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Report on Poas (Costa Rica) — December 1992


Bulletin of the Global Volcanism Network, vol. 17, no. 12 (December 1992)
Managing Editor: Lindsay McClelland.

Poas (Costa Rica) Vigorous gas emission; rising crater lake covers terraces and fumaroles

Please cite this report as:

Global Volcanism Program, 1992. Report on Poas (Costa Rica) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 17:12. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199212-345040


Costa Rica

10.2°N, 84.233°W; summit elev. 2697 m

All times are local (unless otherwise noted)

The level of the turquoise-green crater lake rose, covering terraces of sulfur and mud on its SE, NE, and NW sides. Lake temperature was 70°C, and pH was 1.3. Fumaroles remained active on small islands in the lake, producing gas columns that rose 500 m above the crater floor and were carried W and SW by prevailing winds. Continuous noise from the fumaroles was audible from the overlook on the crater rim. Vents in the center of the lake that had produced small phreatic eruptions have disappeared. A light-gray terrace in the NW part of the lake generated sporadic phreatic activity that ejected material to 1-2 m height.

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: E. Fernández and J. Barquero, OVSCIORI.