Report on Poas (Costa Rica) — May 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 5 (May 1981)
Managing Editor: Lindsay McClelland.
Poas (Costa Rica) Incandescence observed
Please cite this report as:
Global Volcanism Program, 1981. Report on Poas (Costa Rica). In: McClelland, L (ed.), Scientific Event Alert Network Bulletin, 6:5. Smithsonian Institution. https://dx.doi.org/10.5479/si.GVP.SEAN198105-345040.
10.2°N, 84.233°W; summit elev. 2708 m
All times are local (unless otherwise noted)
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  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.
Geologic Background. The broad, well-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, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 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 the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.
Information Contacts: J. Barquero H., Univ. Nacional, Heredia.