RAPID PROPOSAL: Accelerating unrest within a large rhyolitic magma system: Laguna del Maule, Chile

这次RAPID将调查智利南安第斯山脉顶部拉古纳德尔毛勒火山场动荡的原因。这个火山场包含了地球上最集中的年轻的，冰川后（2万到2000年）流纹岩熔岩流，这些熔岩流从24个喷口喷发出来，以23 x 16.5公里的椭圆模式排列，拉古纳del Maule在中心。在湖盆两侧喷发的贫晶流纹岩在成分上几乎相同，通过晶液分馏与前体安山岩和流纹岩熔岩有关。这种岩石学和构造构型表明，目前整个湖盆下部有一大块高温流纹岩熔体，这些熔体是从一个岩心尺度的晶体泥状储层中提取出来的；一个类似的储层被认为是560平方公里的主教凝灰岩超级喷发的来源。从2007年开始的卫星大地测量（InSAR和GPS）表明，流纹岩圆顶圈的西侧开始以2012年底加速到300毫米/年的速度上升。此外，在直径20 km的膨胀区发生横向伸展，膨胀中心上方形成了一系列正断层。目前的抬升速度是最近在Yellowtone火山口或Santorini火山口检测到的4-5倍，这表明动荡可能会产生大量流纹岩的爆炸性喷发。该项目旨在了解流纹岩岩浆系统在喷发前的基本动力学，并评估拉古纳德尔毛勒火山场的火山危险性。这将通过用重力和二氧化碳测量来补充现有的观测来完成。重力基线将有助于确定地下密度和质量的变化。二氧化碳的测量将限制岩浆脱气的范围和强度，这可能反映了最近向浅层地壳中添加的新岩浆。这两项分析都将与南方安第斯天文台Volcanológico （OVDAS）合作进行。来自威斯康星大学麦迪逊分校的四名研究生将参与该项目的各个方面，包括在智利的实地考察和数据分析。2011年6月，智利Cordon Caulle的流纹岩爆发表明，即使是流纹岩的小规模爆发也可能引发大面积的洪水和火山灰，对智利和阿根廷的基础设施和农业造成相当大的破坏。该项目将提供有关拉古纳德尔毛勒火山场行为的重要信息，改进该地区火山危险性的评估，并最终有助于更好地了解喷发（或超级喷发）前的大型活动流纹岩岩浆系统。该项目由地球物理计划、岩石学和地球化学计划以及国际和综合活动办公室支持。

This RAPID will investigate the cause of unrest at the Laguna del Maule volcanic field on the crest of the southern Andes, Chile. This volcanic field comprises the greatest concentration of young, post-glacial (20,000 to 2,000 year old) rhyolitic lava flows on Earth that have erupted from 24 vents, arranged in a 23 x 16.5 km elliptical pattern with Laguna del Maule in the center. Crystal-poor rhyolites erupted on opposite sides of the lake basin are nearly identical in composition and are related to precursor andesitic and rhyodacitic lavas by crystal-liquid fractionation. This petrologic and structural configuration suggests that a large body of high temperature rhyolitic melt, extracted from a batholith-scale crystal-mush reservoir, currently underlies the entire lake basin; a similar reservoir is postulated to have been the source of the 560 km3 Bishop Tuff super-eruption. Beginning in 2007 satellite geodesy (InSAR and GPS) indicates that the western side of the rhyolite dome circle began uplifting at rates that have accelerated to 300 mm/yr by late 2012. Moreover, the inflating region, 20 km in diameter, is undergoing lateral extension and a series of normal faults are forming above the center of inflation. The current rate of uplift is 4-5 times higher than recently detected at the Yellowtone or Santorini calderas and indicates unrest that could produce a large-volume explosive eruption of rhyolite. The project seeks to understand the fundamental dynamics of a rhyolitic magma system prior to eruption and to assess the volcanic hazard in the Laguna del Maule volcanic field. This will be accomplished by complementing the existing observations with gravity and CO2 measurements. The gravity baselines will help determine the subsurface density and mass changes. The CO2 measurements will constrain the extent and magnitude of magmatic outgassing that might reflect a very recent addition of new magma into the shallow crust. Both analysis will be done in collaboration with the Observatorio Volcanológico de los Andes del Sur (OVDAS). Four graduate students from the University of Wisconsin-Madison will participate in all aspects of this project, including fieldwork in Chile, and data analysis.As demonstrated by the ongoing rhyodacite eruption at Cordon Caulle, Chile, that began in June, 2011, even a minor explosive eruption of rhyolite could trigger extensive flooding and ash fall, causing considerable damage to infrastructure and agriculture in both Chile and Argentina. This project will provide important information about the behavior of the Laguna del Maule volcanic field, improving the assessment of the volcanic hazard in the region, and ultimately helping to better understand large active rhyolitic magma systems prior to an eruption (or super-eruption).This project is supported by the Geophysics Program, the Petrology and Geochemistry Program and the Office of International and Integrated Activities.