Report on Poas (Costa Rica) — March 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 3 (March 1981)
Managing Editor: Lindsay McClelland.
Poas (Costa Rica) Explosions; incandescence on dome
Please cite this report as:
Global Volcanism Program, 1981. Report on Poas (Costa Rica) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 6:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198103-345040
10.2°N, 84.233°W; summit elev. 2697 m
All times are local (unless otherwise noted)
Activity at Poás had increased, with explosions observed  September and 26 December 1980. As of mid-March, ICE and the Univ. Nacional were keeping the volcano under continuous observation. The temperature of the "dome" in the crater lake was 650-750°C and some red areas were seen along fissures in the dome. Lake water temperatures were 50°C, similar to temperatures in the fall of 1980. The pH of the lake had decreased to 0.1. Fumaroles emitted large quantities of water vapor and SO2. Many landslides had occurred in the walls of the main 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.
Information Contacts: G. Avila, ICE; J. Barquero H., Univ. Nacional, Heredia.