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Open-vent volcanic systems with active degassing are particularly effective at producing infrasound that exhibits resonant tones controlled by the geometry of the volcano's crater. Changes in the infrasound character can thus provide constraints on a crater's lava level, which may vary dynamically in the lead-up to an eruption. Here we show that the increasing frequency content and damping characteristics of the resonant infrasound at Volcan Villarrica (Chile) relate to lava lake position in its crater/conduit preceding its 2015 eruption. We model the acoustic response of Villarrica's crater to determine that the lake began to rise on 27 February and reached the flared upper part of Villarrica's crater before oscillating during the two days prior to the 3 March paroxysm and 1.5 km-high lava fountain. This study demonstrates the utility of remote infrasound monitoring for future eruptions of Villarrica and other analogous open-vent volcanoes.

Plain Language Summary Volcanic craters are like giant musical instruments in the sense that the sounds that volcanoes produce can change as the crater size evolves. We track the changing tones and resonance of low-frequency infrasounds at Volcan Villarrica (Chile) and attribute their variations as a response to the rise of the lava lake in the crater. We use infrasound data and employ numerical models to track the depth of the lava lake over time. Significant changes in infrasound character occurred just prior to a violent 3 March 2015 eruption leading us to propose that monitoring volcano infrasound resonance is important for forecasting future volcanic eruptions.