OVSICORI-UNA reported that periodic phreatic explosions at Rincón de la Vieja continued to be recorded by the seismic network and webcams during 20-26 May; most plumes rose no higher than 500 m above the crater rim. Some of the events were not visually confirmed by webcams because of weather conditions or darkness. A small ash eruption at 0537 on 21 May produced a plume that rose 1 km above the crater rim and contained less water vapor than plumes from eruptive events in the past weeks. Plumes from eruptive events at 1624 on 22 May and 0608 on 24 May rose 1 km above the rim. An event at 0325 on 25 May was heard by residents to the N but was no visually confirmed due to darkness. Later that day, in the early evening, an hour-long sequence of eruptions produced gas-and-steam plumes and ejected tephra no higher than the crater rim (except for an event at 1738).

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)