Excess degassing drives long-term volcanic unrest at Nevado del Ruiz

This study combines volcanic gas compositions, SO₂ flux and satellite thermal data collected at Nevado del Ruiz between 2018 and 2021. We find the Nevado del Ruiz plume to have exhibited relatively steady, high CO₂ compositions (avg. CO₂/S_T ratios of 5.4 ± 1.9) throughout. Our degassing models support that the CO₂/S_T ratio variability derives from volatile exsolution from andesitic magma stored in the 1-4 km depth range. Separate ascent of CO₂-rich gas bubbles through shallow (< 1 km depth), viscous, conduit resident magma causes the observed excess degassing. We infer that degassing of ~ 974 mm³ of shallow (1-4 km) stored magma has sourced the elevated SO₂ degassing recorded during 2018-2021 (average flux ~ 1548 t/d). Of this, only < 1 mm³ of magma have been erupted through dome extrusion, highlighting a large imbalance between erupted and degassed magma. Escalating deep CO₂ gas flushing, combined with the disruption of passive degassing, through sudden accumulation and pressurization of bubbles due to lithostatic pressure, may accelerate volcanic unrest and eventually lead to a major eruption.