INGEOMINAS reported that observations of Galeras during an overflight on 3 December revealed thermal anomalies in the main crater measuring 155 degrees Celsius. During 8-11 December, seismic activity decreased, although some seismic signals resembled patterns seen prior to previous eruptions. Sulfur dioxide emissions were low. Seismicity increased on 12 December; earthquakes M 2.2 and less were detected within 2 km of the summit and at depths up to 3 km below the summit during 12-15 December. The Alert Level was raised 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)