OVSICORI-UNA reported periodic phreatic explosions at Rincón de la Vieja during 22-28 April. At 0535 on 22 April a phreatic event was recorded by the seismic network; weather conditions prevented good visual observations, though a steam plume was seen rising 1.5 km above the crater rim. There were five events recorded by the seismic network during the morning of 24 April, with most producing steam-and-gas plumes that rose 300-500 m above the crater rim. The largest event, recorded at 1020, ejected water and solid material 300 m above the crater rim and a steam plume that rose 1 km. An event at 1547 on 26 April ejected sediment 200 m above the rim and plumes 300 m above the rim. A plume with no ash rose 1 km at 1720 on 27 April.

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)