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爆发，如果爆发，会发生什么。 然而，火山学家面临的另一个问题是区分喷发的真正结束和喷发活动的短暂停顿。在这个项目中，建议通过详细研究厄瓜多尔基洛托阿火山一个不寻常的富含晶体的沉降物存款来解决这个问题。 大约800年前，基洛托阿火山猛烈喷发，其风格与1991年皮纳图博火山喷发非常相似，皮纳图博火山摧毁了菲律宾的克拉克美国空军基地。 然而，Quilotoa重复了两次阵发性喷发，时间很短（几天到几周？）两次喷发之间的间隙。 为什么第一次喷发结束时，有足够的岩浆驱动第二次事件后不久？ 当时通风口发生了什么？ 在间歇期，火山口区域是如何出现的？我们能否识别出第二次大喷发即将来临的迹象？ 这项研究将检查开始第二次喷发的排气口清理爆炸的沉积物，因为它们含有似乎在中断期间堵塞排气口的物质，因此应该为我们提供有关该时期状况的最多信息。 该小组将重点关注一个基底分选良好的次斜长石或火山岩瀑布存款，其中包含一些浪涌存款附近的基地在近端的地方。爆炸散布的灰烬已经被很好地分类，所有的玻璃都从晶体中分离出来。 非常有限的分选距离，连同浓度的“干净”的晶体，表明一些过程研磨和/或溶解的颗粒喷发前。据推测，晶体和岩屑碎屑的来源是第一次喷发后堵塞喷口的物质，或者是在两次喷发之间重新填充管道的幼年岩浆。 在这两种情况下，似乎源受到强酸性气体喷射，研磨和蚀刻的材料，淘析的玻璃质部分，并留下一个晶体和结石丰富的质量堵塞通风口。 然后，潜水或潜水岩浆爆炸一定会将通气孔堵塞物喷出。在本项目中，调查人员将通过进行详细的实地调查和收集感兴趣的矿床来检验这一假设。 他们将表征样品的粒度（以及单独的晶体尺寸）、成分（包括斑晶比例）、晶体和岩屑形状以及表面纹理（通过SEM，以寻找磨损和/或点蚀的证据，从而提供物理或化学制备的证据）。 他们还将分析晶体中的主要元素，以及熔融夹杂物和粘附的基质玻璃中的主要元素和挥发性成分（使用EPMA和FTIR）。 然后，将结构（晶体尺寸分布）和成分数据与现有的数据进行比较，为其他存款的喷发，以及与旧的存款暴露在破火山口的墙壁。这些技术将允许在中断期间的管道条件的特性，以及重新启动喷发的过程。 这一项目将导致更好地了解复杂的火山喷发，特别是关于岩浆管道内的条件，可能导致暂时终止和爆炸活动的中止。 这个项目是与厄瓜多尔，法国和意大利的同事和学生参与该项目将受益于这些互动以及北方亚利桑那大学和俄勒冈州大学之间的合作。