Activity at Tungurahua was very low during 9-12 September, with low gas emission (50 tons of SO2 on 12 September) and negligible seismic activity. On the 12th at 1300 a sudden ash emission occurred that marked the onset of a new eruptive phase. A dark ash column reached 1 km above the crater and drifted W. About 1.5 hours later the column turned white and roaring indicated that sustained Strombolian activity was taking place in the crater. Intense Strombolian activity was seen at night, with large amounts of ballistic material being thrown out of the crater and ash falling on the volcano's flanks. SO2 gas emission was relatively high (1,200 tons per day). By the next day activity began to decline; low-level Strombolian activity occurred and 700 tons of SO2 were emitted that day. On the 14th around 0100, long-lived and high-amplitude tremor suddenly started at the volcano. It was followed by an intense explosive phase that began near 1000 and continued through at least the 15th at 2200. During the eruptive episode, continuous gas-and-ash emissions and short-lived Vulcanian explosions occurred. Dark ash plumes occasionally rose 3-4 km above the crater and drifted SW. Large blocks were thrown as high as 700 m above the crater and landed as far as 2 km from the crater. Lava fountaining also occurred.

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: Institut de Recherche pour le Développement (IRD), Washington Volcanic Ash Advisory Center (VAAC)