Collaborative Research: Boiling-Over Pyroclastic Flows

火山爆发经常产生火山碎屑密度流，这是颗粒和热气体的混合物，比周围大气密度大。火山碎屑密度流在大气的最底层迅速传播，在那里它们可以直接影响人口中心和建筑物，是一些最危险的火山现象。1877年在科托帕希爆发的火山碎屑密度流，以及2006年和2008年在通古拉瓦爆发的火山碎屑密度流，都是由一个鲜为人知的过程产生的，这个过程被称为“沸腾”。在沸腾的喷发过程中，火山碎屑和气体的密集泡沫溢出火山口边缘或通过火山口的缺口，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丘或高对流柱崩塌的形成机制在许多重要方面有所不同，而且这些沉积物是独特的。沸腾喷发很可能是常见的，但在地质记录中没有得到充分认识，因为大多数喷发都与相对小体积的富含挥发物的基性岩浆到中间岩浆喷发有关。该项目由岩石地球化学项目和美洲项目（OISE）支持，是一个多学科的国际合作项目，其中数值模拟和实地研究将结合起来，以表征沸腾覆盖沉积物，确定这种喷发方式发生的原因，并了解由此产生的密度流的运输。特别地，本提案聚焦于两个基本问题，旨在促进我们对火山碎屑密度流的理解：什么管道条件会导致沸腾over动力学？2. 侵蚀底物和夹带空气在沸腾喷发中的相对贡献是什么？这如何影响火山碎屑密度流动力学？建议使用二维多相数值模型来测试封闭峰顶火山口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进行溶出和微岩结晶。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实地研究的岩石学和挥发物含量观测结果作为建模程序保真度的关键测试。他们将验证一个假设，即在离开山顶火山口后，火山碎屑流中的一个颗粒密集的床载区迅速发展。该层载区域由颗粒组成，这些颗粒与基材进行多次持久的接触，可能会增强侵蚀和气体孔隙压力，抑制环境空气夹带，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于检查夹带在这些流动传播中的作用。提出的研究的许多新颖方面之一是使用热代理，包括热磁化，结合数值模型来约束夹带的热后果。

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.