INGEOMINAS reported that an earthquake swarm from Galeras which began on 20 August had been preceded by increased gas emissions a few days prior. During 21-22 August seismicity remained high. Five volcano-tectonic earthquakes were felt by local residents and caused windows to vibrate. They were located within a 300-900 m radius from the crater, at depths of less than 2 km. The largest event was M 4.3. On 23 August a M 4.6 earthquake was located E of Galeras at a depth of 2 km. The Alert Level was raised to II (Orange; "probable eruption in term of days or weeks"). An eruption began on 25 August, prompting INGEOMINAS to raise the Alert Level to I (Red; "imminent eruption or in progress"). Meteorological cloud cover initially prevented visual observations of the summit, although an eruption plume was seen amongst the clouds, and thermal anomalies were detected by an infrared camera. Ashfall was reported in areas 7-12 km NW. Observers in Pasto (~ 10 km E) reported that gas-and-ash plumes rose 300 m above the crater.

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)