Are eruptions from linear fissures and caldera ring dykes more likely to produce pyroclastic flows?
Are eruptions from linear fissures and caldera ring dykes more likely to produce pyroclastic flows?
复制标题
DOI：
10.1016/j.epsl.2016.09.005
复制
发表时间：
2016-11-15
影响因子：
5.3
通讯作者：
Roche, O.
中科院分区：
文献类型：
--
作者：
Jessop, D. E.;Gilchrist, J.;Roche, O.
湍急的火山喷流是由高能量的爆炸性喷发产生的，可能会形成漂浮的羽流，上升到几十公里外的大气中形成伞状云，或者坍塌产生紧贴地面的火山碎屑流。向大气中注入的火山灰可以运输数百公里，有可能影响气候，扰乱全球航空旅行，并造成呼吸道健康问题。相比之下，火山碎屑流对火山附近的人口和基础设施可能是灾难性的。发生这两种行为的关键是形成喷流边缘的大(卷吸)涡将环境空气机械卷吸和混合到射流中的程度改变了空气-火山灰混合物的密度：低卷吸速率导致火山碎屑流，而高卷吸速率产生浮力羽流。最近对来自扩张和直边圆形开口的颗粒(多相)火山喷流的实验表明，产生浮力羽流的可能性将强烈取决于喷口区域的形状和内部几何形状。火山喷流的命运对喷口结构的这种新认识的敏感性是喷流和夹带的固体颗粒之间复杂的动态耦合的结果，这一影响在以前的研究中通常被忽视。在这项工作的基础上，我们在这里使用了一系列关于来自模拟线性裂缝和环状裂缝的多相湍流射流的广泛实验，以探索灾害性火山口形成(CCF)喷发期间的限制性喷口几何形状是否最终会导致相对更频繁的火山碎屑流，而不是层状火山上圆形喷口的喷发。我们的结果通过标度分析和一维理论模型得到理解，结果表明，当粒子运动为卷吸涡贡献角动量时，卷吸增强。然而，由于卷吸涡的大小与排气口宽度大致成正比，因此卷吸卷吸的程度随着排气口宽度变小而减小。因此，我们的工作表明，对于特定的质量喷发速率，CCF事件典型的高长宽比喷口更有可能产生火山碎屑流。我们认为，地质记录中最大的CCF喷发产生火山碎屑流的神秘趋势是它们通过连续或分段连续的破火山口环状断裂喷发的预期结果。(C)2016爱思唯尔B.V.保留所有权利。
Turbulent volcanic jets are produced by highly-energetic explosive eruptions and may form buoyant plumes that rise many tens of kilometres into the atmosphere to form umbrella clouds or collapse to generate ground-hugging pyroclastic flows. Ash injected into the atmosphere can be transported for many hundreds of kilometres with the potential to affect climate, disrupt global air travel and cause respiratory health problems. Pyroclastic flows, by contrast, are potentially catastrophic to populations and infrastructure close to the volcano. Key to which of these two behaviours will occur is the extent to which the mechanical entrainment and mixing of ambient air into the jet by large (entraining) eddies forming the jet edge changes the density of the air-ash mixture: low entrainment rates lead to pyroclastic flows and high entrainment rates give rise to buoyant plumes. Recent experiments on particle-laden (multi-phase) volcanic jets from flared and straight-sided circular openings suggest that the likelihood for buoyant plumes will depend strongly on the shape and internal geometry of the vent region. This newly recognised sensitivity of the fate of volcanic jets to the structure of the vent is a consequence of a complex dynamic coupling between the jet and entrained solid particles, an effect that has generally been overlooked in previous studies. Building on this work, here we use an extensive series of experiments on multi-phase turbulent jets from analogue linear fissures and annular ring fractures to explore whether the restrictive vent geometry during cataclysmic caldera-forming (CCF) eruptions will ultimately lead a relatively greater frequency of pyroclastic flows than eruptions from circular vents on stratovolcanoes. Our results, understood through scaling analyses and a one-dimensional theoretical model, show that entrainment is enhanced where particle motions contribute angular momentum to entraining eddies. However, because the size of the entraining eddies scales approximately with vent width, the extent of entrainment is reduced as the vent width becomes small in comparison to its length. Consequently, our work shows that for specified mass eruption rates, the high length-to-width ratio vents typical of CCF events are more likely to produce pyroclastic flows. We suggest that the enigmatic trend in the geological record for the largest CCF eruptions to produce pyroclastic flows is an expected consequence of their being erupted through continuous or piece-wise continuous caldera ring fractures. (C) 2016 Elsevier B.V. All rights reserved.