Low-level activity persisted in November and December 1997. Significant seismic events during this period included 13 long-period (LP) "Tornillo" or screw-type events. The screw-type events are characterized as quasi-monochromatic wave-forms with slowly decaying coda. They preceded eruptions in 1992 and 1993, although they have also occurred without clear surface manifestations.

Volcano-tectonic (VT) earthquakes recorded during November and December were concentrated in three zones (figure 86): around the active cone at shallow levels, 2-8 km to the NE, and 7 km to the SW. These events occurred at depths of 1-18 km below the summit. "Mariposa" or "Butterfly" earthquakes were also recorded. These are a hybrid of VT and LP events located at shallow levels. Three small episodes of spasmodic tremor were registered in December.

Figure 86. Epicenters of volcano-tectonic events recorded during September-October 1997. Circle diameters are proportional to depth below the summit. Courtesy of OVP-INGEOMINAS.

Electronic tiltmeters installed in various locations around Galeras were stable through November and December. No changes were recorded. Venting of gases at the active cone continued from the fumaroles Anganoy, Urcunina, Alteradas, Deformes, and from the craters El Paisita, Las Chavas, and La Joya (figure 87).

Figure 87. Recent morphology of the active cone at Galeras volcano. Jagged lines represent fractures and black circles represent fumaroles. Courtesy of OVP-INGEOMINAS.

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 and Diego Gomez Martinez, Observatorio Vulcanologico y Sismologico de Pasto (OVP), INGEOMINAS, Carrera 31, No. 18-07 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).