About dawn on 26 December 1980, an explosion from the S portion of the crater lake ejected ash, small blocks, and hot water that rose above the level of the crater rim (~320 m above the lake surface). This was the first such activity since a similar explosion on [12] September from the same area, near the wall of the "dome" in which fumarolic activity has been concentrated. Both the volume of vapor emitted and the amount of SO₂ and other toxic gases in the vapor have increased since mid-1980, forcing the use of gas masks near the fumaroles. In mid-January 1981, Parque Nacional Poás employee Geiner Chacón reported seeing incandescence during the night in the N wall of the "dome." In February, volcanologists descended to the "dome" and observed incandescent areas, estimating temperatures of 700-800°C from their color. Significant quantities of sulfur had been deposited, some of which had been melted and turned a yellow-orange color by the heat. Many landslides had occurred in the crater walls, forming talus slopes at their base. The incandescence was continuing as of late May.

Further Reference. Barquero-H., J., 1998, Volcan Poas: San Jose, Costa Rica, 42 p.

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.

Information Contacts: J. Barquero H., Univ. Nacional, Heredia.