The explosivity of volcanic eruptions: Quantifying the critical role of permeability in magmas

火山爆发是最强大和最壮观的自然现象之一，即使是相对较小的爆发也会在全球范围内产生重大的社会，环境和经济影响。世界上约有9%的人口直接受到火山爆发的影响，因此火山学的主要目标是能够预测火山爆发的类型，以便脆弱的社区能够安全疏散。然而，在火山动荡时期，我们仍然不能肯定会发生什么类型的喷发，因此它可能有多危险。随着岩浆在火山内部上升，溶解在岩浆中的液体开始形成气泡。喷发的爆炸性主要由这些流体从岩浆中逸出的能力所控制。我们知道，控制岩浆中流体运动的主要因素是它的渗透性。渗透率被定义为流体可以通过介质移动的容易程度，并且由岩浆的物理性质（化学成分、晶体含量和孔隙度）以及改变这些性质的管道中发生的环境条件和过程控制。一般认为流体运动有两种模式，一种与岩浆破裂有关，另一种与气泡链的形成有关。然而，这些模型没有量化或验证。在火山动荡时期，内部管道过程产生地球物理信号，如地震活动。了解地球物理信号和产生它们的过程之间的关系可以帮助我们做出更可靠的火山爆发预测。我们经常在火山上记录的某种类型的地震活动被认为是由流体运动产生的。这些被称为低频（LF）地震事件。然而，低频地震活动的震源过程有多种模型，这阻碍了我们解释它们的能力。由于我们无法访问火山管道，我们使用天然岩石样本进行实验室实验，以模拟火山内部发生的情况，并产生模拟地球物理数据（声发射，AE，是天然地震活动的实验室模拟），以更好地解释这些过程。然而，火山管道的环境条件是极端的，不幸的是，大多数已发表的实验研究并没有完全复制这些条件（温度，压力，流体和应力）同时。这是由于高温、高压实验的困难和现有设备设计的局限性。因此，我们仍然不了解流体如何在岩浆中移动，或者是什么特定的过程产生了LF地震活动类型。利用独特的高温三轴变形实验，将在模拟火山的温度、压力和应力条件下，首次对天然岩浆样品的渗透性进行全面研究。在这些实验中，变形过程常见的火山管道和假设有显着的影响渗透率将被复制，使用施加的轴向载荷。AE将在所有实验中使用新设计的传感器阵列进行监测，该传感器阵列将允许在岩浆温度下进行高空间分辨率AE测量。了解流体如何在岩浆中移动，以及实验室模拟地球物理数据的生成将打开一扇通往火山系统的窗户。将确定控制岩浆中流体运动的关键过程和产生的地球物理信号。这些独特的实验数据将把决定渗透率演变的过程与火山系统的爆炸潜力联系起来，并更准确地定义产生LF地震活动的源过程，支持新一代模型，以预测火山系统的状态和未来潜在的喷发活动。

Volcanic eruptions are one of the most powerful and spectacular natural phenomena and even relatively small eruptions can have major social, environmental and economic impacts, on a global scale. Around 9% of the world's population are directly affected by volcanic eruptions and so it is a primary goal of volcanology to be able to forecast the style of volcanic eruptions so that vulnerable communities can be safely evacuated. Yet, in times of volcanic unrest, we still cannot say with any certainty what type of eruption will occur and therefore how hazardous it might be.As magma rises inside the volcano, fluids that are dissolved in the magma begin to form bubbles. Eruption explosivity is principally controlled by the ability of these fluids to escape from magma. We know that a primary control of the movement of fluids in magma is its permeability. Permeability is defined as the ease at which fluids can move through a medium and, is controlled by the physical properties of the magma (chemical composition, crystal content and porosity) and, the environmental conditions and processes occurring in the conduit that modify these properties. Two models of fluid movement are generally considered, one related to the fracture of magma and a second related to the formation of bubble chains. However, these models are not quantified or verified.During times of volcanic unrest, internal conduit processes produce geophysical signals, such as seismicity. Understanding the relationship between geophysical signals and the processes that generate them can help us to produce more robust eruption forecasts. A certain type of seismicity that we frequently register at volcanoes are proposed to be generated by the movement of fluids. These are called low frequency (LF) seismic events. However, there are multiple models for the source processes of LF seismicity, hindering our capacity to interpret them.As we cannot visit a volcanic conduit, we use laboratory experiments with natural rock samples to simulate what is happening inside a volcano and to produce analogue geophysical data (acoustic emission, AE, are the laboratory analogue of natural seismicity) to better interpret these processes. However, the environmental conditions in volcanic conduits are extreme and unfortunately the majority of published experimental studies do not fully replicate these conditions (temperature, pressure, fluids and stress) simultaneously. This is due to the difficulties of high-temperature, high-pressure experiments and the limitations of current equipment designs. Therefore, we still do not understand how fluids move through magma or what specific processes generate types of LF seismicity.Using unique, high-temperature, triaxial deformation experiments; the first comprehensive study of the permeability of natural magma samples, under simulated volcanic conditions of temperature, pressure and stress will be performed. Within these experiments, deformation processes common to volcanic conduits and hypothesised to have significant effect on permeability will be replicated, using an applied axial load. AE will be monitored throughout all experiments using a newly designed sensor array that will allow high spatial-resolution AE measurements to be made at magmatic temperatures. Understanding how fluids move in magma and, the generation of laboratory analogue geophysical data will open a window into the volcanic system. Key processes that control the movement of fluids in magmas and the geophysical signals produced will be identified. These unique experimental data will link the processes dictating permeability evolution to the explosive potential of a volcanic system and more accurately define the source processes that generate LF seismicity, underpinning a new generation of models to forecast the state of the volcanic systems and potential future eruptive activity.