OVSICORI-UNA reported that during 23-26 February seismicity at Poás was dominated by low-frequency events. Robust gas emissions rose from four fumarolic vents in the area previously covered by a lake, though the most vigorous emissions originated from vent A (Boca Roja). Minor incandescence from vent A was sometimes visible at night. A sulfur odor was reported in areas downwind including Naranjo, Zarcero, and Grecia (16 km SW). Particles of molten sulfur were included in ash deposits collected in Naranjo. Ashfall was reported in Canoas de Alajuela on 26 February.

Geological Summary. The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

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