INGEOMINAS reported that on 25 January an emerging seismic pattern from Galeras, characterized by "tornillo-type" earthquakes, was similar to patterns detected prior to past eruptions. Staff noted a strong sulfur gas odor and observed emissions from various areas of the crater that drifted N. Based on changes in seismicity and observed gas emissions, INGEOMINAS raised the Alert Level to II (Orange; "probable eruption in term of days or weeks"). Scientists again observed emissions from various areas of the crater during an overflight on 27 January, but with a slight increase in the number of vents. Gas plumes drifted NW. "Tornillo-type" earthquakes ceased on the morning of 30 January.

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)