Reconstructing Explosive Magma Vesiculation

1912年阿拉斯加诺瓦鲁普塔火山的普林尼火山喷发是历史上最大的爆发之一，也是本研究的主题。普林尼火山喷发具有非常高的爆炸强度和破坏性。它们可以持续数天，单个的普林尼相持续数小时，并产生数十公里高、持续的热气体和火山颗粒柱。这些火山柱可以在半球或全球范围内分散，对社会产生重大影响。产生普林尼火山喷发的关键机制是岩浆挥发物（如水、二氧化碳和硫）的“爆炸性”释放，以及喷发岩浆的“爆炸性”碎裂成火山碎屑。后者是大小不一的淬火岩浆颗粒，包括火山灰。岩浆挥发物的爆炸性释放与岩浆碎裂导致普林尼火山喷发之间的相互关系是本项目的重点。该项目将包括对1912年Novarupta火山喷发的火山碎屑中测量的囊泡大小分布进行数值模拟。气泡大小分布提供了岩浆上升条件的直接记录，可以通过模拟上升岩浆中的气泡成核和生长来研究，这两者都是岩浆在上升驱动的减压过程中挥发性析出的结果。一个上升岩浆包的时间压力历史是决定模型与观测到的气泡大小分布是否吻合的关键参数。这项研究将验证普林尼火山喷发期间减压率惊人的假说，以及气泡成核和岩浆破碎之间的关系。火山碎屑中以小泡形式保存下来的大量气泡，其成核所需的大减压速率显然只能维持很短的时间，大概是在岩浆碎裂之前或碎裂期间。据认为，破碎是气泡超压积累的结果，超过了岩浆的抗拉强度。该项目将通过全面的定量分析，将现有的喷发产物观测分析与气泡成核和岩浆破碎的理论和实证工作结合起来。1912年Novarupta火山碎屑样本所记录的岩浆减压、气泡成核、气泡生长和最终岩浆破碎的重建，有望为我们对爆炸性火山作用的理解，特别是对普林尼式火山喷发的理解，带来新的进展。

The 1912 Plinian eruption of Novarupta volcano, Alaska is one the largest historical explosive eruptions and the subject of this study. Plinian eruptions are of very high explosive intensity and destructiveness. They can last for days with individual Plinian phases lasting for hours and producing tens of kilometers tall, sustained columns of hot gases and volcanic particles. These volcanic plumes can become dispersed on a hemispherical or global scale with significant societal impact. Key mechanisms to producing a Plinian eruption are the 'explosive' release of magmatic volatiles, such as water, carbon dioxide and sulfur, in conjunction with the 'explosive' fragmentation of the erupting magma into pyroclasts. The latter are variably sized particles of quenched magma, including volcanic ash. The interrelation between this explosive release of magmatic volatiles and magma fragmentation to result in a Plinian eruption is the focus of this project.This project will involve the numerical modeling of vesicle size distributions measured in pycroclasts from the 1912 Novarupta eruption. Vesicle size distributions provide a direct record of magma ascent conditions that can be investigated through modeling of bubble nucleation and growth in the ascending magma, both a consequence of volatile exsolution from the magma during ascent-driven decompression. The time-pressure history of an ascending parcel of magma, from which a given pyroclast is derived, is the key parameter determining the fit of modeled to observed vesicle size distributions. This investigation will test the hypothesis that decompression rates are surprisingly large during Plinian eruptions, as well as the question of how bubble nucleation and magma fragmentation are related. The large decompression rates apparently required to nucleate the large numbers of bubbles preserved as vesicles in pyroclasts can only be sustained for very short times, presumably immediately prior to or perhaps during magma fragmentation. It is thought that fragmentation is the consequence of accumulated bubble overpressure in excess of the magma's tensile strength. This project will integrate existing observational analyses of eruptive products with theoretical and empirical work on bubble nucleation and magma fragmentation through a comprehensive quantitative analysis. The resultant reconstruction of magma decompression, bubble nucleation, bubble growth and ultimately magma fragmentation, as recorded by the well-documented 1912 Novarupta pyroclast samples, is expected to lead to new advances in our understanding of explosive volcanism in general and Plinian style volcanic eruptions in particular.