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，以及如果喷发，喷发将产生什么影响。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。在这个项目中，建议通过对厄瓜多尔奎洛托亚火山一种不寻常的富含晶体的尘埃沉积物的详细研究来解决这个问题。大约800年前，奎洛托亚以与1991年皮纳图博火山喷发非常相似的方式猛烈喷发，那次喷发摧毁了位于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？)两次喷发之间的间歇期。为什么第一次喷发结束时，有足够的岩浆驱动不久之后的第二次喷发？当时通风口里发生了什么事？喷口区域是如何在间歇期出现的，我们能识别出第二次大喷发即将到来的迹象吗？这项研究将检查开始第二次喷发的喷口清理爆炸的沉积物，因为它们含有似乎在喷口间歇期间堵塞喷口的物质，因此应该会为我们提供关于那段时间喷口状况的最多信息。该团队将把重点放在一个基本的、分选良好的陆下或火山岩系秋季矿床上，该矿床在其近端的底部附近含有一些激增的矿床。爆炸分散了一种已经被很好地分类的火山灰，所有的玻璃都从晶体中分离出来。非常有限的距离分选，以及“干净”晶体的浓度，表明某些过程在喷发前碾磨和/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事件后堵塞喷口的物质，要么是重新填充事件之间管道的年轻岩浆。在任何一种情况下，看起来源头都受到了强酸性气体喷射的影响，粉碎和蚀刻了材料，淋洗了玻璃部分，并留下了堵塞通风口的富含晶体和岩石的物质。然后一定是潜水或潜水岩浆爆炸排出了通风口堵塞。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感兴趣的存款来检验这一假设。他们将对样品进行粒度(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的表征(通过扫描电子显微镜，以寻找磨损和/或点蚀的证据，以提供物理或化学准备的证据)。他们还将分析晶体中的主要元素和熔融夹杂物，以及粘合研磨玻璃中的主要元素和挥发性成分(使用EPMA和FTIR)。然后，结构(晶体大小分布)和成分数据将与喷发的其他沉积物的现有数据进行比较，并与火山口墙壁中暴露的更古老的沉积物进行比较。这些技术结合在一起，将能够描述中断期间的管道条件以及重新引发喷发的过程。该项目将有助于更好地了解复杂的硅质喷发，特别是关于岩浆管道内可能导致爆炸活动暂时终止和恢复的情况。该项目是厄瓜多尔、法国和意大利同事的合作项目，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。

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.