Report on Galeras (Colombia) — May 1995
Bulletin of the Global Volcanism Network, vol. 20, no. 5 (May 1995)
Managing Editor: Edward Venzke.
Galeras (Colombia) Gas emission pressures remain high, but seismicity is low
Please cite this report as:
Global Volcanism Program, 1995. Report on Galeras (Colombia) (Venzke, E., ed.). Bulletin of the Global Volcanism Network, 20:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199505-351080.
1.22°N, 77.37°W; summit elev. 4276 m
All times are local (unless otherwise noted)
Volcanic activity remained low during May. Seismicity was characterized by 3-4 fracture events/day associated with a source ~3-5 km NNE of the volcano at depths of 3-13 km. Most of these M <=2.2 events were detected only by seismometers of the Galeras network. The two most notable events during the month were on 18 May at 1011 (M 4.2) and 1124 (M 3.8). These were felt by the inhabitants of Pasto and nearby towns. This activity was similar to that in April and November 1993 (BGVN 18:04 and 18:11), when there were swarms of ~500 earthquakes from a source NNE of the volcano. The activity also resembled the >2,000 aftershocks of the 4 March (M 4.5) earthquake (BGVN 20:03).
Screw-type events, characterized by a semi-monochromatic record with a slowly decaying coda associated with fluid movement, disappeared after 2 May. Conventional long-period events continued. Butterfly-type events similar to previous months were observed with sporadic variations in background tremor and small pulses having a possible volcanic association.
Increased pressure of gas emissions, which started in April (see BGVN 20:04), persisted, especially at the craters and W-sector fumaroles of the active cone. Aerial reconnaissance indicated that strong rains in April and May contributed to the increase of gas and vapor emission in the main crater. COSPEC measurements revealed low SO2 emission levels. Deformation stations recorded no significant changes.
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-Observatorio Vulcanologico y Sismologico de Pasto, AA 1795, San Juan de Pasto, Narino, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).