Report on Galeras (Colombia) — 9 November-15 November 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 9 November-15 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, 9 November-15 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)
INGEOMINAS reported that during 9-14 November, a large number of tornillo earthquakes (long-period seismic events related to pressurized fluid flow at shallow depth) continued to occur below Galeras. In the previous 2 weeks, 25 tornillos were recorded. The earthquakes had characteristics similar to those that occurred before eruptions in 1992-1993. A small amount of deformation was recorded at Galeras during the report period. Activity in the previous month suggested that the volume of magma beneath the volcano was greater than that inferred to have been present during the 1992-1993 eruptions. Due to the increased activity at Galeras, the Alert level was raised from 3 (changes in the behavior of volcanic activity have been noted) to 2 (probable eruption in days or weeks).
According to new reports, on 14 November local authorities recommended the voluntary evacuation of as many as 9,000 people living in towns near the volcano, including in parts of Pasto (to the W), La Florida (to the N), and Nariño (to the N).
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.