The Observatorio Vulcanológico y Sismológico de Costa Rica-Universidad Nacional (OVSICORI-UNA) reported continued activity at Poás during 10-17 June. Due to a sustained decrease in seismicity, a shift from inflation to deflation of the crater, gas compositions that appear less magmatic, and a lack of significant eruptions since 19 May, the Alert Level was lowered from 3 to 2 (the second lowest level on a four-level scale) and the Aviation Color Code was lowered from Orange to Yellow (the second lowest color on a four-color scale) on 13 June. Moderate emissions of ash, water vapor, and sulfur-rich gas continued during 11-17 June, primarily from vent A. Seismic and acoustic tremor, thermal anomalies, and SO2 degassing (up to 500 tons/day) continued to be observed at levels consistent with recent weeks.

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)