OVSICORI-UNA reported that a phreatic eruption from Poás was recorded at 1008 on 20 June. The explosion generated a plume 200 m above the crater lake. The plume was mainly water vapor with entrained lake sediments (which contained sulfur precipitates and altered rocks). Other gases such as SO₂, H₂S, HCL, HF, and others were also included.

This was the second phreatic explosion to occur in June. There was no visual record of that event, but routine inspections by the National Park staff on 2 June determined that during the night, there had been a large explosion evidenced by significant fall deposits around the crater lake shoreline. A solar panel had also been damaged by falling rocks associated with that explosion. Seismicity suggests indicated explosive signatures at 1954 on 1 June.

On 18 June, observatory staff noted that between 1000 and 1343, small phreatic eruptions were occurring within the lake. One of these explosions released a strong sulfur smell and large waves. The water temperature was recorded at 44.6°C with a pH of -0.49.

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)