Report on Galeras (Colombia) — 17 November-23 November 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 17 November-23 November 2004
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2004. Report on Galeras (Colombia). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 17 November-23 November 2004. Smithsonian Institution and US Geological Survey.
1.22°N, 77.37°W; summit elev. 4276 m
All times are local (unless otherwise noted)
Galeras volcano erupted explosively on 21 November at 1544, Instituto Colombiano de Geología y Minería reported. The eruption produced a shock wave felt as far away as Cimarrones (18 km N of the volcano), Chachagui (17 km N of the volcano), and Laguna de La Cocha (20 km SW of the volcano). Effects of the shock wave varied from a loud roar, to the vibration of large windows, to the feeling of an earthquake. Ballistic rock blocks were expelled and fell nearly 3 km from the volcano on its eastern flank, producing short-lived forest fires. The eruption produced an ash-and-gas column that rose to an estimated height of 9-10 km a.s.l and drifted to the S and W. The Washington VAAC reported that satellite imagery through 2015 on 21 November revealed that high level ash estimated to be near 9 km a.s.l had moved to the W, while low level ash estimated to be near 4-5 km a.s.l remained in the vicinity of the summit of the volcano and showed little motion.
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.