Our last report on Galeras discussed ongoing explosions and ash plumes during February-June 2009 (BGVN 34:07). That report concluded with a rise in the alert level to II (orange; "probable eruption in terms of days or weeks") on 26 June 2009. That change followed elevated seismicity suggestive of precursory behavior similar to that of previous eruptions. This report continues coverage of activity from July 2009 to January 2010, including eruptions on 30 September and 30 November 2009, and 2 January 2010.

Overflight observations from 12 and 13 July 2009 found reduced crater temperatures, including a decrease from 220 to 100°C in one small area. In a 28 July report, INGEOMINAS reported earthquakes of up to M 1.6. Due to continuing low levels of activity, on 4 August the Alert Level was decreased to III (on a scale of I-IV, with I being high). It was also noted that there were active fumaroles on the W flank. An episode of tremor lasting ~3 hours was reported on 10 August, and an overflight on 23 August observed an increase in fumarolic activity since 12 July. On 8 September 2009 INGEOMINAS reported recent earthquakes of up to M 1.5.

Eruption of 30 September 2009. At 0914 on 30 September, INGEOMINAS reported an explosive eruption prompting the rise in Alert Level to I (red; "imminent eruption or in progress"). A second INGEOMINAS report stated that National Park personnel observed two explosions and ejection of incandescent material at the active cone, as well as an ash plume rising to ~12 km altitude that drifted E and later N. The SO₂ measurements between 0930 and 1000 included values between 1,100 and 9,300 tons/day. Ashfall was reported in Sandoná (15 km NW), Ancuya, Linares, La Llanada, and Sotomayor (40 km NW). Seismicity decreased after the eruption and the Alert Level was lowered on 1 October to II and on 6 October to III.

On 30 October, INGEOMINAS reported that degassing had decreased and seismicity had increased, a previous indication of possible eruptions; the Alert Level was raised to II. A 3 November report described decreasing SO₂ emissions and seismicity, and INGEOMINAS reported on 10 November that seismicity continued to decrease and SO₂ was not detected. An overflight on 14 November detected low rates of gas discharge and thermal anomalies inside the main crater measuring 110°C.

Eruption of 20 November 2009. INGEOMINAS reported an explosive eruption on 20 November at 2037. Five explosions were reported by residents in San Cayetano, and incandescence was observed at the summit. The Washington Volcanic Ash Advisory Center (VAAC) reported an eruptive column to ~14 km altitude that drifted N, and ashfall occurred in Nariño, La Florida, and Bellavista. INGEOMINAS stated that seismicity levels associated with the eruption were lower than those during the 30 September eruption. Seismicity increased after the eruption but then gradually decreased. The Alert Level had been raised to I during the eruption but was dropped to II on 21 November and to III on 27 November.

Overflights on 26 November and 3 December revealed fumarolic activity in the main crater, with respective estimated temperatures up to 200°C and on the latter date, 155°C. The Alert Level was raised to II.

Earthquakes with magnitudes of up to 2.2 were measured during 12-15 December, at distances of up to 2 km from the crater and at depths of up to 3 km. INGEOMINAS noted that seismicity included both tornillo (BGVN 18:04) and pseudo-tornillo earthquakes. The seismicity was similar to the behavior prior to the eruptions on 30 September and 20 November. In a 29 December report, INGEOMINAS noted that these types of earthquakes have preceded the majority of the explosive eruptions of Galeras from 1992 to 2009.

Eruption of 2 January 2010. An explosive eruption at Galeras on 2 January at 1943 lasted ~30 minutes and prompted INGEOMINAS to raise the Alert Level to I. Ash emissions, summit incandescence, and ejected incandescent blocks that ignited surrounding areas of the volcano were observed. The Washington VAAC reported an eruptive column to 12 km altitude that drifted W and NW, with ashfall observed in Sandoná, Consacá, Ancuya, Linares, Samaniego, Santacruz-Guachavéz and La Llanada. Seismicity declined after the end of the eruptive event and INGEOMINAS lowered the Alert Level to II. An overflight on 3 January revealed diffuse white-colored gas plumes from the main crater. On 5 January ashfall was reported in areas as far as 110 km to the W, with falling blocks having reached distances of 3.2-3.5 km from the crater.

INGEOMINAS reported eight tornillo-type seismic events between 16-18 January, similar to those observed before previous eruptions, followed by low-to-moderate SO₂ emissions beginning on 19 January and continuing low through at least 26 January.

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: Instituto Colombiano de Geología y Minería (INGEOMINAS), Observatorio Vulcanológico y Sismológico de Popayán, Popayán, Colombia.