IG reported that at night during 16-17 July observers noted incandescent blocks falling onto Tungurahua's flanks. Cloud cover often prevented observations. An explosion was heard in Ambato (31 km N) on 16 July. Explosions were detected on 17 July, and white ashfall was reported in Choglontus (SW). Steam-and-ash plumes were observed rising 1.5 km and drifting W. During 18-19 July Strombolian activity ejected blocks that rolled 500 m down the flanks. Ash fell in Choglontus. Seismicity remained high during 17-19 July; 18-33 long-period earthquakes, 53-82 tremors indicting emissions, and 3-6 explosions were recorded per day.

On 19 July an ash plume rose 1 km and drifted SW. The geodetic monitoring system indicated an inflationary trend on the N flank and deflation SW of the volcano, indicating the presence of a magma body about 2 km below the crater. During 19-20 July ashfall was reported in Choglontus and El Manzano (8 km SW). On 20 July 127 long-period earthquakes, 71 tremors indicting emissions, and 43 explosions were detected.

Seismicity again increased on 21 July; 220 long-period earthquakes, three periods of tremor indicating emissions, and 15 explosions were detected. The three periods of tremor were characterized by two 1-hour-long sessions and a third period lasting at least eight hours. Explosions vibrated nearby structures, and ejected blocks onto the upper parts of the flanks. Gas-and-ash plumes rose as high as 5 km, and produced ashfall in Cevallos (23 km NW), Tisaleo (29 km NW), Mapayacu (SW), Choglontus, and El Manzano. Strombolian activity overnight during 21-22 July ejected blocks that rolled 500 m down the flanks. Strong explosions again vibrated structures, and ash emissions rose 1 km. Ashfall was noted in El Manzano, Pillate, Chacuaco and Cahuaji. On 23 July ash plumes rose 1.5 km and drifted WSW. Strombolian activity was observed overnight and roaring was heard. Ashfall was reported in Cahuají and Choglontus. Seimscity decreased but still remained high during 22-23 July; 22-40 long-period earthquakes, 7-12 tremors indicting emissions, and 4-9 explosions were detected per day.

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.

Source: Instituto Geofísico-Escuela Politécnica Nacional (IG-EPN)