Collaborative Research: Crystal- and Lithic-rich Fallout at Quilotoa Volcano, Ecuador: Vent Processes During Short-Lived Hiatuses in Violent Eruptions

合作研究：厄瓜多尔基洛托阿火山富含晶体和岩屑的沉降物：剧烈喷发中短暂中断期间的喷发过程

• 批准号: 0739087
• 负责人: Michael Ort
• 金额: $ 8.58万
• 依托单位: Northern Arizona University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-07-01 至 2011-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0739087&HistoricalAwards=false
• 关键词: Collaborative; Research; Crystal; Lithic; rich

Volcanologists commonly focus on predicting when a volcano will erupt and, if it does, what that eruption will do. However, another problem that faces volcanologists is distinguishing between the true end of an eruption and a short-lived pause in eruptive activity. In this project, it is proposed to address this question through a detailed study of an unusual crystal-rich fallout deposit from Quilotoa volcano, Ecuador. Quilotoa erupted violently about 800 years ago in a style very similar to that of the 1991 Pinatubo eruption, which devastated Clark US Air Force base in the Philippines. Quilotoa, however, repeated the paroxysmal eruption twice, with a short (days to weeks?) hiatus between the two eruptions. Why did the first eruption end when there was sufficient magma to drive a second event a short time later? What happened in the vent at that time? How might the vent area have appeared during the hiatus, and could we recognize the signs that a second huge eruption was imminent? This study will examine the deposits from the vent-clearing explosions that began the second eruption, as they contain the material that appears to have been clogging the vent during the hiatus and thus should give us the most information on its condition during that period. The team will focus on a basal well-sorted subplinian or vulcanian fall deposit that contains a few surge deposits near its base in proximal localities. The explosions distributed an ash that was already well sorted and had had all glass separated from the crystals. Very limited sorting with distance, together with the concentration of 'clean' crystals, suggest that some process milled and/or dissolved the grains prior to eruption. It is hypothesized that the source of crystals and lithic clasts was either material that clogged the vent after the first eruptive episode or juvenile magma that re-filled the conduit between events. In either case, it appears that the source was subjected to strong acidic gas jetting that milled and etched the material, elutriated the vitric portion, and left a crystal- and lithic-rich mass clogging the vent. Phreatic or phreatomagmatic explosions must have then ejected the vent clog. In this project, the investigators will test this hypothesis by conducting a detailed field survey and collection of the deposits of interest. They will characterize the samples for grain size (and, separately, crystal size), componentry (including phenocryst proportions), crystal and lithic shapes, and surface textures (by SEM, to look for evidence of abrasion and/or pitting that would provide evidence of either physical or chemical preparation). They will also analyze the crystals for major elements and both melt inclusions and adhering groundmass glass for major elements and volatile components (using EPMA and FTIR). The textural (crystal size distribution) and compositional data will then be compared with existing data for components of the other deposits of the eruption, as well as with older deposits exposed in the caldera walls. Together these techniques will allow the characterization of the conduit conditions during the hiatus as well as the processes that reinitiated the eruption. This project will result in improved understanding of complex silicic eruptions, particularly with regard to conditions within magma conduits that might lead to temporary terminations and resumptions of explosive activity. This project is a collaboration with Ecuadorian, French, and Italian colleagues and students involved in the project will benefit from these interactions as well as the collaboration between Northern Arizona University and University of Oregon.

火山学家通常专注于预测火山何时会爆发，如果这样做，那爆发会做什么。 但是，面对火山学家的另一个问题是区分喷发的真实末端和喷发活动中短暂的停顿。在这个项目中，建议通过详细研究来自厄瓜多尔Quilotoa Volcano的不寻常的水晶辐射沉积物的详细研究。 大约800年前，Quilotoa的风格与1991年的Pinatubo爆发非常相似，该风格摧毁了菲律宾的克拉克美国空军基地。 然而，Quilotoa在两次喷发之间进行了短短（天到几周）的短速度（几天到几周）的阵发两次。 当不久后有足够的岩浆驾驶第二个事件时，为什么第一次爆发结束了？ 当时的通风口发生了什么？ 在中断期间，通风口区域如何出现，我们能否认识到第二次巨大爆发的迹象？ 这项研究将检查开始第二次喷发的排气清除爆炸的沉积物，因为它们包含似乎在裂隙期间堵塞通风口的材料，因此在此期间应该为我们提供有关其状况的最多信息。 该团队将专注于基础良好的Subplinian或Vulcanian Fall矿床，该矿床在其近端地区的基地附近包含一些激增的沉积物。爆炸分布了一个已经分类良好的灰烬，并且将所有玻璃与晶体分开。 与距离的分类非常有限，以及“清洁”晶体的浓度，表明某些过程在喷发前铣削和/或溶解了谷物。假设晶体和岩性碎屑的来源是在第一次喷发发作后堵塞通风口的材料，或者是重新填充事件之间导管的少年岩浆。 无论哪种情况，看来该源都经受了强酸性气喷射的影响，从而磨碎了材料并蚀刻了材料，放大了玻璃体，并留下了晶体和岩石富含晶体的质量。 然后必须弹出液压或phreatomagmagmatic爆炸。在该项目中，调查人员将通过进行详细的现场调查并收集感兴趣的存款来检验这一假设。 他们将表征晶粒尺寸（以及分别是晶体尺寸），成分（包括表晶的比例），晶体和岩性形状以及表面纹理的样品（通过SEM，以寻找磨损和/或斑点的证据，这些证据都可以提供物理或化学制备的证据）。 他们还将分析主要元素的晶体，以及熔融夹杂物和粘附的地面玻璃（使用EPMA和FTIR）的主要元素和挥发性成分。 然后，将将纹理（晶体尺寸分布）和组成数据与喷发其他沉积物的组成部分以及暴露于Caldera壁中的较旧沉积物进行比较。这些技术将允许在裂口期间的导管条件以及重新启动喷发的过程进行表征。 该项目将提高人们对复杂硅质喷发的理解，特别是关于岩浆导管中可能导致临时终止和爆炸活性恢复的条件。 该项目是与厄瓜多尔，法语和意大利同事的合作，参与该项目的学生将受益于这些互动，以及北亚利桑那大学和俄勒冈大学的合作。