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

合作研究：仔细观察 1980 年 5 月 18 日的浮石平原沉积物：对评估喷发条件和火山碎屑密度电流动态的影响

• 批准号: 0948588
• 负责人: Brittany Brand
• 金额: $ 24.13万
• 依托单位: University of Washington
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-01-01 至 2013-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0948588&HistoricalAwards=false
• 关键词: Collaborative; Research; Closer; Look; May

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.

火山碎屑密度流（PDC）是热气体和火山碎屑物质的地面混合物，以高速沿着火山侧翼传播。 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，因为在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动力学的大部分理解来自对它们产生的沉积物的分析。1980年5月18日在圣海伦斯火山爆发有一个显着的影响，对世界的认识和理解爆炸性喷发，并在这次喷发产生的喷发产物的研究，帮助我们解释存款从许多其他喷发。早期对这次喷发过程中产生的PDCs的研究与喷发强度和行为的变化有关。然而，从那时起，深深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电流的重要信息，允许对这些沉积物进行更全面的研究。该项目代表了一个多学科的方法，以更好地了解PDC动态在一个最好的记录喷发。计划结合联合收割机对质量通量的估计、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，以更好地约束5月18日产生的PDC的动态。这项建议特别集中在三个有关运送残疾人士发展中心的问题。1)流动强度和浓度与岩屑碎屑的搬运能力有何关系？这又是如何导致流的分离和密度随离源距离的分层的？许多PDC矿床的一个显著观察结果是嵌在细粒基质中的非常大（1 m）的浮石和岩屑碎屑。新的多相数值技术的出现允许更好地核算颗粒轨迹从通风口到其最终沉积。 这些信息将与最近暴露的沉积物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，这如何影响随后的动力学？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百分比的细灰。 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态，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详细的圆度研究，这反过来又会产生有关这些流量的小规模动量转移的信息的一个很好的候选人。 3)PDCs的自通道化是如何影响输运距离的？据推测，自我通道化，最近沉积的松散的火山碎屑的侵蚀容易的帮助下，发挥了作用，在一些最大的圣海伦火山PDCs。新暴露的沟壑横截面、探地雷达和多相模拟的改进地图将提供关于圣海伦斯PDC自渠化和流量反馈的进一步信息。总之，对圣海伦斯山进行的模拟和实地研究将提高对沉积物中的迁移和沉积的总体认识。 这项工作的结果将推进我们的知识和能力，以评估危险与PDCs在类似的爆炸性喷发在其他地方，包括相对跳动的距离和量化的细灰在横向运输过程中产生的，这具有重要的相关性航空危险评估和潜在的气候影响。