Report on Galeras (Colombia) — November 1993
Bulletin of the Global Volcanism Network, vol. 18, no. 11 (November 1993)
Managing Editor: Richard Wunderman.
Galeras (Colombia) Conditions remain stable, but seismicity increases
Please cite this report as:
Global Volcanism Program, 1993. Report on Galeras (Colombia) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 18:11. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199311-351080.
1.22°N, 77.37°W; summit elev. 4276 m
All times are local (unless otherwise noted)
Seismicity . . . measured 5.8 km SE of the crater was moderate to low in November. Seismic energy due to high-frequency events remained low for most of November, but rose dramatically at the end of the month. The largest number of high-frequency events took place on the 24th (26 events) and 29th (10 events). Low-frequency events occurred more uniformly during the month, with the largest number of low frequency events on 5-10 November (averaging 2.3 events/day).
About half the high-frequency events were of sufficient amplitude and frequency to be locatable. They were placed ~3.5 km N of the crater margin at 4-8 km depths. No events exceeded M 3.9. Ten events were felt at settlements 5-8 km from the epicenters.
In November no significant change was indicated by either direct visual observations of the crater, or dry tilt measured at instruments located 0.9 and 1.6 km from the crater. On 5 November the peak value of SO2 flux, estimated using on a 1 m/s (3.6 km/hour) wind speed, was 418 t/d. This is tiny compared to the values seen in May 1989: up to 5,000-10,000 t/d.
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.