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

2010年10月火山喷发。默拉皮火山是印度尼西亚S最活跃的火山之一，它展示了世界各地活跃的层状火山系统即使是中到小规模的喷发也可能造成人类生命的破坏和突然损失。能够观测Merapi上的当前活动，以及由于长期活动而保存下来的喷发序列的存在，使得能够详细检查熔岩穹顶生长和重大穹顶坍塌事件期间的挥发性脱气动力学。它还提供了一个机会来制定这样的系统如何随着时间的推移而演变的一般模型。由于熔岩穹顶的生长和坍塌是产生危险和不可预测的火山现象(如火山碎屑密度流、火山灰落下)的主要机制，这些现象对当地和区域人口都具有破坏性影响，因此了解触发穹顶坍塌事件和影响火山碎屑灾害产生的物理参数对于改进这座持续活跃的层状火山的减灾战略至关重要。在探索安山期层状火山重大穹顶坍塌事件的原因时，该项目为喷发产物中这些即将发生的事件寻找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态如何有助于产生火山碎屑密度流和火山灰坠落灾害的特定特征。从Merapi收集的现有样本将作为重点分析的目标，以限制单个喷发事件之间在特定特征上的相似或差异，以及喷发活动长期过程中的时间变化，包括：1.坠落和流动单元的粒度分布以及火山喷发最有效的碎裂部分内的火山灰颗粒形态。熔岩样品中的微晶数量密度和微晶形态、斜长石斑晶边缘成分、边缘结构和角闪石反应边缘厚度。3)熔岩泡孔、泡泡形态、气泡大小分布和透气网络在穹顶生长过程中的发展。4)岩浆上升和穹隆喷出过程中矿物和熔体中挥发性元素(H2O、CO2、Li)的行为。通过比较单个层状火山系统内几个产生穹顶的事件随时间的变化，本研究试图量化导致主要穹顶坍塌事件的流变参数，试图解释为什么发生这些坍塌事件，为什么它们在不同的时间尺度上发生，以及挥发性元素的行为和这些元素的脱气将如何影响由此产生的火山碎屑灾害的特征。量化火山导管内的物理参数，并了解这些参数如何随着时间的变化而变化，从而导致重大的穹顶坍塌事件，将使研究人员能够更准确地模拟(并最终预测)火山行为。这将导致改进的风险评估战略，不仅对印度尼西亚的默拉皮火山来说是一种重大的火山灾害，而且对世界各地其他100个表现出类似行为的活跃的层状火山中心也是如此。

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.