Dr. Brad Singer - Professor, Department of Geoscience, University of Wisconsin Madison
The Laguna del Maule volcanic field (LdM), central Chile, comprises the highest concentration of post-glacial rhyolite in the world and currently exhibits extraordinary unrest manifest as uplift at >20 cm/year since 2007. At least 50 explosive and effusive rhyolitic-andesitic eruptions during the past 26,000 years provide a sampling through time of the development of this large silicic magma system that allows for the evaluation of its spatial and temporal evolution. Ongoing surface deformation, earthquake distribution, and gravity changes suggest that magma is currently intruding at 0.05 km3/yr at ~5 km depth. Zircons reveal that compositionally distinct domains developed concurrently within the LdM magma reservoir, which produced two episodes of rhyolitic eruptions at 23–19 and 8–2 ka. Zircons record 160 kyr of magma emplacement and crystallization, resulting in a large reservoir that has been imaged via seismic, gravity, and electrical resistivity methods. A 3-D shear-wave velocity model that delineates a shallow magma reservoir (~2-8 km below surface) of significant volume (~450 km3) and average melt fraction (~5%) is compatible with the gravity and geodetic observations, as well as with plagioclase diffusion chronometry indicating that rhyolitic eruptions reflect rapid melt extraction (decades) from relatively cool crystalline mush. During the Holocene, a paleoshoreline was warped upward >60 m where rhyolitic eruptions were concentrated. Based on an intrusion model to explain the current rate of surface inflation, we find that ~13 km3 of magma recharged the reservoir at ~7 km depth via several high-flux events during the Holocene, accompanied by ~9 km3 of rhyolitic eruptions.
Although the long-term rate of magma input is consistent with reservoir freezing and pluton formation, trace element zoning in zircon and crystallization rates derived from a model of diffusion-limited zircon growth suggest the erupted rhyolite batches originate from long-lived hot zones embedded within the much larger cool reservoir. These observations imply that large, but mostly cool, systems are incubated and grow at shallow depth via localized, episodic high-flux injections of hot magma. These contrasting, coeval magma storage conditions obviate a simple hot vs cold storage dichotomy for large silicic magma systems. Although few people live or work within 10 km of the rhyolite vents, known hazards include extensive pyroclastic falls and flows that could trigger lahars focused into major river channels toward hydroelectric infrastructure, agriculture, and major population centers.