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: 0738891
• 负责人: Katharine Cashman
• 金额: $ 2.01万
• 依托单位: University of Oregon Eugene
• 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 2010-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0738891&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喷发，如果喷发，将会产生什么后果。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喷发活动的短暂暂停。在该项目中，建议通过对厄瓜多尔基洛托阿火山中不寻常的富含晶体的沉降物沉积物的详细研究来解决这个问题。大约 800 年前，基洛托阿火山爆发了猛烈的喷发，其风格与 1991 年皮纳图博火山喷发非常相似，那次喷发摧毁了菲律宾克拉克美国空军基地。然而，基洛托阿重复了两次阵发性喷发，两次喷发之间有短暂的（几天到几周？）间歇期。为什么第一次喷发会结束，而有足够的岩浆来驱动短时间内的第二次喷发？当时通风口里发生了什么？在间歇期期间喷口区域会如何出现？我们能否识别出第二次大规模喷发即将发生的迹象？这项研究将检查第二次喷发开始时的喷口清理爆炸中的沉积物，因为它们含有在间歇期期间似乎堵塞喷口的物质，因此应该为我们提供有关该时期喷口状况的最多信息。该团队将重点关注底部分类良好的亚普林尼或火山坠落沉积物，该沉积物在邻近地区的底部附近含有一些涌动沉积物。爆炸释放出的灰烬已经分类良好，并且所有玻璃都与晶体分离。距离的分类非常有限，加上“干净”晶体的浓度，表明在喷发之前有一些过程研磨和/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，要么是在事件之间重新填充管道的幼年岩浆。在这两种情况下，源似乎都受到了强酸性气体喷射的影响，研磨和蚀刻了材料，淘洗了玻璃质部分，并留下了富含晶体和岩屑的物质，堵塞了通风口。然后一定是潜水或岩浆爆炸喷射了通风孔堵塞物。在这个项目中，研究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。他们将表征样品的晶粒尺寸（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（通过 SEM，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）和成分数据与喷发其他沉积物成分的现有数据以及火山口壁中暴露的较旧沉积物进行比较。这些技术结合在一起将能够表征间歇期期间的管道条件以及重新喷发的过程。该项目将提高对复杂硅质喷发的了解，特别是关于可能导致爆炸活动暂时终止和恢复的岩浆管道内的条件。该项目是与厄瓜多尔、法国和意大利同事合作的，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之间的合作。