Report on Galeras (Colombia) — 6 May-12 May 2009
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 6 May-12 May 2009
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2009. Report on Galeras (Colombia). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 6 May-12 May 2009. Smithsonian Institution and US Geological Survey.
1.22°N, 77.37°W; summit elev. 4276 m
All times are local (unless otherwise noted)
On 6 May, INGEOMINAS reported that gas-and-ash plumes from Galeras rose to an altitude of 5.8 km (19,000 ft) a.s.l. and drifted NE. An overflight revealed incandescence from a vent, 90-100 m in diameter, in the main crater that corresponded to a 500 degree Celsius temperature anomaly. Blocks, 9-13 m in diameter, scattered on the S and SE flanks were part of the 2008 lava dome that had been ejected during the 24 April 2009 eruption. White plumes originated from multiple points inside and outside of the crater. Volcanic tremor seldom occurred during the previous week. The Alert Level was lowered to III (Yellow; "changes in the behavior of volcanic activity").
On 9 May, a M 2.2 volcano-tectonic earthquake occurred 6 km to the NE of the main crater at a depth of 10 km. On 11 May, seismicity increased, and hybrid earthquakes and tremor were detected. The recent seismicity, along with incandescence in the crater, and low sulfur dioxide values suggested to INGEOMINAS that the volcano may become overpressurized. The Alert Level was raised to II (Orange; "probable eruption in term of days or weeks"). Steam plumes rose 250 m and drifted NW on 12 May.
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.