Logo link to homepage

Report on Poas (Costa Rica) — July 1981


Scientific Event Alert Network Bulletin, vol. 6, no. 7 (July 1981)
Managing Editor: Lindsay McClelland.

Poas (Costa Rica) Medium-to-high temperature gases collected

Please cite this report as:

Global Volcanism Program, 1981. Report on Poas (Costa Rica) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 6:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198107-345040


Costa Rica

10.2°N, 84.233°W; summit elev. 2697 m

All times are local (unless otherwise noted)

Between 14 June and 11 July, personnel from PIRPSEV, CNRS, and the volcano observation section of IPG sampled and analyzed medium-to-high temperature gases from Poás (table 1). Maximum gas temperatures measured were 940°C.

Table 1. Analytical means of 28 gas samples from Poás (dry, HCl excluded).

Gas Mean Amount
SO2 55.79%
CO2 26.06%
H2 17.90%
H2S 0.52%
N2 1.98% in air
CO 0.24%
CH4 84.3 ppm
He 52 ppm
COS 25.8 ppm

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: H. Delorme, Univ. de Paris; J.L. Cheminée, IPG, Paris.