Report on Galeras (Colombia) — 23 November-29 November 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 23 November-29 November 2005
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2005. Report on Galeras (Colombia). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 23 November-29 November 2005. Smithsonian Institution and US Geological Survey.
1.22°N, 77.37°W; summit elev. 4276 m
All times are local (unless otherwise noted)
On 24 November at 0246 seismicity was recorded at Galeras that was associated with the beginning of an eruption. Ash from the eruption fell in the towns of Fontibon, San Cayetano, Postobon, and in north Pasto (E of the volcano). INGEOMINAS raised the Alert Level from 2 (probable eruption in days to weeks) to 1 (eruption imminent or occurring). The Washington VAAC observed a small puff of ash NE of the volcano at a height around ~4.6 km (15,000 ft) a.s.l. Activity decreased by the next day, so the Alert Level was reduced to 2. Thousands of people had been evacuated from the vicinity of the volcano during the week prior to the eruption.
Geologic Background. 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 horseshoe-shaped 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 historical eruptions since the time of the Spanish conquistadors.