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.
Explosive eruptions of large volume rhyolitic magma systems are common in the geologic record and pose a major potential threat to society. The Laguna del Maule Volcanic Field, Chile, is one such volcanic field; since 2007, the crust there has been inflating at an astonishing rate of 25 cm/yr. Our one (1) year grant proposal was to establish baseline measurements of gravity and CO2 emissions and formalize collaborative ties with scientists of the Southern Andes Volcanic Observatory (OVDAS). The project supported installation of a network of ~35 stations, designed for monitoring of gravitational acceleration. Gravity measurements provide the only direct way to determine how mass moves in the subsurface. The results indicate widespread and small positive gravity changes (gravity is pulling slightly more) throughout the lake basin during the ten months between occupations, indicating that the sub-surface density is increasing (presumably through intrusion of basaltic magma). We also measured volcanic CO2 emissions, and found readings of 0.5-1.0 % (vol.) throughout the volcanic field, and up to 7 % (vol.) along the northern lakeshore of Laguna del Maule. These high readings are also consistent with the intrusion of basaltic magma. Finally, we established plans for collaboration with seismologists working in the Chilean governmental organization responsible for volcanic monitoring, OVDAS, through a visit to their volcano monitoring observatory in Temuco.
