OVDAS-SERNAGEOMIN reported that during 13-19 May activity at Calbuco fluctuated at low levels and continued to decline. Inclement weather prevented daily observations of the summit area, although incandescence at the crater was observed during 17-18 May. According to ONEMI, the number of evacuees within the 20-km evacuation zone remained at 6,685 on 18 May. On 19 May the Alert Level was lowered to Orange (the second highest level on a four-color scale), and the exclusion zone was changed to a 10-km radius.

Geological Summary. Calbuco is one of the most active volcanoes of the southern Chilean Andes, along with its neighbor, Osorno. The late-Pleistocene to Holocene andesitic volcano is immediately SE of Lake Llanquihué in the Chilean lake district. Guanahuca, Guenauca, Huanauca, and Huanaque, all listed as synonyms of Calbuco (Catalog of Active Volcanoes of the World), are actually synonyms of nearby Osorno volcano (Moreno 1985, pers. comm.). The edifice is elongated in a SW-NE direction and is capped by a 400-500 m wide summit crater. The complex evolution included collapse of an intermediate edifice during the late Pleistocene that produced a 3-km3 debris avalanche that reached the lake. It has erupted frequently during the Holocene, and one of the largest historical eruptions in southern Chile took place from Calbuco in 1893-1894 that concluded with lava dome emplacement. Subsequent eruptions have enlarged the lava-dome complex in the summit crater.

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