Report on Galeras (Colombia) — February 1993
Bulletin of the Global Volcanism Network, vol. 18, no. 2 (February 1993)
Managing Editor: Edward Venzke.
Galeras (Colombia) Low seismicity and SO2 levels; episodes of long-period tremor
Please cite this report as:
Global Volcanism Program, 1993. Report on Galeras (Colombia). In: Venzke, E. (ed.), Bulletin of the Global Volcanism Network, 18:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199302-351080.
1.22°N, 77.37°W; summit elev. 4276 m
All times are local (unless otherwise noted)
Seismic activity at Galeras continued in February at the low levels that followed the 14 January eruption. The majority of earthquakes were located slightly to the W of the caldera at depths <1.5 km. COSPEC measurements of SO2 remained low and the [Crater Station] electronic tiltmeter showed no significant changes. Continuous gas emissions came from the main crater and fumaroles located to the W and SW. No seismicity could be correlated with periodic reports of roaring noises or explosions from the active crater.
Beginning on 13 February, long-period, "screw-type" seismic events referred to as tornillos were detected, their occurrence increasing towards the end of the month. As of 12 March, twenty-six of these events had been recorded. During the last days of February the tornillos changed character. Though still remaining low-frequency and long-duration, the signature varied or the dominant frequency increased. Some of the tornillos were located near the surface towards the W flank of the volcano.
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: M. Calvache, INGEOMINAS, Pasto.