Report on Poas (Costa Rica) — April 1992

Bulletin of the Global Volcanism Network, vol. 17, no. 4 (April 1992)
Managing Editor: Lindsay McClelland

Poas (Costa Rica) Stronger gas emission; crater lake level falls

Please cite this report as:

Global Volcanism Program, 1992. Report on Poas (Costa Rica). In: McClelland, L (ed.), Bulletin of the Global Volcanism Network, 17:4. Smithsonian Institution. http://dx.doi.org/10.5479/si.GVP.BGVN199204-345040.

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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 crater lake's water level dropped approximately 8 m between January and 7 April, revealing sediments (gypsum, amorphous silica, and sulfur) and numerous mud pots around its edges. The lake was emerald green, with a temperature of 77°C (7 April). Emissions from fumaroles in the lake increased in April, producing jet-engine sounds audible at the tourist overlook, and a 1-km-high plume. Fumaroles on the dome S of the lake had temperatures of <83.5°C. The increase in emissions was reflected in an increase in the effects of acid rain, which damaged trees in the national park, and coffee plantations, cypress trees, and pasture beyond the park boundaries. Residents on the W and SW flanks reported sulfur odors, and skin and eye irritation. A daily average of 250 low-frequency earthquakes was recorded in April (at the UNA station POA2, 2.7 km SW).

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