On 24 November 2005 an eruption began at Galeras that resulted in local ash fall (BGVN 31:01and 31:03). This report discusses behavior through mid-August 2006.

Through December 2005 to the end of March 2006, the lava dome in the main crater continued to grow and seismicity remained elevated. Because of an increase in tremor at Galeras on 28 March 2006, Instituto Colombiano de Geología y Minería (INGEOMINAS) raised the Alert Level from 3 (changes in the behavior of volcanic activity have been noted) to 2 (likely eruption in days or weeks). Although the seismic activity apparently decreased on 29 March, Galeras remained at Alert Level 2.

INGEOMINAS reported that Galeras remained at a critical state during April and May 2006, with a partially solidified lava dome in the main crater. Seismicity, deformation, gas emissions, and temperatures all decreased. During 10-17 April, there were small gas emissions from the volcano. During 9-15 May, there were small gas and sporadic ash emissions. During 12-19 June, ash columns reached heights of 0.6-1.4 km above the summit.

According to Reuters and BBC reports, an increase in volcanic activity 12 July prompted the Colombian government to order the evacuation of ~ 10,000 people living near Galeras. INGEOMINAS reported an increase in seismic activity and at least two explosive eruptions. Ash accumulated in the towns of La Florida and Nariño, about 10 km N, and in the town of Genoy, 5 km NE. The Alert Level was increased from 2 (likely eruption in days to weeks) to 1 (eruption imminent or occurring). On 13 July, because of decreased activity, the Alert Level was lowered from 1 to 3. Approximately 2,000 people had been taken to shelters.

On 17 July, INGEOMINAS reported that after the 12 July eruption of Galeras, seismic activity decreased considerably. Observations of the dome and secondary craters in the W sector after 12 July showed minor physical changes. Weak gas plumes were observed without associated seismic activity. Through the first two weeks of August 2006, seismic activity remained at low levels. Gas and steam emissions from the main crater continued. Galeras remained at Alert Level 3 (changes in the behavior of volcanic activity have been noted).

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: Diego Gomez Martinez, Observatorio Vulcanológico y Sismológico de Pasto (OVSP), INGEOMINAS, Carrera 31, 1807 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html; Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); El Pais (URL: http://elpais-cali.terra.com.co/paisonline/); Reuters; British Broadcasting Company (BBC) (URL: http://www.bbc.co.uk/).