An explosive eruption from Galeras detected by the seismic network on 2 January prompted INGEOMINAS to raise the Alert Level to I (Red; "imminent eruption or in progress"). An ash plume rose to an altitude of 12 km (39,400 ft) a.s.l. and drifted W and NW. Ashfall was reported in areas downwind, as far away as 110 km W. Ejected incandescent blocks fell onto the flanks 3.2-3.5 km away from the summit and ignited fires. An overflight on 3 January revealed diffuse gas plumes from the main crater. Fires started the previous day continued to burn on the N flank. The Alert Level was lowered to II (Orange; "probable eruption in term of days or weeks").

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)