Dynamics of a large, restless, rhyolitic magma system at Laguna del Maule, southern Andes, Chile

Explosive eruptions of large-volume rhyolitic magma systems are common in the geologic record and pose a major potential threat to society. Unlike other natural hazards, such as earthquakes and tsunamis, a large rhyolitic volcano may provide warning signs long before a caldera-forming eruption occurs. Yet, these signs—and what they imply about magma-crust dynamics—are not well known. This is because we have learned how these systems form, grow, and erupt mainly from the study of ash flow tuffs deposited tens to hundreds of thousands of years ago or more, or from the geophysical imaging of the unerupted portions of the reservoirs beneath the associated calderas. The Laguna del Maule Volcanic Field, Chile, includes an unusually large and recent concentration of silicic eruptions. Since 2007, the crust there has been inflating at an astonishing rate of at least 25 cm/yr. This unique opportunity to investigate the dynamics of a large rhyolitic system while magma migration, reservoir growth, and crustal deformation are actively under way is stimulating a new international collaboration. Findings thus far lead to the hypothesis that the silicic vents have tapped an extensive layer of crystal-poor, rhyolitic melt that began to form atop a magmatic mush zone that was established by ca. 20 ka with a renewed phase of rhyolite eruptions during the Holocene. Modeling of surface deformation, magnetotelluric data, and gravity changes suggest that magma is currently intruding at a depth of ~5 km. The next phase of this investigation seeks to enlarge the sets of geophysical and geochemical data and to use these observations in numerical models of system dynamics. INTRODUCTION Caldera-scale rhyolitic volcanoes can rapidly deposit hundreds of cubic kilometers of ash over several million square kilometers, threatening people and agriculture at the scale of an entire continent (Sparks et al., 2005; Lowenstern et al., 2006; Self, 2006). Sooner or later, Earth will experience another eruption of this magnitude (Lowenstern et al., 2006; Self and Blake, 2008); consequently, there is a need to gather comprehensive information and create multi-scale models that realistically capture the dynamics leading to these destructive events. Most of our current understanding of this type of volcanic system has been gleaned from the study of eruptive products long after the catastrophic eruption, including voluminous ash flow deposits, such as the Bishop, Bandelier, Huckleberry Ridge, and Oruanui Tuffs (Lowenstern et al., 2006; Hildreth and Wilson, 2007; Bachmann and Bergantz, 2008; Wilson, 2008). The most recent rhyolitic “super-eruption” produced the Oruanui Tuff 26,500 years ago in New Zealand. Even in this relatively recent case, the geologic evidence has been partly obliterated by caldera-collapse, erosion, and burial (Wilson et al., 2005). Moreover, probing the present-day structures beneath a number of calderas using seismic tomography (e.g., Romero et al., 1993; Steck et al., 1998; Farrell et al., 2014) or other geophysical measures (e.g., Lowenstern et al., 2006; Battaglia et al., 2003; Tizzani et al., 2009) has not detected eruptible domains of crystal-poor melt in the shallow crust, nor has it captured the dynamics that preceded these large eruptions. This paper focuses on the Laguna del Maule Volcanic Field, Chile, a large, potentially hazardous, rhyolitic magmatic system, where an alarming rate of surface uplift for the past seven years and concentrated swarms of shallow earthquakes prompted Observatorio Volcanologico de los Andes del Sur (OVDAS) to declare in March 2013 a yellow alert, signaling a potential eruption within months or years. Straddling the Andean range crest at 36° S (Fig. 1A), this volcanic field features: (1) 13 km of rhyolite that erupted both explosively and effusively during the past 20 k.y.; (2) a zone of low electrical resistivity in the shallow crust below the deforming area; (3) widespread elevated CO 2 concentrations; and (4) a negative (~10 mGal) Bouguer anomaly and preliminary evidence for a positive dynamic gravity signal indicating mass addition. The underlying magma system has been sampled by eruptions numerous times since its apparent inception in the late Pleistocene, including a dozen crystal-poor, glassy rhyolitic lavas during the Holocene. Linking the assembly and evolution of this