In June and July, fumarolic activity on the SW flank of the central cone increased significantly. A new fumarolic vent (named Las Portillas) opened on the inner W wall of the central crater. The vent was ~2 m in diameter and vigorously emitted gas with temperatures >470°C on 22 July. Both Las Portillas and the 10-m-wide La Chava vent emitted superheated gases, producing a loud noise. During the last two weeks in July, numerous small fumarolic fractures on the S rim of the crater, and small radial and concentric fractures on the W crater rim, were observed.

Seismic activity during June and July was nearly stable (figure 3), except for a slight decline (20%) in the daily average of B-type events between mid-May and the last week of July. The seismic energy release associated with B-type events was also stable. The number of A-type events was low until 23 July when a swarm of 30 earthquakes occurred between 0811 and 1500, including a M 3 event at 1233. The swarm was centered about 2 km W of the crater, at 1-7.5 km depth (figure 4). The number of low-frequency events had increased slightly before the swarm. Dry tilt showed no significant changes, although electronic tilt varied slightly on the 21st (figure 5). SO₂ emissions increased slightly with an average of 600 t/d in July (see figure 12).

Figure 3. Daily seismic energy release (top) and number of events (bottom) at Galeras, 22 April-31 July. Courtesy of the Observatorio Vulcanológico de Colombia.

Figure 4. Map showing epicenters (top) and an E-W cross-section (bottom) of seismic events at Galeras in July. Courtesy of the Observatorio Vulcanológico de Colombia.

Figure 5. Deformation measurements at Galeras, July 1989, showing radial (E-W) tilt at Peladitos station, at about 3,800 m elevation 2 km E of the crater at Galeras (top), and tangential (E-W) tilt at Telecom station, at about 3,900 m elevation 2 km S of the crater (bottom). Increasing values indicate uplift to the W. Courtesy of the Observatorio Vulcanológico de Colombia.

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.