The Servicio Geológico Minero Argentino (SEGEMAR) reported that at 0944 on 4 December a gas-and-ash plume at Planchón-Peteroa rose 850 m above the crater rim and drifted NE. The plume dissipated near the summit. The Alert Level remained at Yellow (the middle level on a three-color scale). According to the Sistema y Servicio Nacional de Prevención y Repuesta Ante Desastres (SENAPRED) the communities of Molina (66 WNW), Curicó (68 km NW), Romeral (75 km NW), and Teno (68 km NW) continued to be under a “Preventive Early Warning” (since 18 July 2025) and that a security perimeter 4 km from the craters was in effect.

Geological Summary. Planchón-Peteroa is an elongated complex volcano along the Chile-Argentina border with several overlapping calderas. Activity began in the Pleistocene with construction of the basaltic andesite to dacitic Volcán Azufre, followed by formation of the basaltic and basaltic andesite Volcán Planchón, 6 km N. About 11,500 years ago much of Azufre and part of Planchón collapsed, forming the massive Río Teno debris avalanche, which traveled 95 km to reach Chile's Central Valley. Subsequently, Volcán Planchón II was formed. The youngest volcano, andesitic and basaltic andesite Volcán Peteroa, consists of scattered vents between Azufre and Planchón, and and contains a small steaming crater lake. Reported eruptions from the complex have been dominantly explosive, although lava flows were emplaced in 1837 and 1937.

Sources: Servicio Geológico Minero Argentino (SEGEMAR), Sistema y Servicio Nacional de Prevención y Repuesta Ante Desastres (SENAPRED), Servicio Geológico Minero Argentino (SEGEMAR), Buenos Aires Volcanic Ash Advisory Center (VAAC)