SERNAGEOMIN reported continuing explosive and effusive activity at Nevados de Chillán’s Nicanor Crater during 16-31 August, though weather conditions often prevented visual confirmation. Explosions generated ash plumes that rose as high as 1.1 km above the crater rim and were denser towards the end of August. Crater incandescence was sometimes visible at night, and though not intense, it brightened during explosive periods. The L5 and L6 lava flows continued to advance, though at a very low rate, averaging 1 m/h for L5. The L5 lava flow was 1,380 m long and L6 was 850 m long based on satellite images, measured from the rim of Nicanor Crater to the distal end of the flows. A decrease in thermal anomalies over the flows identified in satellite images suggested that the flows were cooling. The average temperature was 73 degrees Celsius with a maximum of 100 degrees for L5 and an average of 79 degrees Celsius with a maximum of 100 degrees for L6. Temperatures at the vents at Nicanor Crater averaged 115 degrees Celsius and were as high as 252 degrees during explosive phases. Sulfur dioxide emissions measured from local DOAS stations abruptly decreased and remained low. There was a total of five thermal anomalies, all with low radiance values. On 29 August pyroclastic flows traveled 560 m NE and collapses of L5’s middle and distal parts of the flow were observed. 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)