As noted by the IG in their daily reports covering 25-31 July, Tungurahua emitted a substantial number of small ash-bearing explosions and several unusually large ones as well. Based on ground observer estimates, many plumes during the reporting interval rose to 2-3 km above the crater rim (up to ~1 mile above the crater) and dropped ash on towns located on the volcano's flanks.

On the 26th, the IG reported one of the larger explosions, the biggest since March 2007 (its seismic signal yielded a reduced displacement of 9.2 cm2). Associated ashfalls affected some parts of the volcano. The explosion took place at night and the plume height was not estimated.

On the 30th IG observers witnessed another strong explosion that generated a heavily ash-laden plume. The dense portion of the plume rose 400 m above the crater rim. A similar plume had not been seen since 16 August 2006. The associated column of less dense material rose to 3 km and visible portions of dense material appeared as a curtain of ash deposited to the W. Some blocks associated with the outburst rolled up to 0.5 km below the crater's rim. Visibility hampered further observations that day but the many emission noises included the hammering of bouncing blocks.

Tungurahua's 25-31 July activity spurred numerous VAAC reports, but satellite analysts generally had great difficulty with cloudy conditions and few if any plumes were clearly detected.

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)