Report on Galeras (Colombia) — October 1994
Bulletin of the Global Volcanism Network, vol. 19, no. 10 (October 1994)
Managing Editor: Richard Wunderman.
Galeras (Colombia) Sporadic screw-type seismic events; SO2 flux of 38-832 metric tons/day
Please cite this report as:
Global Volcanism Program, 1994. Report on Galeras (Colombia) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 19:10. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199410-351080.
1.22°N, 77.37°W; summit elev. 4276 m
All times are local (unless otherwise noted)
During October activity at Galeras remained low. In terms of seismicity, on 20 October sporadic "screw-type" events reappeared. Screw-type events are comparatively monochromatic and with slowly decaying coda (late arriving) waves. They were so-named because their seismograph records look similar to the profile of a finely threaded screw. They are considered significant because they preceded five of the six eruptions between July 1992 and June 1993; on the other hand they have also occurred without being followed by an eruption. During October, seismic stations located 0.9-2.4 km from the active crater detected seven screw-type events. The codas of the screw-type events had durations of 31-63 seconds and a computed damping coefficient of 0.02. The seismic signals detected at all three stations had the same dominant frequency, ~ 2.5 Hz, and the spectra ranged from ~ 2.4 to 10.3 Hz.
Small earthquakes (M<2.4) took place at depths up to 5 km. These earthquakes had epicenters clustered beneath and around the active crater, most plotting within a radius of ~4 km. Butterfly-type events also took place. The SO2 flux obtained by the mobile COSPEC method showed fairly low values: 38-832 t/d. Degassing continued to be concentrated chiefly on the active cone's W fringe with smaller fumaroles at the interior of the main crater.
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.
Information Contacts: INGEOMINAS, Pasto.