Report on Poas (Costa Rica) — December 1984
Scientific Event Alert Network Bulletin, vol. 9, no. 12 (December 1984)
Managing Editor: Lindsay McClelland.
Poas (Costa Rica) Fumaroles active, but temperatures lower
Please cite this report as:
Global Volcanism Program, 1984. Report on Poas (Costa Rica). In: McClelland, L (ed.), Scientific Event Alert Network Bulletin, 9:12. Smithsonian Institution. https://dx.doi.org/10.5479/si.GVP.SEAN198412-345040.
10.2°N, 84.233°W; summit elev. 2708 m
All times are local (unless otherwise noted)
"Dartmouth geologists visited Poás on 6 and 10 December. Extensive fumarolic activity continued. Fumaroles had lower temperatures than measured on previous trips, not exceeding 525°C. Lake water sampled 100 m NW of the fumarole bank had a temperature of 36°C and a pH of 0.22 at the surface, and a temperature of 38°C and a pH of 0.08 at 4 m depth."
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: B. Barreiro, H.R. Naslund, R. Stoiber, P. Turner, Dartmouth College.