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

火山学家通常专注于预测火山何时喷发，如果喷发，将会造成什么后果。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短暂的喷发活动暂停。在这个项目中，建议通过对厄瓜多尔Quilotoa火山不寻常的富含晶体的沉降沉积物的详细研究来解决这个问题。大约800年前，基洛托亚火山猛烈喷发，其喷发方式与1991年的皮纳图博火山喷发非常相似，那次火山喷发摧毁了菲律宾的克拉克美国空军基地。然而，Quilotoa火山重复了两次爆发，两次爆发之间有短暂的间歇（几天到几周？）为什么第一次喷发结束时，有足够的岩浆在短时间内引发第二次喷发？当时通风口里发生了什么？在间歇期，喷发区是如何出现的？我们是否能识别出第二次大喷发即将来临的迹象？这项研究将检查从第二次喷发开始的清理喷口的爆炸中产生的沉积物，因为它们含有在间歇期间似乎堵塞了喷口的物质，因此应该为我们提供有关该时期情况的最多信息。该小组将把重点放在一个基底分选良好的次纪或火山坠落矿床上，该矿床在其近端基部附近含有一些涌动矿床。爆炸分散的火山灰已经被很好地分类，所有的玻璃都从晶体中分离出来。距离非常有限的分选，加上“干净”晶体的浓度，表明在喷发之前，一些过程研磨和/或溶解了颗粒。据推测，晶体和岩屑的来源要么是第一次喷发后堵塞喷口的物质，要么是在两次喷发之间重新填充通道的幼年岩浆。无论是哪种情况，似乎都表明该来源受到强酸性气体喷射的影响，这些气体研磨和蚀刻了材料，洗脱了玻璃部分，并留下了富含晶体和岩石的物质堵塞了通风口。然后必然是潜水或潜水岩浆爆炸把通风口堵塞了。在这个项目中，调查人员将通过进行详细的实地调查和收集利息存款来检验这一假设。他们将对样品的晶粒尺寸（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进行表征（通过扫描电镜，寻找磨损和/或点蚀的证据，这些证据将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，熔体夹杂物和粘附的底质玻璃中的主要元素和挥发性成分（使用EPMA和FTIR）。然后将其结构（晶体大小分布）和成分数据与其他喷发沉积物成分的现有数据以及暴露在火山口壁上的较旧沉积物进行比较。总之，这些技术将允许表征管道条件在中断期间以及重新启动喷发的过程。这个项目将使人们更好地了解复杂的硅质喷发，特别是岩浆通道内可能导致爆炸活动暂时终止和恢复的条件。该项目是与厄瓜多尔、法国和意大利的同事和学生合作进行的，参与该项目的学生将从这些互动以及北亚利桑那大学和俄勒冈大学之间的合作中受益。

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.