This compilation re-visits the May 18, 1980 Plinian eruption sequence of Mount St. Helens and correlates new and existing observations and measurements for a revised narrative of when and how eruption behavior changes occurred. It presents new observations of vesicle textures in white pumice and analyses of lithic materials that are correlated with observed eruption behavior. These and the compositional and seismic research by others provide constraints to a proposed process of how the conduit system was modified during the Plinian sequence. The eruption sequence is sub-divided into 3 periods interpreted to represent changes in magma supply and transport as a result of conduit changes. Based on observed pumice textures and composition, the microlite-free, white pumice in the earliest Plinian tephra is interpreted to have been emplaced in a poorly-interconnected, conduit system on or about May 16. The Morning period began with the landslide and lateral blast and initial conduit decompression. The early Plinian column then discharged conduit-resident, white pumice through an effective conduit of ≥20 m radius that produced small, irregular tephra clasts with abundant, small tubular vesicles. Small pyroclastic flows observed about 1010 h started the Transition period and the discharge of small batches of magma from the magma reservoir. Increasingly larger pumice clasts and large, coalesced vesicle textures are attributed to the increased ascent path from the magma reservoir and increased, average ascent rates that increased the rates and extent of bubble growth and coalescence. The eruption intensity that occurred at 1215 h is attributed to an increased magma supply through a modestly, improved conduit. The Afternoon period started at 1300 h as the vertical, eruption column dissipated. Episodic pyroclastic flows from collapsed fountains increased in volume until climax, as a preferred path in the complex, lower conduit was established. Lithic fragments comprise about 40% of deposits of the Pumice Plain and about 70% of the glass was lost as very-fine ash during emplacement. It is proposed that syn-eruptive conduit modification incrementally consolidated adjacent dikes and fractures, improved local interconnections among conduit segments, and removed the lithic debris, so that the conduit configuration was simplified and the effective conduit radius enlarged to ≥43 m during the Climactic phase. The Climactic phase after 1500 h resulted from full conduit interconnection, and magma supply and transport from deeper parts of the reservoir.

该汇编重新审视了 1980 年 5 月 18 日圣海伦斯山的普林尼亚火山喷发序列，并将新的和现有的观察和测量结果关联起来，以修订关于喷发行为发生变化的时间和方式的叙述。它提出了对白色浮石中囊泡结构的新观察，并对与观察到的喷发行为相关的岩屑材料进行了分析。这些以及其他人的成分和地震研究为在 Plinian 序列期间如何修改管道系统的拟议过程提供了限制。喷发序列被细分为 3 个时期，解释为代表由于管道变化而导致的岩浆供应和运输的变化。根据观察到的浮石质地和成分，不含微晶石，最早的 Plinian tephra 中的白色浮石被解释为在 5 月 16 日或大约 5 月 16 日被放置在一个连接不良的管道系统中。早晨时期开始于山体滑坡和侧向爆炸以及初始管道减压。早期的 Plinian 柱然后通过半径≥20 m 的有效管道排放管道驻留的白色浮石，该管道产生小的、不规则的火山碎屑和丰富的小管状囊泡。大约 1010 小时观察到的小火山碎屑流开始了过渡期，小批量的岩浆从岩浆库中流出。越来越大的浮石碎屑和大的、聚结的囊泡纹理归因于从岩浆库增加的上升路径和增加的平均上升率，增加了气泡生长和合并的速度和程度。发生在 1215 小时的喷发强度归因于通过适度改进的管道增加的岩浆供应。随着垂直的喷发柱消散，下午时段开始于 1300 小时。来自倒塌喷泉的偶发性火山碎屑流体积增加，直到达到高潮，因为在复杂的下部管道中建立了首选路径。岩屑碎片约占浮石平原沉积物的 40%，约 70% 的玻璃在就位过程中以极细的灰烬形式损失。建议 建立了下层管道。岩屑碎片约占浮石平原沉积物的 40%，约 70% 的玻璃在就位过程中以极细的灰烬形式损失。建议 建立了下层管道。岩屑碎片约占浮石平原沉积物的 40%，约 70% 的玻璃在就位过程中以极细的灰烬形式损失。建议同喷式河道改造逐步巩固相邻的堤坝和裂缝，改善了河道段之间的局部互连，并去除了岩屑，从而简化了管道配置，并在高潮期将有效管道半径扩大到≥43 m。1500 小时后的气候阶段是由全管道互连以及来自储层较深部分的岩浆供应和输送造成的。