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首页> 外文期刊>Journal of Volcanology and Geothermal Research >The controls and consequences of substrate entrainment by pyroclastic density currents at Mount St Helens, Washington (USA)
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The controls and consequences of substrate entrainment by pyroclastic density currents at Mount St Helens, Washington (USA)

机译：美国华盛顿州圣海伦斯山的火山碎屑密度流夹带底物的控制及其后果

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Evidence in the deposits from the May 18, 1980 eruption at Mount St Helens demonstrates that pyroclastic density currents (PDCs) produced during the afternoon of the eruption became intermittently erosive. Using detailed componentry and granulometry we constrain the sources for lithic blocks in the deposits and identify deposits from PDCs that became locally erosive. The componentry of the lithics in the fall deposits is used as a proxy for vent erosion and assumed to represent the starting componentry for PDCs prior to entrainment from any other source. We find little evidence in the PDC deposits nearest to the base of the volcano for entrainment from the steep flanks; however, significant evidence indicates that PDCs eroded into the debris avalanche hummocks, suggesting that entrainment is favored as PDCs interact with highly irregular topography. Evidence for locally entrained material downstream from debris avalanche hummocks decreases with height in the outcrop, suggesting that less entrainment occurs as local relief decreases and upstream topography is buried. The prevalence of lithofacies containing locally entrained material at the base of unit contacts and only 10s of meters downstream from debris avalanche hummocks suggests that the majority of entrainment occurs at or near the head of the current. Occasionally, entrained material is located high above unit contacts and deposited well after the initial head of the current is inferred to have passed, indicating that entrainment can occur during periods of non-deposition either from the semi-sustained body of the current or from a pulsating current. Additionally, self-channelization of PDCs, either by levee deposition or scouring into earlier PDC deposits, occurs independently of interaction with topographic obstacles and can affect carrying capacity and runout distance. While we begin to explore the mechanisms and effects of erosion on current dynamics, additional laboratory and numerical studies are necessary to fully understand these processes. (C) 2016 Elsevier B.V. All rights reserved.
机译：1980年5月18日在圣海伦斯火山喷发中产生的沉积物证据表明，喷发下午产生的火山碎屑密度流（PDC）呈间歇性侵蚀。通过使用详细的成分和粒度分析，我们可以约束矿床中岩性块的来源，并识别来自局部侵蚀的PDC的矿床。秋季沉积物中的岩性成分被用作发泄口侵蚀的代名词，并假定代表从任何其他来源夹带之前PDC的起始成分。我们发现在最接近火山底部的PDC沉积物中没有证据表明从陡峭的侧翼带走了。但是，大量证据表明，PDC侵蚀了崩塌的雪崩山丘，这表明，随着PDC与高度不规则的地形相互作用，青睐夹带。碎屑雪崩山口下游的局部夹带物质的证据随露头高度的增加而减少，这表明随着局部浮雕的减少和上游地形被掩埋而发生的夹带较少。在单位接触点底部以及夹杂物雪崩山口下游仅十米处的含岩相的岩相普遍存在，这表明大部分夹带发生在洋流的顶部或附近。有时，夹带的材料位于单元触点上方的较高位置，并在推断出电流的初始水头通过之后很好地沉积，这表明夹带可能发生在非沉积期间，无论是从电流的半维持体还是从流脉动电流。此外，通过堤坝沉积或冲刷到较早的PDC沉积物中，PDC的自沟道化独立于与地形障碍物的相互作用而发生，并且会影响承载能力和跳动距离。虽然我们开始探索腐蚀对当前动力学的机理和影响，但要充分了解这些过程，还需要进行其他实验室和数值研究。 （C）2016 Elsevier B.V.保留所有权利。

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