The number of seismic events, SO₂ emission rate, and deformation were all low in March. Instruments detected a total of 2,247 "butterfly-type" events. These were characterized by small magnitudes, associated with rock fracturing and fluid movement at depths of <2 km within the active cone, and influenced by earth tidal movements and external agents such as rain. Rock fracture events of M <2.5, were located predominantly in the W and NNE sectors of the active cone. Background tremor was variable. There were also new occurrences of the long-period "screw-type" events that are associated with pressurization of the system. These events are important because they were registered before most of the explosive eruptions at Galeras between July 1992 and June 1993, when volcanic activity was low. Measurements of SO₂ emission obtained by the mobil COSPEC method remained low (<780 t/d). Aerial observations of the active volcanic cone revealed no changes; gas emission continues to be concentrated in the W sector of the main crater. Electronic tiltmeters showed no deformation changes.

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: INGEOMINAS, Pasto.