Volcanic tremor and long-period earthquakes from Tungurahua began at approximately 2100 on 23 February. On 24 February at 0310, tremor amplitude increased. Incandescent material was ejected 800 m above the summit and fell on the flanks about 1 km below the summit. An eruption plume drifted NW and roaring noises were audible. Gravel and sand-sized ash reportedly fell at Pillate (8 km W) and San Juan (40 km WSW) and in places accumulated up to 3 mm thick. Deposits of ash 2 mm thick were reported from Bilbao (8 km W), Cotaló (8 km NW), Manzano (8 km SW), and Choglontus (W).

On 25 February, 12 moderate to large explosions occurred according to seismic interpretation. Based on satellite imagery, MWO, pilot reports, and the IG, the Washington VAAC reported that ash plumes rose to altitudes of 7.6-12.2 km (25,000-40,000 ft) a.s.l. during 24-25 February. Plumes drifted SW and NW.

On 26 February, a plume with no ash content rose to 6 km (19,700 ft) a.s.l. and drifted W. Later that day, two explosions produced ash plumes that 7 km (23,000 ft) a.s.l. and again drifted W. On 27 February, incandescent material was ejected above the summit and fell on the flanks about 500 m down the flanks. Noises produced by material rolling down the flanks and "cannon shots" were heard during 25-27 February.

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