Turrialba, in Costa Rica, is a large stratovolcano that covers an area of 500 km². Three well-defined craters occur at the upper SW end of a broad 800 x 2,200 m summit depression that is breached to the NE. Five major explosive eruptions have occurred during the past 3,500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Minor eruptive activity has been frequent since 2010. The following report summarizes activity from September 2021-February 2022 and is based primarily on information from periodic reports published by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) and satellite data. During this period, activity continued at a low level, similar to that observed since the beginning of September 2020 (BGVN 46:09), with occasional weak ash explosions and ash emissions.

Based on OVSICORI-UNA’s 2021 annual report, activity continued to decrease in 2021, a trend that began in 2019. Frequent rim collapses into the bottom of the crater were evident. Sporadic ash emissions occurred between 13 June and 23 July 2021 (BGVN 46:09), and again beginning on 3 November continuing into late February 2022. Neither the seismic nor geodetic data suggested a new supply of magma, and petrological observations from ash samples revealed a large amount of lithic material and less than 12% of fresh magmatic material. These observations strongly suggested to OVSICORI-UNA that the explosions were phreatic (caused by the interaction between rainwater and a hot body or shallow magma).

A one-minute explosion from the West Crater at 0646 on 3 November produced a plume that rose 200 m above the crater rim and drifted W. Ash from the plume contained very few fragments of juvenile glassy material (less than 5%), and abundant porous and lithic material (~85%), indicating to OVSICORI-UNA that the explosion was mainly phreatic. Another small explosion was recorded on 7 November. On 11 November, a small area of incandescence was visible on the internal SW wall of the summit vent, which was vigorously degassing. An explosion at 0624 on 23 November lasted less than a minute and produced a plume that rose less than 500 m above the crater rim that drifted SW.

Incandescence on the NW inner wall of the West Crater, which had been periodically visible since mid-November, was visible during 26-27 December, suggesting that fumarolic temperatures exceeded 300°C. OVSICORI-UNA reported three weak ash explosions on 28 December. The first, at 0644, was one minute long and produced an ash emission that rose 50 m above the crater rim and drifted W. Another, recorded at 1105 by the seismic and infrasound networks, was also heard by authorities in the Parque Nacional Volcán Turrialba. No data were reported for the third explosion.

An OVSICORI-UNA report on 14 January 2022 indicated that atmospheric SO₂ levels measured by the TROPOMI Sentinel-5P satellite showed an almost daily detection from the volcanic plume. However, detected values were relatively low (less than 50 tons) except for a few days, particularly on 29 December 2021 (~126 tons) and 7 January 2022 (84 tons).

Incandescence from the West Crater was visible overnight during 15-16 January 2022. Four explosions were recorded on 17 January, including one at 2126 and another at 2131. The second explosion was the strongest since June 2021 and generated an ash-rich plume that rose about 1 km above the summit and was accompanied by a pyroclastic flow that extended more than 400 m. Ashfall and a sulfur odor were reported by residents in Coronado, Tres Rios (30 km SW), Alajuela (50 km W), and Santa Ana (46 km WSW). The ash contained less than 3% juvenile material, indicating that the explosion was phreatic. Another explosion at 1115 on 18 January generated a plume that rose 300 m and drifted SW.

During 19-24 January, periodic explosions lasting 1-2 minutes each were recorded at the West Crater. At 0546 on 19 January an explosion produced an ash plume that rose 200 m above the summit and drifted W. In addition, a small explosion at 1052 generated a plume that rose 50 m and drifted NW. At 0706 on 24 January, an explosion generated a plume that rose 100 m and drifted SW. Two points of incandescence on the internal SW crater wall were also visible. Three areas of incandescence in the West Crater were visible during 26-27 January. A weak explosion was recorded on 30 January.

According to OVSICORI-UNA, on 6 February at 1259, a weak phreatic explosion at West Crater produced an ash plume that rose 100 m above the crater rim. Two incandescent points in the crater were visible in overnight webcam images during 7-8 February. Another explosion at the West Crater on 19 February at 0412 produced a small ash plume that rose 100 m above the summit and drifted W. Minor ashfall was reported by park rangers in Parque Nacional Volcán Irazú. A weaker explosion was recorded later that day at 1624, though weather clouds prevented visual confirmation.

A point of incandescence on the SW wall of the West Crater was visible on 23 February, coincident with the area of strongest gas emissions. An explosion at 1955 on 27 February produced a diffuse ash plume that rose 300 m above the summit and drifted NE. Rumbling sounds heard on 28 February were sometimes coincident with minor ash emissions.

Geological Summary. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr).