Explosive-effusive volcanic eruption transitions caused by pyroclast sintering

该项目将解决火山学中的一项首要挑战--了解在最危险的喷发期间对喷发方式和强度的转变的控制。大多数硅质喷发始于高能、高危险的爆炸阶段，然后要么减弱并停止，要么过渡到混合和喷发行为，产生相对较短距离的熔岩流动，而潜在的危险要低得多。了解这些转变的时间以及喷发结束的时间，是影响危险评估和喷发反应的一个重大挑战。现有的模型假设，从爆炸性行为到溢出性行为的转变是由岩浆上升速率的变化或受压气体的可渗透释放从下向下驱动的，有效地“化解”了爆炸潜力。然而，这些自下而上的模型未能解释硅质火山作用的两个基本方面：(I)2011-12年智利Cordon Caulle火山喷发期间直接目睹的同时的爆炸-喷发行为，随后在其他地方推断；(Ii)保存在这些喷发类型所有阶段的沉积物中的管道内火山碎屑烧结的广泛证据。项目组成员利用这一证据为硅质喷发中的爆炸-喷发转变开发了一种新的范例(Wadsworth等人，2020年)，在这种转变中，碎裂岩浆的浅焊接和浅管道的闭塞从上方驱动了转变。在这种“神秘的碎裂”模式中，所有的硅质喷发在深度上都是爆炸性的，即使表面上看起来是热情洋溢的。这一新想法要求对硅质火山系统进行大规模的重新评估。我们的新模型提出，表面上喷出的熔岩是由火山爆发直接产生的，这些火山作用是由地壳最浅部分的火山导管中的热火山灰和浮石粘性融合--烧结而成的(参见CFS)。神秘的碎裂模型是根据贫晶硅系统的证据而开发的。在这项新的研究中，我们更进一步，并提出该模型也适用于富含晶体的中间体系，这是更常见的，并构成全球危险。这一假说是基于来自富含晶体的系统的大量证据，类似于上面总结的。该项目将提供：(1)利用来自多个实验室的现有样本，对世界各地形成穹顶和富含晶体的熔岩进行新的分析。我们将限制基质中的纹理--重点放在表明烧结成岩作用的孔隙纹理上--以及与破碎和烧结相关的宏观纹理，如部分烧结颗粒的断裂。这项新的结构工作，加上分析和岩石物理测量，将为我们将神秘的碎裂模型扩展到含晶体的岩浆系统奠定基础。(2)一套新的多相岩浆颗粒烧结率的实验火山学测量--带晶体的玻璃。依靠PI大量的实验和理论烧结工作，我们将开发新的实验验证的模型，用于在高压、岩浆挥发分存在和剪切应力下的系统中的晶体烧结率。这将第一次将烧结理论推向岩浆条件，并允许首次对火山的烧结率进行定量测试。(3)我们将通过与火山观察站的利益相关者接触，将这些烧结速率方程应用于过去在收集样本套件的相同地点的含晶体活动火山喷发，重点是科利马火山(墨西哥)。由烧结引起的火山喷发的爆炸性-热液性转变。科学进步，第6(39)页，第7940页

This project will address a first-order challenge in volcanology - understanding the controls on transitions in eruption style and intensity during the most hazardous eruptions. Most silicic eruptions begin with a high-energy, high-hazard explosive phase, then either wane and stop, or transition to hybrid and effusive behaviour that produces relatively short-range lava flows with much lower hazard potential. Understanding the timing of these transitions, and of the end of an eruption, is a major challenge that impacts hazard assessment and eruption response. Existing models assume that a transition from explosive to effusive behaviour is driven from below by a change in either the magma ascent rate or by the permeable release of pressurised gas, effectively 'defusing' the explosive potential. However, these bottom-up models fail to explain two fundamental aspects of silicic volcanism: (i) simultaneous explosive-effusive behaviour that was witnessed directly during the 2011-12 eruption of Cordon Caulle (Chile) and subsequently inferred elsewhere, and (ii) widely documented evidence for in-conduit pyroclast sintering preserved in the deposits from all phases of these eruption types. Members of the project team have used this evidence to develop a new paradigm for explosive-effusive transitions in silicic eruptions (Wadsworth et al., 2020) in which transitions are driven from above by shallow welding of fragmented magma and occlusion of the shallow conduit. In this 'cryptic fragmentation' paradigm, all silicic eruptions are explosive at depth, even when apparently effusive at the surface. This new idea demands a wholesale re-evaluation of silicic volcanic systems.Our new model proposes that apparently effusive lava is generated directly from explosive volcanism, assembled by the viscous amalgamation - sintering - of hot volcanic ash and pumice in the volcanic conduit in the shallowest parts of the Earth's crust (see CfS). The cryptic fragmentation model was developed in response to evidence from crystal-poor silicic systems. In this new study we go further, and propose that the model also applies to crystal-rich intermediate systems, which are much more common, and pose a global hazard. This hypothesis is based on abundant evidence from crystal-rich systems, similar to that summarized above. This project will deliver:(1) New analysis of dome-forming and crystal-rich lavas worldwide using existing samples from multiple laboratories. We will constrain the textures in the groundmass - with a focus on pore-textures indicative of sintering petrogenesis - and macro-scale textures associated with breaking and sintering, such as fractures will with partially sintered particles. This new textural work, coupled with analytical and petrophysical measurements, will underpin our extension of the cryptic fragmentation model to crystal-bearing magma systems. (2) A comprehensive suite of new experimental volcanology measurements of sintering rates with multiphase magmatic particles - glass with crystals. Relying on the PI's large body of experimental and theoretical sintering work, we will develop new experimentally-validated models for sintering rates with crystals in systems under elevated pressures, in the presence of magmatic volatiles, and under shear stresses. For the first time, this will push sintering theory to magmatic conditions and allow the first quantitative test of sintering rates at volcanoes. (3) We will apply these sintering rate equations to active crystal-bearing volcanic eruptions of the past at the same sites from which the sample suites were collected, with a focus on Colima volcano (Mexico) via engagement with stakeholders at volcano observatories.Cryptic fragmentation model reference:Wadsworth, F.B., Llewellin, E.W., Vasseur, J., Gardner, J.E. and Tuffen, H., 2020. Explosive-effusive volcanic eruption transitions caused by sintering. Science advances, 6(39), p.eaba7940