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(PDC)是热气体和火山碎屑物质的混合物，它们沿着火山侧翼高速传播。虽然火山喷发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火山灰阻碍了对内部流动动力学的目测观察。因此，对PDC动力学的大部分了解都来自于对它们所产生的矿床的分析。1980年5月18日的圣海伦斯火山喷发对世界对爆炸性喷发的认识和理解产生了重大影响，对这次喷发产生的喷发产物的研究有助于我们对许多其他喷发的沉积的解释。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，深度切割的排水沟提供了新的、广泛的暴露，其中包含有关产生它们的洋流的重要信息，从而使人们能够对这些沉积物进行更全面的研究。这个项目代表了一种多学科的方法，以更好地了解PDC在一次记录最好的喷发中的动态。计划将质量通量的估计、最近暴露的地层的详细测量，包括探地雷达研究，以及多阶段数值模拟技术结合在一起，以更好地约束5月18日产生的PDC的动力学。特别是，这项提案侧重于关于私营部门的运输的三个问题。1)水流强度和浓度与碎屑岩的运移能力有何关系，又是如何导致距源头较远的水流分异和密度分层的？对许多PDC矿床的一个引人注目的观察是嵌入在细粒基质中的非常大的(1米)浮石和岩屑碎屑。新的多相数值技术的出现使粒子从喷口到最终沉积的轨迹有了更好的解释。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屑所需的条件。2)在PDC运输过程中发生了多少碎屑粉碎，这对后续的动力学有何影响？当火山颗粒相互碰撞或滑过时，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，并在水流中产生更高比例的细灰。早期的模型表明，较高的细灰比例可以在这些流动中产生紧急动力，从而导致更大的跳动距离。这些单位和震源条件之间的详细关联使圣海伦斯火山成为详细圆度研究的极佳候选者，这反过来将产生关于这些流动中的小规模动量转移的信息。3)PDC的自信道化对传输距离有何影响？据推测，在最近沉积的松散的火山碎屑很容易受到侵蚀的帮助下，自通道作用在一些最大的圣海伦斯山PDC中发挥了作用。改进的新暴露的沟壑横截面地图、探地雷达和多相模拟将提供有关圣海伦斯PDC自渠化和水流反馈的进一步信息。总而言之，对圣海伦斯山的模拟-现场联合研究将提高对PDC中运输和沉积的总体理解。这项工作的结果将提高我们的知识和能力，以评估在其他地方类似的爆炸性喷发中与PDCs有关的危险，包括相对超限距离和横向运输过程中产生的细火山灰的量化，这对航空危险评估和潜在的气候影响具有重要意义。

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.