Galeras was last reported on in BGVN 30:09, covering the period from July 2004 to mid-October 2005. During July through October 2004, eruptions generated ash and gas plumes that caused ashfall in surrounding areas. On 21 November 2004 Galeras erupted explosively. During January - September 2005, low-level relatively shallow seismicity and small gas-and-ash emissions continued. Occasional steam plumes were visible from Pasto in October 2005. Seismicity fluctuated and some instrumentally measured deformation continued.

During the first week of November 2005, low-level seismicity included several tornillo earthquakes (long-period seismic events related to pressurized fluid flow at shallow depth). Small amounts of deformation were recorded at the volcano. During 9-14 November, a large number of tornillo earthquakes were reported by Instituto Colombiano de Geología y Minería (INGEOMINAS). The earthquakes were similar to those that occurred before eruptions in 1992-93. Activity during October suggested that the volume of magma beneath the volcano was greater than that inferred to have been present during the 1992-93 eruptions. Due to increased activity, the Alert Level was raised to 2 (probable eruption in days or weeks) on 14 November.

According to news reports, on 14 November local authorities recommended an evacuation of as many as 9,000 people living in towns near the volcano, including Pasto (to the E), La Florida (to the N), and Nariño (to the N). Heightened seismicity continued during 16-22 November. According to news articles, only ~ 1,000 residents had actually left as of 18 November.

On 24 November at 0246 seismic signals indicated the beginning of an eruption. Ash fell in the towns of Fontibon, San Cayetano, Postobon, and in north Pasto. Around this time, INGEOMINAS raised the Alert Level to 1 (eruption imminent or occurring). The Washington VAAC observed a small puff of ash NE of the volcano at ~ 4.6 km altitude. Activity decreased by the next day, so the Alert Level was reduced to 2. Thousands of people had been evacuated during the week prior to the eruption.

Due to a decrease in activity, on 28 November INGEOMINAS reduced the Alert Level to 3. Low levels of seismicity and deformation were continuing. Although poor weather conditions obscured the volcano most of the time, steam and gas emissions were photographed on 2 December coming from several locations on the active cone, including the main crater. The plume rose 1 km above the summit on 3 December.

Through 12 December, seismicity indicated fluids moving within the volcano, small changes in deformation occurred, and gas rose to a height of ~ 500 m. Based on information from the US Geological Survey, the Washington VAAC reported that a pilot observed an ash plume from Galeras on 23 December at an altitude of ~ 7.3 km and drifting W.

During 23 December to 2 January 2006 there were emissions of gas and small amounts of ash. On 23 December four ash plumes rose to ~ 3 km altitude and drifted to Consacá. A cluster of 33 volcano-tectonic earthquakes, reaching a maximum M 1.2, occurred beneath the volcano's crater during 29-30 December. The SO₂ flux varied between 300 and 1,500 metric tons per day (t/d).

Gas emissions with small amounts of ash, and heightened seismicity, continued through 9 January. The SO₂ flux at the volcano varied between 490 and 1,500 t/d. A lava dome was visible in the main crater during an overflight on 13 January. Around this time, there was an increase in the amount of seismicity and deformation. The Washington VAAC reported that a pilot observed an ash plume on 23 December at an altitude of ~ 7.3 km and drifting W.

During 23 January to 6 February, the lava dome in the main crater continued to grow; seismicity associated with the movement of fluids continued, with an average of 200 small earthquakes per day, and slight deformation was recorded. SO₂ flux of about 300 t/d was measured. Strong degassing occurred in several sectors of the active cone and around the lava dome. Steam rose to 900 m above the volcano. During a field visit on 8 February, scientists found pyroclastic-flow deposits high on the SE flank.

The rate of seismicity the week of 13-20 February averaged 190 small earthquakes per day, while the SO₂ flux was about 200 metric tons per day. Steam rose to ~ 1.1 km above the volcano on 19 February and incandescence was visible at parts of the lava dome. The volume of the dome in the main crater was approximately 1.5 times larger than when it was first observed on 13 January. Seismicity increased to an average of 280 small earthquakes per day during 20-27 February. SO₂ flux also rose, to about 600 t/d. On 26 February a cluster of earthquakes included an M 4.8 volcano-tectonic earthquake followed by 35 smaller earthquakes. Slight deformation was recorded at the volcano. Steam and gas rose to ~ 700 m above the volcano. Galeras remained at Alert Level 3 through February 2006.

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/).