The Observatorio Vulcanológico y Sismológico de Costa Rica-Universidad Nacional (OVSICORI-UNA) reported continuing gas-and-steam emissions that occasionally contained ash at Poás during 14-20 May. Activity generally decreased during the week, and deformation data indicated deflation. Vigorous gas-and-steam emissions continued at Boca A and incandescence was visible at night. Water that had pooled at Boca C continued to bubble; torrential rains during the beginning of the week continued to fill the crater. A moderate phreatic eruption at 0408 on 18 May generated a gas-and-steam plume with minor amounts of ash that rose 1 km above the crater rim and drifted SW. Sulfur dioxide emissions detected in satellite data were estimated to be 650 tons per day (t/d) on 18 May, 350 t/d on 19 May, and 598 t/d on 20 May. The volcano’s Alert Level remained at 3 (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest color on a four-color scale).

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)