POVI reported that observations of Nevados de Chillán's Volcán Arrau dome complex during 7-11 December indicated phreatic explosions, ash emissions, and a new sulfur deposit on the flank. On 11 December Servicio Nacional de Geología and Minería (SERNAGEOMIN) reported that explosive activity generated an ash plume to 3 km above the complex. The Alert Level remained at Yellow, the middle level on a three-color scale, and the public was reminded not to approach the craters within a 3-km radius.

Geological Summary. The compound volcano of Nevados de Chillán is one of the most active of the Central Andes. Three late-Pleistocene to Holocene stratovolcanoes were constructed along a NNW-SSE line within three nested Pleistocene calderas, which produced ignimbrite sheets extending more than 100 km into the Central Depression of Chile. The dominantly andesitic Cerro Blanco (Volcán Nevado) stratovolcano is located at the NW end of the massif. Volcán Viejo (Volcán Chillán), which was the main active vent during the 17th-19th centuries, occupies the SE end. The Volcán Nuevo lava-dome complex formed during 1906-1945 on the NW flank of Viejo. The Volcán Arrau dome complex was then constructed on the SE side of Volcán Nuevo between 1973 and 1986, and eventually exceeded its height. Smaller domes or cones are present in the 5-km valley between the two major edifices.

Sources: Proyecto Observación Villarrica Internet (POVI), Servicio Nacional de Geología y Minería (SERNAGEOMIN)