Degassing Dynamics that Lead to Repeated Lava Dome Growth and Collapse at Persistently Active Stratovolcanoes

2010年10月，印尼火山之一的默拉皮火山爆发。在世界上最活跃的火山中，即使是中等到小型的平流层火山系统的活跃喷发也可能造成破坏和人类生命的突然丧失。观察默拉皮火山当前活动的能力，以及由于长期活动而保存下来的喷发序列的存在，使我们能够详细检查熔岩穹丘生长和主要穹丘崩塌事件期间的挥发性脱气动力学。它还提供了一个机会，来形成这样的系统如何随时间演变的一般模型。由于熔岩穹丘的生长和崩塌是产生危险和不可预测的火山现象（如火山碎屑密度流、火山灰坠落）的主要机制，这些火山现象对当地和区域人口都有破坏性影响，因此，了解触发穹丘崩塌事件和影响火山碎屑灾害产生的物理参数，对于改进这种持续活跃的层状火山的减灾战略至关重要。在探索安山岩层状火山的主要圆顶崩塌事件的原因时，本项目在喷发产物中寻找这些即将发生事件的地球化学和结构信号的证据，目的是确定熔岩喷出过程中挥发性脱气的动力学如何有助于产生火山碎屑密度流和灰烬坠落危险的特定特征。从默拉皮火山收集的现有样本将被重点分析，以限制单次喷发事件与喷发活动长期过程中的时间变化之间特定特征的相似性或差异性，包括：崩落单元和崩落单元的粒度分布及灰粒形态在草本最有效破碎部分的分布。熔岩样品中的微岩数量密度和微岩形态、斜长石斑晶边缘组成、边缘结构和角闪孔反应边缘厚度。3)熔岩囊泡的形态、气泡大小分布，以及穹顶生长过程中气体逸出的渗透网络的发育。4)岩浆上升和穹丘喷发过程中矿物和熔体中挥发性元素（H2O、CO2、Li）的行为。通过比较同一层状火山系统中不同时间的几个圆顶生成事件，本研究试图量化导致主要圆顶坍塌事件的流变参数，试图解释这些坍塌事件发生的原因，为什么它们发生在不同的时间尺度上，以及挥发性元素的行为和这些元素的脱气将如何影响火山碎屑危害的最终特征。量化火山导管内的物理参数，并了解这些参数如何随时间变化而导致主要的圆顶坍塌事件，将使研究人员能够更准确地建模（并最终预测）火山行为。这不仅将改善默拉皮火山的危害评估策略，因为默拉皮火山对印度尼西亚来说是一个重大的火山危害，而且还将改善世界上其他100个表现出类似行为的活跃平流层火山中心的危害评估策略。

The October 2010 eruption of Mt. Merapi, one of Indonesia?s most active volcanoes, demonstrates the devastation and sudden loss of human life that can result from even moderate to small eruptions of active stratovolcanic systems throughout the world. The ability to observe current activity on Merapi, and the existence of preserved erupted sequences resulting from long-lived activity, allow for a detailed examination of volatile degassing dynamics during lava dome growth and major dome collapse events. It also provides an opportunity to formulate a general model of how such systems evolve over time. Because lava dome growth and collapse is a dominant mechanism for the production of dangerous and unpredictable volcanic phenomena (e.g., pyroclastic density currents, ash fall) that have devastating effects on both local and regional populations, understanding the physical parameters that trigger dome collapse events and influence the generation of pyroclastic hazards is paramount to improving the hazard mitigation strategies at this persistently active stratovolcano. In exploring the causes of major dome collapse events at andesitic stratovolcanoes, this project seeks evidence for geochemical and textural signals for these impending events in erupted products, with the objective of determining how the dynamics of volatile degassing during lava effusion contributes to particular characteristics of resulting pyroclastic density currents and ash fall hazards. Existing samples collected from Merapi will be targeted by a focused range of analyses in order to constrain similarities or differences in particular characteristics between single eruptive events and temporal variations over the long-term course of eruptive activity, including: 1. Grain size distributions of fall and flow units and ash grain morphologies within the most efficiently fragmented portion of the tephra.2. Microlite number density and microlite morphology within lava samples, plagioclase phenocryst rim compositions, rim textures, and amphibole reaction rim thicknesses. 3) Lava vesicularity, vesicle morphology, bubble size distributions, and development of permeable networks for gas escape during dome growth. 4) Behavior of volatile elements (H2O, CO2, Li) in minerals and melts during magma ascent and dome effusion. By comparing several dome-producing events over time within a single stratovolcanic system, this study seeks to quantify the rheological parameters leading up to major dome collapse events, attempting to explain why these collapse events occur, why they occur on variable time scales, and how the behavior of volatile elements and the degassing of these elements will influence the resulting characteristics of pyroclastic hazards. Quantifying the physical parameters within volcanic conduits and understanding how these parameters vary over time to cause major dome collapse events will allow researchers to more accurately model (and ultimately, forecast) volcanic behavior. This will lead to improved hazard assessment strategies for not only Merapi, which represents a significant volcanic hazard for the nation of Indonesia, but for the other 100 active stratovolcanic centers throughout the world that display similar behavior.