On 22 November IG concluded that the minor ash emissions recorded at Cotopaxi on 21 October were due to magma in the volcano’s conduit, though not from new magma entering the system after the 2015 eruption. An average of one seismic event per day was recorded based on long-term seismic rates. In the months prior to the 21 October event, the rate had gradually increased to 1.5 events per day, though after the ash emission the rate fell back to one event per day. Most of the seismicity was located beneath the summit. Minor deformation was recorded during August-November, but it could not be conclusively linked to the eruptive activity. Sulfur dioxide emissions increased in October and gas-emission analysis indicated a magmatic origin. Nearly continuous emissions of gas-and-steam had been rising from the main crater since 21 October, as high as 2 km above the rim. The heights of emissions averaged 200 m and were as high as 800 m in 2021.

At 1848 on 25 November the seismic network recorded a tremor signal associated with a gas emission that drifted NNW. At approximately 0310 on 26 November a new episode of tremor was associated with a gas-and-ash emissions that lasted for several hours. The plume drifted 85 km NNW, passing over Quito (55 km N), and caused ashfall in El Pedregal (60 km N), Tambillo (32 km NNW), Guamaní (42 km NNW), Amaguaña (33 km NNW), Chillogallo (44 km NNW), Quitumbe (41 km NNW), Solanda (46 km NNW), Lloa (48 km NNW), Conocoto (41 km N), Mercado Mayorista (45 km NNW), Villaflora (47 km NNW), and Rumipamba (55 km N). Moderate levels of seismic tremor were recorded until about 1050. Servicio Nacional de Gestión de Riesgos y Emergencias (SNGRE) maintained the Alert Level at Yellow (the second lowest level on a four-color scale).

Geological Summary. The symmetrical, glacier-covered, Cotopaxi stratovolcano is Ecuador's most well-known volcano and one of its most active. The steep-sided cone is capped by nested summit craters, the largest of which is about 550 x 800 m in diameter. Deep valleys scoured by lahars radiate from the summit of the andesitic volcano, and large andesitic lava flows extend to its base. The modern edifice has been constructed since a major collapse sometime prior to about 5,000 years ago. Pyroclastic flows (often confused in historical accounts with lava flows) have accompanied many explosive eruptions, and lahars have frequently devastated adjacent valleys. Strong eruptions took place in 1744, 1768, and 1877. Pyroclastic flows descended all sides of the volcano in 1877, and lahars traveled more than 100 km into the Pacific Ocean and western Amazon basin. Smaller eruptions have been frequent since that time.

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