Report on Galeras (Colombia) — June 1991
Bulletin of the Global Volcanism Network, vol. 16, no. 6 (June 1991)
Managing Editor: Lindsay McClelland.
Galeras (Colombia) Frequent tephra emissions; more long-period seismicity; small summit inflation
Please cite this report as:
Global Volcanism Program, 1991. Report on Galeras (Colombia) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199106-351080.
1.22°N, 77.37°W; summit elev. 4276 m
All times are local (unless otherwise noted)
Frequent ash and lapilli emissions continued in June, as indicated by seismic signals and confirmed by periodic observations during clear weather (2, 17, 18, and 20 June). Gas plumes rose to 800-1,700 m height (1 and 17 June) during periodic pulses of activity. Significant increases in the incandescent area were noted near the crater floor and on the W wall. The SO2 flux fluctuated between low and moderate values.
Fewer low-frequency events but more long-period events were recorded in June than in May. The majority of high-frequency events were centered 6 km E of the crater at 0.9-2.3 km depths. Some tremor episodes and long-period events were associated with ash emissions and increases in plume height. Deformation measurements continued to show low levels of inflation (20 µrad in June) at the "Crater" tiltmeter, . . . .
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-OVP.