Collaborative Research: Boiling-Over Pyroclastic Flows

爆炸性火山爆发经常产生火山碎屑密度流，这是颗粒和热气体的地面混合物，其密度比环境大气更大。火山碎屑密度流在大气层的最低层迅速传播，在那里它们可以直接影响人类的人口中心和结构，并且是一些最危险的火山现象。1877年在科托帕希和2006年和2008年在通古拉瓦爆发的火山碎屑密度流是由一种鲜为人知的过程产生的，这种过程被称为“沸腾”。在沸腾喷发过程中，火山碎屑和气体的密集泡沫从火山口边缘或火山口的缺口涌出，形成火山碎屑密度流。沸腾火山碎屑密度流的形成机制与熔岩穹丘或高对流柱崩塌所产生的密度流的形成机制有很大的不同，而且沉积物也很独特。沸溢喷发很可能是常见的，但在地质记录中认识不足，因为大多数都与挥发性丰富的镁铁质到中等岩浆的相对小体积喷发有关。 该项目由岩石学地球化学计划和美洲计划（OISE）支持，是一项多学科的国际合作努力，其中将集成数值模拟和实地研究，以表征沸腾沉积物，确定为什么会发生这种类型的喷发，并了解由此产生的密度流的运输。特别是，这个建议侧重于两个基本问题，旨在促进我们的火山碎屑密度流的理解：1。什么样的管道条件会导致沸腾动力学？2.在沸腾喷发中，侵蚀基底和夹带空气的相对贡献是什么？这是如何影响火山碎屑密度流动力学的？建议使用二维多相数值模型来测试是否存在一个封闭的山顶火山口提高减压速率，允许在有限的空间域的快速序列的出溶和微晶结晶。这项研究的结果将普遍适用于世界各地的火山。研究小组还将使用在科托帕希和通古拉瓦进行的实地研究中获得的岩相学和挥发物含量观测结果，作为对建模程序保真度的关键测试。他们将测试这样一种假设，即在离开山顶火山口后，火山碎屑流中的颗粒密集的推移质区域迅速发展。由与基底进行多次和持久接触的颗粒构成的该底床负荷区域可以增强侵蚀和气体孔隙压力，并抑制环境空气夹带，从而导致这些流的更长的流出距离。详细分析的存款架构，古地磁测量，粒度沿着与三维颗粒多相计算方法将被用来检查夹带在这些流动的传播中的作用。所提出的调查的许多新的方面之一是使用热代理，包括thermorecentric磁化，结合数值模型，以约束夹带的热后果。

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.