Rincón de la Vieja, a remote volcanic complex in NW Costa Rica with an acid lake, exhibits numerous weak phreatic (hydrothermal) explosions along with associated gas-and steam emissions and an occasional plume (BGVN 46:11). This report covers activity during September 2021-June 2022, and is based primarily on weekly bulletins and occasional daily reports from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) and satellites.

During September 2021-June 2022, the level of activity continued as before, with frequent weak phreatic explosions every month (table 7), except that no explosions were reported between 21 October and 25 December 2021. Based on OVSICORI-UNA reports, plumes rose no more than 500 m above the crater rim, with the exception of stronger explosions on 25 January, 7 April, and 6 June. Explosions produced lahars on 5 and 13 January. OVSICORI-UNA reports suggested that an explosion on 5 January may have been phreatomagmatic in nature (i.e., interaction between water and magma).

Table 7. Summary of activity at Rincón de la Vieja during September 2021-June 2022. Explosions were weak and phreatic. Courtesy of OVSICORI-UNA (compiled from weekly bulletins and occasional daily reports).

Geological Summary. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

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/, https://www.facebook.com/OVSICORI/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html).