IG reported that although visual observations were occasionally limited due to cloud cover, ash-and-steam and ash plumes from Tungurahua rose to altitudes of 6-7 km (19,700-23,000 ft) a.s.l. during 11-18 December. Roaring noises and "cannon shots" were heard. During 10-11 December, seismic signal interpretation was characterized by explosions and almost constant emission of ash plumes. Incandescent blocks were expelled from the summit and rolled down the flanks. On 11 December, explosions vibrated windows and the ground in areas near the volcano. During 10-14 December, ashfall was reported from areas downwind, including areas to the SW, W, and NW, and was almost constant during 10-12 December. On 13 December, incandescent blocks rolled down the flanks. Noises indicating blocks rolling down the flanks were heard on 15 December, but were not observed due to cloud cover.

During 16-18 December, explosions rattled windows in areas around the volcano, including Tungurahua Observatory (OVT) in Guadalupe, 11 km N, on 16 December. Ash plumes drifted SW. On 18 December, a steam plume rose to an altitude of 8.5 km (27,900 ft) a.s.l. and drifted NE. Constant steam-and-ash plumes were observed during an overflight. Ashfall was reported in areas to the NE. Based on pilot reports, observations of satellite imagery, and information from IG, the Washington VAAC reported that ash plumes rose to altitudes of 6.7-13.7 km (22,000-45,000 ft) a.s.l. and drifted E and NW on 18 December.

Geological Summary. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II collapsed about 3,000 years ago and produced a large debris-avalanche deposit to the west. The modern glacier-capped stratovolcano (Tungurahua III) was constructed within the landslide scarp. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.

Sources: Instituto Geofísico-Escuela Politécnica Nacional (IG-EPN), Washington Volcanic Ash Advisory Center (VAAC)