Reconstructing Explosive Magma Vesiculation

重建爆炸岩浆囊泡

• 批准号: 1019872
• 负责人: Helge Gonnermann
• 金额: $ 23.17万
• 依托单位: William Marsh Rice University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-07-01 至 2014-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1019872&HistoricalAwards=false
• 关键词: Reconstructing; Explosive; Magma; Vesiculation

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.

1912年阿拉斯加州诺瓦鲁塔火山的普林尼安喷发是历史上最大的爆炸性喷发之一，也是本研究的主题。普林尼亚火山喷发具有极高的爆炸性和破坏性。它们可以持续几天，个别的普林尼安阶段持续数小时，并产生数十公里高的持续热气和火山颗粒柱。这些火山羽流可以在半球或全球范围内扩散，产生重大的社会影响。产生普林尼安火山喷发的关键机制是水、二氧化碳和硫磺等岩浆挥发物的“爆炸性”释放，以及喷发的岩浆“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。这种岩浆挥发分的爆炸性释放和岩浆碎裂导致普林尼安火山喷发之间的相互关系是这个项目的重点。这个项目将涉及1912年诺瓦鲁塔火山喷发以来在火山碎屑中测量到的气泡大小分布的数值模拟。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，这两者都是上升驱动减压过程中挥发分从岩浆中逸出的结果。岩浆上升地块的时间-压力历史决定了模拟的泡孔尺寸分布是否与观测到的泡孔尺寸分布相吻合。这项调查将检验在普林尼亚火山喷发期间减压速度惊人地大的假设，以及气泡成核和岩浆破碎如何相关的问题。在火山碎屑中以小泡的形式保存下来的大量气泡成核所需的高减压速率只能维持很短的时间，可能是在岩浆破碎之前，也可能是在岩浆破碎之前。认为碎裂是累积的气泡超压超过岩浆抗张强度的结果。该项目将通过全面的定量分析，将现有对喷发产物的观测分析与关于气泡成核和岩浆破碎的理论和经验工作结合起来。1912年诺瓦鲁塔火山碎屑样品记录了岩浆减压、气泡成核、气泡生长和最终岩浆碎裂的结果，预计这将导致我们对一般爆炸性火山作用，特别是普林式火山喷发的理解取得新的进展。