OVSICORI-UNA reported that about 900 small phreatic eruptions at Poás were recorded by the seismic network and often visually observed during 4 March to around 0800 on 8 March; no additional eruptions were recorded at least through 12 March. The small eruptive events ejected material no higher than 50 m. The water at the bottom of the crater had almost disappeared leaving a couple of muddy water puddles. Boca C remained underwater and frequently bubbled. Weak incandescence from sulfur combustion at Boca A (SW part of crater floor) was frequently observed at night. Gas-and-steam emissions rose from the vents and caused a sulfur odor in the vicinity of the volcano during the earlier part of the week.

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)