On 22 November INGEOMINAS reported that the Alert Level for Galeras was increased from 3 (changes in the behavior of volcanic activity have been noted) to 2 (probable eruption in days to weeks) on a scale of 4-1, due to seismic patterns previously indicative of eruptive episodes. Emissions of gas and steam were seen coming from the periphery of the main crater during aerial observations on 4, 5, 10, and 11 December.

Geological Summary. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Source: Servicio Geológico Colombiano (SGC)