OVSICORI-UNA reported that on 20 April a dense water vapor plume rose from a vent in the newly-forming pyroclastic cone at the site of the old dome in the hot lake at Poás. Gas flux increased from 1,000 tons/day (t/d) on 13 April to 2,500 t/d on 20 April. During 20-22 April Strombolian activity ejected tephra that fell around the vent within a 300-m radius. Gas-and-ash plumes rose 200 m above the vent. The Red Cross of Grecia reported ashfall in Alajuela, Fraijanes, San Miguel, Carbonal, Cajón, San Francisco, San Roque, and San Juan Norte de Poás. Events at 1316 and 1603 on 22 April produced plumes of unknown height. Several more eruptive events were recorded that day; an event at 2212 was very intense, ejecting bombs large distances. An event at 1215 on 23 April generated a plume of unknown height.

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)