OVSICORI-UNA reported that during September, Laguna Caliente, a summit lake of Poás, was mostly gray in color and produced gas columns that reached the crater rim. The level of the lake had dropped 5 cm with respect to August measurements and had a temperature of 46 degrees Celsius. On 25 October, a phreatic eruption produced a plume that drifted 12 km SW of the crater. Fumarolic activity from a pyroclastic cone on the floor of the crater produced gas plumes that drifted W and SW. New points of gas discharge were noted from the crater floor, the SE and NE crater walls, the N terrace, and the NE edge of the crater.

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)