Report on Galeras (Colombia) — October 1993
Bulletin of the Global Volcanism Network, vol. 18, no. 10 (October 1993)
Managing Editor: Edward Venzke.
Galeras (Colombia) Fumarolic activity continues; seismicity remains low
Please cite this report as:
Global Volcanism Program, 1993. Report on Galeras (Colombia) (Venzke, E., ed.). Bulletin of the Global Volcanism Network, 18:10. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199310-351080.
1.22°N, 77.37°W; summit elev. 4276 m
All times are local (unless otherwise noted)
Volcanic activity remained low in September and October. Electronic tiltmeters located 0.9 and 1.6 km from the summit on the E flank showed no changes, continuing the stability of recent months. Fumarolic activity was concentrated in the W sector of the summit, in the Chava crater and Florencia fumarole. During overflights of the active cone, gas emissions in the W sector were unchanged. The SO2 concentration in the gas column was low at the end of October.
The amplitude of minor background tremor varied at some seismic stations after 13 September with a dominant peak at 3.4 Hz. Additionally there were tremor episodes on 28 and 29 September, with durations of 20 minutes and 9 minutes, respectively, which coincided with periods of heavy rain. Similar to August, most of the seismicity in September consisted of "butterfly-type" events (hybrid between high-frequency and long-period). A total of 1,783 "butterfly-type" events occurred in September, occasionally as swarms with up to 30 events/hour. There were 20 sporadic long-period "screw-type" events with long, slowly decaying codas. One occurrence on 14 September lasted for 127 seconds with a dominant peak amplitude of 3.8 Hz.
Seismicity in October consisted of sporadic long-period, high-frequency, and tremor events, which were accompanied by background tremor. In total, there were 1,097 "butterfly-type" events registered, the most common type of event during the month. Only 9 long-period "screw-type" events occurred in October. Locations of high-frequency earthquakes were dispersed around the crater of the active cone. Three earthquakes of M 2.2-2.6, felt in Jenoy (6 km NNE) and Nariño (7.5 km N), were associated with the seismic source 3 km N of the active crater that produced ~300 earthquakes in April 1993.
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.