Seismicity during March and April remained low, similar to previous months, and was characterized by fracture events at the seismogenic source 2-8 km NE of the main crater, generally at 6-10 km depths. Long-period events and tremor associated with gas movement also remained at low levels during this interval. Surface activity continued to be concentrated in craters and fumaroles on the W sector of the active cone, mainly at Las Chavas and La Joya fumaroles (figure 80). The two electronic tiltmeters and the shortline leveling network did not show significant changes. Measurements of SO2 done with the COSPEC method registered emission rates <100 tons/day in March, and <130 tons/day in April. Radon concentrations measured in March decreased with respect to recent months.

Figure 80. Fumaroles located around the main crater of Galeras, April 1996. Courtesy of INGEOMINAS.

There were 11 Tornillo events (see BGVN 18:04) detected during 3-16 April with durations of 20-70 seconds, and dominant frequencies of 3.0-3.2 Hz. These characteristics are similar to those recorded before the 1992-93 eruptions (BGVN 18:01, 18:03, and 18:06). On 22 April a swarm of 11 small (M <1) events, recorded in about 45 minutes, were centered ~2 km NNE of the active crater at depths of 4.5-6.5 km.

Geological Summary. 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 open 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 eruptions since the time of the Spanish conquistadors.

Information Contacts: Pablo Chamorro, INGEOMINAS Observatorio Vulcanologico y Sismologico de Pasto (OVP), A.A. 1795, San Juan de Pasto, Nariño, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).