After 50 days of low activity, Tungurahua entered a new phase of activity on 20 August, characterized by a short sequence of long-period earthquakes followed by gas-and-ash emissions that reached a maximum height of 3 km above the volcano. A small amount of ash fell in the sector of Cusúa. During the evening incandescent volcanic blocks were hurled ~300 m above the volcano and traveled ~1 km down the volcano's flanks. On 21 August emissions of mostly steam and small amounts of ash rose ~1 km above the volcano and drifted W. Ash fell in the sectors of Riobamba, Ambato, and Santa Fé de Galán. On 23 August plumes rose to 0.5-2.5 km above the volcano, and ash fell in the town of Guaranda. On 24 August an explosion at 2133 ejected blocks that traveled ~1 km down the volcano's flanks. The explosion was heard in the town of Baños.

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), Agence France-Presse (AFP), Washington Volcanic Ash Advisory Center (VAAC)