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Report on Poas (Costa Rica) — December 1987


Scientific Event Alert Network Bulletin, vol. 12, no. 12 (December 1987)
Managing Editor: Lindsay McClelland.

Poas (Costa Rica) Phreatic activity continues

Please cite this report as:

Global Volcanism Program, 1987. Report on Poas (Costa Rica) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 12:12. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198712-345040


Costa Rica

10.2°N, 84.233°W; summit elev. 2697 m

All times are local (unless otherwise noted)

Phreatic activity, which has been occurring for the past several months from Thermomineral Lagoon [generally called Crater Lake] in the active crater, continued through November. Activity, such as on 17 November between 2000 and 2030, was characterized by geyser-like eruptions and small bubbles that were observed about every 5 minutes from 21 sites around the lagoon. On 27 November, phreatic eruptions were recorded frequently during 1-2-hour periods, separated by 40-50-minute periods of quiet.

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: Guillermo Alvarado I., Sección Sismología e Ing. Sísmica, Departamento de Geología, Instituto Costarricense de Electricidad (ICE).