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Report on Poas (Costa Rica) — March 1991


Bulletin of the Global Volcanism Network, vol. 16, no. 3 (March 1991)
Managing Editor: Lindsay McClelland.

Poas (Costa Rica) Crater lake dries up again; increase in seismicity and rainwater acidity

Please cite this report as:

Global Volcanism Program, 1991. Report on Poas (Costa Rica) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199103-345040


Costa Rica

10.2°N, 84.233°W; summit elev. 2697 m

All times are local (unless otherwise noted)

Vigorous, sulfur-rich thermal activity, punctuated by occasional phreatic explosions, has continued for many years at Poás. Its crater lake has been shrinking since early 1987, with only small remnants remaining at the ends of the 1989 and 1990 dry seasons.

Gas emission continued from various points on the crater floor in March. The lake had completely dried up, and part of the lake bottom subsided, producing a morphology of tilted blocks with fissures and craters emitting large quantities of gas. Some of the fumaroles had burning sulfur and others ejected small amounts of sediments, depositing them on the W crater wall. The gas plume was carried W and SW of the volcano, where residents reported a sulfurous odor. Measurements of rainwater pH, 2.5 km SW of the crater (Cerro Pelón), showed a decrease from 4.65 in February to 3.25 in March.

Seismicity increased in March to a daily average of 272 low-frequency earthquakes and a monthly total of 25 high-frequency earthquakes. The highest levels of seismicity occurred on 29-31 March, when up to 400 earthquakes were recorded/day.

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, J. Barquero, and V. Barboza, OVSICORI.