IG reported a high level of activity at Sangay during 15-22 September. Seismicity was characterized by high levels of explosions, harmonic tremor, long-period earthquakes, and signals indicating emissions. Weather clouds sometimes prevented visual observations of the volcano, but the Washington VAAC and IG webcams recorded daily ash plumes that rose as high as 2 km above the summit and drifted in multiple directions. Pyroclastic flows descended the SE flank almost daily.

An explosion at 0420 on 20 September was the largest such event in the recent months. Within 10 minutes several satellite images showed a large ash cloud rising 6-10 km above the summit; high-altitude parts of the cloud drifted E while lower parts drifted W. Servicio Nacional de Gestión de Riesgos y Emergencias (SNGRE) reported that 32 districts within the provinces of Chimborazo, Bolívar, Guayas, and Los Ríos were affected by ashfall. Authorities in the districts of Bucay and Cumandá restricted driving, the opening of businesses, and outdoor activities due to ashfall. The José Joaquín de Olmedo Airport in Guayaquil suspended its operations for seven hours to clean the runways. Ashfall was most significant in Chimborazo, particularly in the districts of Guamote, Alausí, Chunchi, Pallatanga, and Cumandá, with photos showing poor visibility and ashfall covering streets, cars, and houses. Ashfall significantly impacted agriculture fields.

Authorities inspected the confluence of the Volcán River (SE flank) and Upano River, and observed significant deposits of tephra, some of which had dammed the river and created an immense lagoon. Normally the Upano was about 25 m wide in that area but because of the deposits it was more than 250 m across and had almost no water in it. After the explosion, IG noted that activity returned to levels similar to previous months with ash plumes rising 1-2 km above the volcano during 20-22 September.

Geological Summary. The isolated Sangay volcano, located east of the Andean crest, is the southernmost of Ecuador's volcanoes and its most active. The steep-sided, glacier-covered, dominantly andesitic volcano grew within the open calderas of two previous edifices which were destroyed by collapse to the east, producing large debris avalanches that reached the Amazonian lowlands. The modern edifice dates back to at least 14,000 years ago. It towers above the tropical jungle on the east side; on the other sides flat plains of ash have been eroded by heavy rains into steep-walled canyons up to 600 m deep. The earliest report of an eruption was in 1628. Almost continuous eruptions were reported from 1728 until 1916, and again from 1934 to the present. The almost constant activity has caused frequent changes to the morphology of the summit crater complex.

Sources: Instituto Geofísico-Escuela Politécnica Nacional (IG-EPN), Servicio Nacional de Gestión de Riesgos y Emergencias (SNGRE), La República, El Comercio