OVSICORI-UNA reported continuing gas-and-steam emissions from vents Boca A and Boca C on the crater floor of Poás during 24-30 April. Although emissions from Boca C often contained low ash content, no ash was present for a period of time during 25-27 April. Plumes intensified on 28 April and rose several hundred meters high. A sulfur odor was reported in Sarchí and Grecia (both about 17 km SW) on 25 April. Incandescence from Boca A was visible at night during 27-28 April and from both Boca A and Boca C at night during 29-30 April.

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)