A field team from OVSICORI-UNA visited Poás on 25 January and found that the level of the volcano's hot acidic crater lake had risen in comparison to the previous month. Intense and sustained rainfall during the previous months caused the water level to increase by ~4 m. The area of the lake increased by ~20%. Flooding occurred in relatively flat areas to the N, E, and SE. Water reached about 150 m towards the SE of the lake. Scattered fumaroles and hot spots at the N base of the lava dome were flooded. Increased steaming was visible from the National Park. The lake temperature remained at 22 degrees C, with hot spots near the rim reaching up to 80 degrees C. OVSICORI-UNA noted that in the past an increase in lake level during a rainy period has been followed by a decrease during the drier months of February to 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)