The IG reported that although visual observations were occasionally limited due to cloud cover during 14-20 May, ash and steam plumes from Tungurahua were spotted most days and rose to altitudes of 6-8 km (19,700-26,200 ft) a.s.l. Ashfall was reported in areas within 8 km to the SW and W during 14-15 and 17-18 May. On 15 May, Strombolian activity was observed and blocks rolled down the flanks. On 17 and 18 May, "cannon shots" and explosions vibrated large windows in areas to the SW and W. Roaring noises were occasionally heard. On 18 May, a lahar possibly descended a drainage to the W. On 19 May, numerous incandescent blocks rolled about 1.6 km down the flanks following a large explosion. Roaring and "cannon shot" noises were audible and windows vibrated in nearby areas after the large explosion and several others that followed throughout the night. Ashfall was reported in areas W and NW during 19-20 May.

Based on information from IG, the Washington VAAC reported that on 20 May an ash plume rose to an altitude of 7.9 km (26,000 ft) a.s.l. and drifted NW.

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)