SERNAGEOMIN reported low levels of activity at Nevados de Chillán during 1-15 May. The area of the lava dome on the floor of Nicanor Crater was 1,626 square meters, similar to measurements from March (1,665 square meters); deformation measurements indicated that extrusion had ceased. Crater incandescence was sometimes visible; thermal anomalies were identified in satellite images on 3, 5, 8, 10, 13, and 15 May. There were 217 explosions in the active crater recorded by the seismic network. The explosions mainly produced steam, though the plumes occasionally contained tephra. Material from explosive activity accumulated in the NE part of the crater. An energetic explosion on 16 May produced an ash plume that rose 900 m and pyroclastic flows that descended the SE, NE, and W flanks as far as 400 m. Seismicity remained at moderate levels. Sulfur dioxide emissions averaged 466 ± 54 tons per day, peaking at 802 tons per day on 7 May. An explosion was recorded on 21 May. 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)