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Report on Poas (Costa Rica) — February 1990

Bulletin of the Global Volcanism Network, vol. 15, no. 2 (February 1990)
Managing Editor: Lindsay McClelland.

Poas (Costa Rica) Intermittent geyser-like activity and sulfur emission from shrinking crater lake

Please cite this report as:

Global Volcanism Program, 1990. Report on Poas (Costa Rica) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 15:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199002-345040.

Volcano Profile |  Complete Bulletin


Costa Rica

10.2°N, 84.233°W; summit elev. 2708 m

All times are local (unless otherwise noted)

February activity was characterized by intermittent geyser-type phreatic eruptions from the center of the hot crater lake, reaching maximum heights of 2-3 m. Sulfur emissions from a vent in the NE part of the lake coated the crater wall, coloring it light yellow. The crater lake has been enriched in sediments transported by surface erosion. As evaporation has exceeded input from precipitation, the level of the lake has continued to fall.

Seismicity increased slightly in February. A total of 9,460 events were recorded the first 27 days of the month, a mean of 350/day. Most of the events were B-type, with some A-type shocks and brief tremor episodes.

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: Mario Fernández and Hector Flores, Univ de Costa Rica.