OVSICORI-UNA reported that small phreatic explosions continued at Rincón de la Vieja during 14-20 December. A small phreatic explosion at 0828 on 14 December produced a steam plume that rose 500 m above the crater rim. An associated sulfur dioxide anomaly was registered at a station located to the W of the crater. Small phreatic events were also recorded at 1433 on 15 December, at 0314 and 2042 on 16 December, at 1958 on 17 December, and 0119 on 20 December, though cloudy weather conditions prevented visual confirmation.

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.

Source: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)