An explosive eruption from Galeras detected by the seismic network on 20 November prompted INGEOMINAS to raise the Alert Level to I (Red; "imminent eruption or in progress"). Residents reported five explosions, sound waves, and incandescence from multiple areas in the crater. Plume modelling from the Washington VAAC suggested that the resulting ash plume may have risen as high as 14.3 km (46,900 ft) a.s.l. and drifted N. Ashfall was reported in areas 10 km E, N, and NNW. Seismicity increased after the eruption and then gradually decreased. On 21 November, INGEOMINAS lowered the Alert Level to II (Orange; "probable eruption in term of days or weeks"). According to news articles, 900-1,000 people out of about 9,000 ordered to evacuate went to shelters.

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.

Sources: Servicio Geológico Colombiano (SGC), Agence France-Presse (AFP)