OVSICORI-UNA reported that during 7-13 June seismicity at Poás was characterized by low-amplitude tremor, discrete volcano-tectonic events, low-frequency events, and some periods of small long-period events with low amplitude. During 7-8 June the webcam recorded strong emissions of water vapor, magmatic gases, and particulates. A sulfur odor was reported in Alajuela, San Ramon, and Barva, and incandescence in the area of the crater was recorded at night. OVSICORI-UNA noted that during 8-9 June a plume of water vapor, magmatic gases, and particulates rose from two vents; the lake had evaporated and exposed the vents. A minor sulfur odor was reported on the campus of the Universidad Nacional in Heredia. Events at 1610 and 1750 on 11 June generated plumes that rose 300 and 600 m above the crater, respectively. Plumes from the vents rose 1 km during 12-13 June. A sulfur odor was noted in Quesada, Santa Ana, San José de Alajuela, and San Juanillo Naranjo.

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)