Volcanic mixed avalanches: A distinct eruption-triggered mass-flow process at snow-clad volcanoes
Volcanic mixed avalanches: A distinct eruption-triggered mass-flow process at snow-clad volcanoes
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火山混合雪崩：雪覆盖火山上独特的喷发引发的质量流过程
DOI：
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发表时间：
1994
期刊：
影响因子：
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通讯作者：
R. Janda
中科院分区：
文献类型：
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作者：
T. C. Pierson;R. Janda
1982年至1989年间，圣海伦斯火山、内华达德尔鲁伊斯火山和Redoubt火山爆发期间，雪和热火山碎屑紧密混合的快速雪崩形成了一种通常未被识别的火山碎屑沉积物。这些“混合雪崩”以高达27米/秒的速度行进了14公里，涉及了多达10.7米的岩石和冰，并留下了厚达5米的未融化的单一流动单元沉积物。在滑坡过程中，从热岩石到雪的热量传递产生的融水使混合物部分饱和，显然使这些混合雪崩的机动性等于或大于类似体积的“干”碎片雪崩。本报告研究的雪崩开始于雪崩，在火山爆发的爆发阶段，由炽热的火山碎屑碎片对雪坡的弹道轰击引发。雪崩的侵蚀将额外的雪、冰川碎片和其他岩石碎片结合在一起。混合雪崩沉积物是块状的，分选非常差，在融化前未分级到反向分级。尽管岩石碎屑占冻结样品的70%，但岩石碎屑的体积含量仅为9%至36%。雪和冰占冷冻样品的36%到72%，其余的体积由空隙构成。冰融化后的变形和固结使沉积物变成松散的多孔层，这些层的厚度只有原始厚度的一小部分，后来也没有什么线索表明它们的起源。在融化和干燥后，沉积物极易受到侵蚀，不太可能在地层记录中保存完好。
A generally unrecognized type of pyroclastic deposit was produced by rapid avalanches of intimately mixed snow and hot pyroclastic debris during eruptions at Mount St. Helens, Nevado del Ruiz, and Redoubt Volcano between 1982 and 1989. These "mixed avalanches" traveled as far as 14 km at velocities up to ∼27 m/s, involved as much as 10 7 m 3 of rock and ice, and left unmelted deposits of single flow units as thick as 5 m. During flow downslope, heat transfer from hot rocks to snow produced meltwater that partially saturated the mixtures, apparently giving these mixed avalanches mobilities equal to or greater than those of "dry" debris avalanches of similar volume. The avalanches studied for this report began as snow avalanches, triggered during explosive phases of the eruptions by ballistic bombardment of snow slopes by hot pyroclastic debris. Erosion by the avalanches incorporated additional snow, fragments of glacier ice, and other rock debris. Mixed-avalanche deposits were massive, very poorly sorted, and ungraded to inversely graded prior to melting. Although rock debris composed as much as 70 wt% of the frozen samples, the bulk volumetric content of rock debris was only 9% to 36%. Snow and ice composed between 36% and 72% of the frozen samples, with void space making up the remainder of the volume. Deformation and consolidation after melting of the ice content reduced the deposits to loose, porous layers that were only a fraction of their original thickness and that later gave little hint of their origin. After melting and desiccation, the deposits are highly susceptible to erosion and unlikely to be well preserved in the stratigraphic record.