OVSICORI-UNA reported that small phreatic eruptions at Poás continued to be observed. Phreatic eruptions at 1922 and 1926 on 25 December and at 0235 on 26 December ejected material 50 m above the lake surface. At 1535 on 27 December a small phreatic event ejected a plume of water and sediment 100 m above the lake and produced a steam-and-gas plume that rose 500 m. Events recorded at 0117 on 29 December, as well as at 0500, 1034, and 1530 on 30 December, ejected sediment, water, and gas no higher than 200 m above the lake.

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)