联合研究将提高对pdc中运移和沉积的总体认识。这项工作的结果将提高我们的知识和能力，以评估与其他地方类似的爆炸性喷发中PDCs相关的危害，包括相对跳动距离和横向运输过程中产生的细灰的量化，这与航空危害评估和潜在的气候影响具有重要的相关性。

Pyroclastic density currents, PDCs, are ground-hugging mixtures of hot gases and pyroclastic material that propagate at high velocities down the flanks of volcanoes. While PDCs are some the most hazardous volcanic phenomenon, they are challenging to study because the voluminous amounts of ash produced during an eruption hinders visual observation of the interior flow dynamics. Consequently much of the understanding of PDC dynamics comes from analysis of the deposits they produce. The May 18, 1980 eruption at Mount St. Helens has had a significant impact on the world's awareness and understanding of explosive eruptions, and study of the eruptive products produced during this eruption has aided our interpretation of deposits from many other eruptions. Earlier studies of the PDCs produced during this eruption were correlated with changes in eruptive intensity and behavior. Since then, however, deeply incised drainages have provided new, extensive exposures that contain important information about the currents that produced them, allowing for a more complete study of these deposits to take place. This project represents a multidisciplinary approach to better understand PDC dynamics at one of the best-documented eruptions. It is planned to combine the estimates of mass flux, detailed measurements of recently exposed strata, including ground penetrating radar studies, and multiphase numerical modeling techniques to better constrain the dynamics of the PDCs produced on May 18th. In particular, this proposal focuses on three questions regarding the transport of PDCs. 1) How is flow intensity and concentration related to the transport capacity of lithic clasts, and how does this lead to current segregation and density stratification with distance from source? One of the striking observations of many PDC deposits is the very large ( 1 m) pumice and lithic clasts that are embedded within the finer-grained matrix. The advent of new multiphase numerical techniques allows for a better accounting of particle trajectories from the vent to their eventual deposition. This information will be combined with a careful analysis of the grain size distribution and architecture of recently exposed deposits to better explain the conditions necessary to transport these large lithics. 2) How much clast comminution occurs during PDC transport, and how does this influence subsequent dynamics? When volcanic particles collide or slide past each other they tend to break up, or comminute. The cumulative effect of this process is rounding of large particles and the production of a higher percentage of fine ash in the current. Early models suggest that this higher fine ash proportion can generate emergent dynamics in these flows, resulting in greater runout distances. Detailed correlation between the units and source conditions make Mount St. Helens an excellent candidate for detailed roundness studies, which in turn will yield information about the small-scale momentum transfer in these flows. 3) How does the self-channelization of PDCs influence transport distance? It is hypothesized that self-channelization, aided by the ease of the erosion of recently deposited unconsolidated pyroclasts, played a role in some of the largest Mount St. Helens PDCs. Improved maps of newly exposed gully cross-sections, ground-penetrating radar, and multiphase simulations will provide further information on St. Helens PDC self-channelization and flow feedback. In summary, the combined modeling-field study of Mount St. Helens will improve the general understanding of transport and deposition in PDCs. The results of this work will advance our knowledge and ability to assess the hazards related to PDCs in similarly explosive eruptions elsewhere, including relative runout distance and quantification of fine ash produced during lateral transport, which has significant relevance aviation hazard assessment and potential climactic effects.