On 11 August SERNAGEOMIN reported that the lava dome on the floor of Nevados de Chillán’s Nicanor Crater had grown taller in the previous few days based on webcam views. The portion of the dome that was visible with the webcam was reddish and rocky. The report noted that an increase in sulfur dioxide emissions and more intense explosions had been detected since 18 July. Similarly, an increase in the intensity and occurrence of thermal anomalies in the crater had been noted since 18 July, though anomalies had further intensified during the recent period of dome extrusion. An explosion at 1041 on 10 August was followed by the most intense thermal anomaly recorded during the last month. The Alert Level remained at Yellow, the second lowest level on a four-color scale. ONEMI stated that Alert Level Yellow (the middle level on a three-color scale) remained in place for the communities of Pinto and Coihueco, noting that the public should stay at least 2 km away from the crater.

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: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Oficina Nacional de Emergencia-Ministerio del Interior (ONEMI)