Report on Poas (Costa Rica) — October 1992

Bulletin of the Global Volcanism Network, vol. 17, no. 10 (October 1992)
Managing Editor: Lindsay McClelland

Poas (Costa Rica) Strong thermal activity in and around crater lake

Please cite this report as:

Global Volcanism Program, 1992. Report on Poas (Costa Rica). In: McClelland, L (ed.), Bulletin of the Global Volcanism Network, 17:10. Smithsonian Institution. http://dx.doi.org/10.5479/si.GVP.BGVN199210-345040.

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Poas

Costa Rica

10.2°N, 84.233°W; summit elev. 2708 m

All times are local (unless otherwise noted)


The level of the crater lake remained unchanged in October. Lake temperature was 75°C, 5° warmer than in September, and pH was 1.4, about the same as in August. Columns produced by small phreatic eruptions from the center of the lake rose no more than 2 m. The terraces on the N side of the lake included at least five very active cones and fumarolic vents; others were found in the hot area on the SE side. Fumarolic activity N and NW of the lake was audible from the overlook. Residents of the S, SW, and W flanks reported a sulfur odor on some days.

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: G. Soto and R. Barquero, ICE; E. Fernández, J. Barquero, and V. Barboza, OVSCIORI.