OVSICORI-UNA reported periodic hydrothermal explosions at Rincón de la Vieja during 8-14 April. A possible small eruption occurred at 0658 on 8 April and was followed by a decrease in the amplitude of tremor signals. Beginning at 0033 on 11 April a small-scale hydrothermal explosion was recorded by the infrasoiund and seismic networks for 20 seconds. Water and sediment was ejected onto the upper flanks of the volcano. The event was recorded by the webcam located in Sensoria, 4 km N of the crater. In the morning local residents noted that the water in the Pénjamo River was milky white. A small hydrothermal explosion at 0251 on 13 April produced a plume of steam and gas that rose 500 m.

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)