Reconstructing Eruption Energetics From Volcanic Ash Morphology and Geochemistry

火山爆发是由岩浆中的气泡膨胀驱动的，当岩浆通过火山下面的“管道”系统上升时。当岩浆上升到地球表面附近较低的压力时，在深处溶解在岩浆中的气体（主要是水）就会从溶液中出来，因为可以溶解在岩浆中的气体的数量取决于压力。当岩浆能容纳的量减少到低于溶解的量时，新的气泡就形成了，来自邻近岩浆的气体通过岩浆迁移到气泡中。随着它们的增长，它们将上覆的岩浆向上推出火山，这导致上覆的气泡增长得更快，导致火山喷口附近表面的上升速度如此之快，以至于气体没有足够的时间慢慢地通过岩浆扩散到现有的气泡中，就在喷发之前又形成了新的气泡。这就产生了一组原始的大气泡和另一组小得多、数量更多的气泡。该项目旨在更好地确定产生第二组气泡的爆炸性阈值，以便如果未来单个火山的喷发行为与过去可以观察到的产生火山灰的火山喷发行为相似，则可以更好地预测喷发能量，从而获得火山危害。这项研究将包括对各种观测到的火山喷发的火山灰进行检查，这些火山灰的能量谱通常以“火山爆发指数”（VEI）为特征。通过扫描电子显微镜（SEM）的详细研究，可以观察到两种气泡的印记。应该有一个喷发能量的阈值，超过这个阈值，在火山灰颗粒中可以发现晚期成核气泡（同步喷发），低于这个阈值，只存在初始的大气泡（喷发前）。该阈值将在本研究中量化。在火山喷发中，水（作为主要的溶解挥发性物质）没有时间扩散到喷发前的气泡和喷发时的气泡中，当喷发的岩浆泡沫破碎成灰烬时，预计会在玻璃中留下仍然溶解在玻璃中的水的浓度梯度，这取决于与喷发前和喷发时的气泡的距离。这种水的浓度可以用微拉曼光谱来观察，它对单个灰颗粒玻璃中O-H键的存在很敏感。该技术的分辨率足以绘制火山灰颗粒上的水浓度图，也可以作为颗粒内部深度的函数。这项技术从未应用于单个灰烬颗粒，因此该项目也将为科学界在未来的相关项目中提供一种新的工具。通过确定同喷发气泡成核的阈值，并进一步探索单个火山灰颗粒中溶解水的浓度梯度，该项目将更好地了解火山危害，以及在火山爆发期间驱动岩浆上升和火山灰形成的过程。

Volcanic eruptions are driven by the expansion of bubbles in magma as it rises through the "plumbing" system beneath a volcano. Gases (mostly water) dissolved in the magma at depth come out of solution as the magma rises to lower pressures near the Earth's surface because the amount of gas that can be dissolved in the magma depends on pressure. When the amount the magma can hold reduces below the amount dissolved, new bubbles are formed, and gas from adjacent magma migrates through the magma into the bubbles. As they grow, they push the overlying magma up and out of the volcano, which causes the overlying bubbles to grow faster, leading to ascent rates at the surface near the vent of the volcano that can be so fast in the case of highly explosive eruptions, that the gases do not have enough time to slowly diffuse through the magma into existing bubbles, and new bubbles form again just prior to eruption. This results in a set of original, large bubbles, and a second set of much smaller and more numerous bubbles. This project is directed toward better determining the threshold of explosivity that can produce this second set of bubbles, so that if future eruptions of individual volcanoes behave like the past ones that produced volcanic ash that can be observed, a better prediction of eruption energy and thus volcanic hazard can be obtained.The study will involve examination of ash from various well observed volcanic eruptions across a spectrum of energies, commonly characterized by "Volcanic Explosivity Index" (VEI). By detailed study using a Scanning Electron Microscope (SEM) the imprints of both populations of bubbles can be observed. There should be a threshold of eruption energy above which late-stage nucleated bubbles (syn-eruptive) are found in ash particles, and below which, only the initial set of large bubbles (pre-eruptive) exist. This threshold will be quantified in this study. In eruptions in which water (as the dominant dissolved volatile) does not have time to diffuse into pre-eruptive bubbles and syn-eruptive bubbles are nucleated, quenching of the erupting magmatic foam as it fragments to ash is expected to leave a concentration gradient of water still dissolved in glass that depends on distance from both pre-eruptive and syn-eruptive bubbles. This water concentration can be observed using Micro-Raman Spectroscopy, which is sensitive to the presence of O-H bonds in the glass of individual ash particles. The resolution of the technique is sufficient to map water concentration across an ash particle and also as a function of depth within the particle. This technique has never been applied to individual ash particles, so the project will also result in a new tool for the scientific community to use in future related projects. By determining the threshold for the nucleation of syn-eruptive bubbles, and further by exploring dissolved water concentration gradients in individual ash particles, this project should result in better understanding of both volcanic hazards, and the processes that drive magma ascent and the formation of ash during explosive volcanic eruptions.