Report on Poas (Costa Rica) — November 1991
Bulletin of the Global Volcanism Network, vol. 16, no. 11 (November 1991)
Managing Editor: Lindsay McClelland.
Poas (Costa Rica) Phreatic explosions end; sulfur emission from crater lake
Please cite this report as:
Global Volcanism Program, 1991. Report on Poas (Costa Rica). In: McClelland, L. (ed.), Bulletin of the Global Volcanism Network, 16:11. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199111-345040.
10.2°N, 84.233°W; summit elev. 2708 m
All times are local (unless otherwise noted)
The crater lake remained quiet in November. The water level rose 1.2 m relative to October, and had a temperature of 70°C. Evaporation at the lake surface remained strong and clots of sulfur dotted the turquoise-green water. Small columns of gas were carried W and SW. Fumaroles on the 1953-55 dome had temperatures of 90°C.
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: E. Fernández and J. Barquero, OVSICORI.