OVSICORI-UNA reported periodic hydrothermal explosions at Rincón de la Vieja during 15-21 April. At 0306 on 15 April a phreatic eruption was recorded by the seismic network; weather conditions prevented visual confirmation, though local residents reported lahars in the Pénjamo, Azufrada, and Azul rivers. A steam plume from a phreatic event at 0929 on 18 April rose 200 m above the crater rim. Several small events were detected during the morning of 19 April but none produced plumes that rose above the rim. At 1014 a phreatic eruption ejected water and sediment 300 m above the crater rim and onto the flank, and produced a steam-and-gas plume that rose 1.5 km above rim. A lahar followed the activity. During 20-21 April several low-energy puffs of gas-and-steam were recorded. A low-energy event at 0626 on 21 April generated a steam plume that rose 1 km above the crater rim.

Geological Summary. Rincón de la Vieja is a volcanic complex in the Guanacaste Range of NW Costa Rica. Sometimes referred to as the Rincon de la Vieja-Santa María Volcanic Complex, it consists of a slightly arcuate 20-km-long ridge of 12 craters and pyroclastic cones 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. The Santa María cone, the highest peak of the complex, is located on the E side of the ridge and has a lake within the 400-m-diameter 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 reported eruptions possibly dating back to the 16th century, have been from the active crater, near the center of the complex, with an acidic 300-m-diameter lake.

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