Crystallization and Degassing Processes during Strombolian Eruptions: A Numerical Approach

人类对火山喷发的兴趣可能和它的存在一样古老，因为一方面，火山为农业提供了极其肥沃的土壤，导致在火山或火山附近有更好的安置地。此外，它们可能是最耐人寻味的自然奇观之一，使它们成为当今对游客极具吸引力的景点。另一方面，它们夺走了人的生命，破坏了基础设施，影响了当地和全球气候。因此，了解和减轻火山活动的危险是一项重大的社会任务。在这项提案中，我们通过详细的模拟研究，解决了火山活动的一个关键方面，即火山喷发前岩浆在火山管道中的运移。我们特别关注斯特龙博勒喷发，因为它们相当频繁，很容易从安全的距离观察，并且有大量不同的数据可用于此类喷发来验证数值模型的各个方面。包括火山碎屑和火山气体喷出在内的闪电爆炸模型要求在携带一定超压的火山管道中上升大型气体段塞(填充气泡的管道，明显长于宽)。在这些喷发之间，观察到的被动脱气目前可以用管道中的两相对流来解释。无论考虑哪种模型，岩浆作为熔体/晶体/气泡混合物的复杂流变学是控制其行为的最基本的控制因素之一。在我们的模型中，岩浆的流变性将直接从运行时的热力学模型推导出来，考虑到熔体在通过管道运输过程中不平衡的冷却/脱气/结晶。在知道确切的组成以及气泡和晶体含量的情况下，我们使用电流模型来计算这种三相体系的粘度。然后将该流变模型应用到计算流体力学模型中，以研究气塞和两相对流状态。模型输入参数将被选择为与意大利斯特龙博利火山岩浆的成分/温度最佳匹配。这个项目将回答的关键问题包括：1)流变学如何影响气体段塞的上升？2)是否有流变学机制促进气塞的发展，从而解释连续的喷发脉冲？3)如果管道不是被建模为刚性管道，而是火山建筑物中的热热异常，火山管道中的传输过程将如何变化？4)反应动力学(非平衡热力学)对火山管道中的传输过程有什么影响？对斯特龙博利火山活动的观测，如喷发速度、斯特龙博利喷发期间的脉冲数量、脱气速度和喷发物中的晶体比例，将用于测试模型结果的有效性。

Mankind’s interest in volcanic eruptions is probably as old as its existence because, on the one hand, volcanoes provide extremely fertile soil for farming leading to preferred settling grounds at or near volcanoes. Furthermore, they are probably one of the most intriguing natural spectacles making them highly attractive spots for tourists nowadays. On the other hand, they claim human lives, destroy infrastructure, and affect the local as well as the global climate. Understanding and mitigating the dangers of volcanic activity is therefore a major societal task. In this proposal, we address one key aspect of volcanic activity, the transport of magma in a volcanic conduit prior to an eruption, through a detailed modeling study. We specifically focus on strombolian eruptions as they are quite frequent, easy to observe from a safe distance, and a huge amount of different data is available for such eruptions to verify various aspects of numerical models. Models for strombolian explosions, including the ejection of pyroclasts and volcanic gases, call for the rise of large gas slugs (conduit filling gas bubbles, significantly longer than wide) in volcanic conduits that carry a certain overpressure. In between those eruptions, the observed passive degassing is currently explained by two-phase convection in the conduit. Regardless of which of these models is considered, the complex rheology of the magma as a mixture of melt/crystals/bubbles is one of the most fundamental controls on its behavior. In our model, rheological properties of the magma will be derived directly from a thermodynamic model during runtime taking into account cooling/degassing/crystallization of the melt in disequilibrium during transport through the conduit. Knowing the exact composition as well as bubble and crystal content, we employ current models for calculating the viscosity of such a three-phase system. This rheological model will then be implemented into a computational fluid dynamics model to study the gas slug and two-phase convection regimes. Model input parameters will be chosen to best match composition/temperature of magma of Stromboli volcano, Italy. Key questions which will be answered by this project include: 1) How does rheology impact the rise of gas slugs? 2) Are there rheological regimes that promote the development of trains of gas slugs that would explain consecutive eruption pulses? 3) How do transport processes in volcanic conduits change if conduits are not modeled as rigid pipes but rather as hot thermal anomalies in a volcanic edifice? 4) What is the effect of reaction kinetics (non-equilibrium thermodynamics) on transport processes in volcanic conduits? Observations of activity at Stromboli volcano, like ejection velocities, number of pulses during a strombolian eruption, degassing rates, and crystal fraction in ejecta, will be used to test the validity of the model results.