Collaborative Research: RUI: Boiling-Over Pyroclastic Flows

火山爆发经常产生火山碎屑密度流，即比周围大气密度更高的颗粒和热气体的地面混合物。火山碎屑密度流在大气的最低层迅速传播，在那里它们可以直接影响人类的人口中心和结构，是一些最危险的火山现象。1877年在科托帕西喷发的火山碎屑密度流，以及2006年和2008年在通古拉瓦喷发的火山碎屑密度流，是由一种鲜为人知的被描述为“沸腾溢出”的过程产生的。在沸腾喷发期间，火山碎屑和气体的密集泡沫从火山口边缘或通过火山口的凹口倾泻而出，形成火山碎屑密度流。沸腾的火山碎屑密度流的形成机制与熔岩穹隆或高对流柱坍塌所产生的机制有重要的不同，沉积也是不同的。沸腾喷发很可能是常见的，但在地质记录中被忽视了，因为大多数喷发与挥发分丰富的镁铁质岩浆到中间岩浆的相对较小的喷发有关。该项目由岩石学与地球化学计划和美洲计划(OISE)支持，是一个多学科的国际合作项目，将整合数值模拟和现场研究，以确定沸腾溢流沉积物的特征，确定这种类型喷发发生的原因，并了解由此产生的密度流的传输。特别是，这个建议集中在两个基本问题上，旨在促进我们对火山碎屑密度流的理解：1.什么管道条件导致沸溢动力学？2.侵蚀底板和夹带空气在沸溢喷发中的相对贡献是什么，这对火山碎屑密度流动力学有何影响？建议使用2D多相数值模型来测试封闭的峰顶陨石坑的存在是否提高了减压速率，从而允许在有限的空间域内快速地进行出溶和微晶结晶。这项研究的结果将普遍适用于世界各地的火山。研究小组还将使用Cotopaxi和Tungurahua现场研究的岩相学和挥发性成分观察结果，作为对建模程序保真度的关键测试。他们将检验这一假设，即在离开峰顶陨石坑后，火山碎屑流中的颗粒密集、推移质区域迅速发展。该推移质区域由与底质进行多次持久接触的颗粒组成，可能会增加侵蚀和气体孔压，并抑制环境空气卷吸，从而导致这些流动的跳动距离更长。详细分析沉积物结构、古地磁测量和粒度测量，以及3D颗粒多相计算方法，将用来研究卷吸作用在这些流动传播中的作用。这项拟议研究的许多新颖方面之一是使用热替代方法，包括热剩余磁化，结合数值模型来限制卷吸的热后果。

Explosive volcanic eruptions often produce pyroclastic density currents, ground-hugging mixtures of particles and hot gas that are denser than the ambient atmosphere. Pyroclastic density currents rapidly propagate in the lowest levels of the atmosphere where they can directly impact human population centers and structures, and are some of the most hazardous volcanic phenomena. Pyroclastic density currents erupted at Cotopaxi in 1877 and at Tungurahua in 2006 and 2008 were produced by a little understood process described as "boiling-over". During a boiling-over eruption, a dense froth of pyroclasts and gas pours over the crater rim or through a notch in the crater, creating a pyroclastic density current. The mechanism for the formation of boiling-over pyroclastic density currents differs in important ways from those that result from the collapse of lava-domes or high convective columns, and the deposits are distinctive. Boiling-over eruptions are likely common but under-recognized in the geologic record, as most are associated with relatively small volume eruptions of volatile-rich mafic to intermediate magmas. This project, which is supported by the Petrology&Geochemistry program and the Americas Program (OISE) is a multidisciplinary, international collaborative effort in which numerical modeling and field studies will be integrated to characterize boiling-over deposits, determine why this style of eruption occurs, and understand the transport of the resulting density currents. In particular, this proposal focuses on two fundamental questions designed to advance our understanding of pyroclastic density currents: 1. What conduit conditions result in boiling-over dynamics? 2. What are the relative contributions of eroded substrate and entrained air in boiling-over eruptions, and how does this influence pyroclastic density current dynamics? It is proposed to use 2D multiphase numerical models to test whether the presence of an enclosed summit crater enhances the decompression rate, allowing for a rapid sequence of exsolution and microlite crystallization over a limited spatial domain. The results of this study will have general applicability to volcanoes worldwide. The research team will also use petrographic and volatile content observations from the field studies at Cotopaxi and Tungurahua as a crucial test of the fidelity of the modeling program. They will test the hypothesis that after exiting the summit crater, a particle-dense, bed load region in the pyroclastic flows develops rapidly. This bed load region, which is constituted of particles that make multiple and enduring contacts with the substrate, may enhance erosion and gas pore pressure, and suppress ambient air-entrainment resulting in longer runout distances of these flows. Detailed analysis of deposit architecture, paleomagnetic measurements, and granulometry along with a 3D granular multiphase computational approach will be used to examine the role of entrainment in the propagation of these flows. One of the many novel aspects of the proposed investigation is the use of thermal proxies, including thermoremanent magnetization, in conjunction with the numerical models to constrain the thermal consequences of entrainment.