An explosive eruption from Galeras on 30 September prompted INGEOMINAS to raise the Alert Level to I (Red; "imminent eruption or in progress"). National Park personnel reported two explosions and incandescent material ejected from the area of the active cone. An ash plume rose to an approximate altitude of 12.3 km (40,400 ft) a.s.l. and drifted E, then N. The sulfur dioxide emission rate was between 1,100 and 9,300 tons per day. Ash was deposited in Sandoná (15 km NW), and Ancuya, Linares, and Sotomayor (40 km NW). Seismicity decreased after the eruption. On 1 October, seismicity was low and the sulfur dioxide emission rate was 300 tons per day. The Alert Level was lowered to II (Orange; "probable eruption in term of days or weeks"), and then to III (Yellow; "changes in the behavior of volcanic activity") on 6 October.

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)