OVSICORI-UNA reported low-to-moderate-amplitude tremor with occasional periods of high-frequency volcano-tectonic events detected at Poás during 28-29 June. Webcams recorded intense incandescence at night from the bottom of the crater. A sulfur odor was noted in San Rafael de Poás and Vara Blanca. An event at 1115 on 19 June generated a plume that rose 1 km above the vents. An event at 1450 may have generated a plume, but poor visibility did not allow for confirmation. During 1-4 July frequent but small Strombolian activity ejected incandescent material that fell around vent “A” (Boca Roja). Plumes of water vapor, magmatic gases, and particulates rose at most 500 m from the vents.

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)