Report on Poas (Costa Rica) — March 1994

Bulletin of the Global Volcanism Network, vol. 19, no. 3 (March 1994)
Managing Editor: Richard Wunderman

Poas (Costa Rica) Fumarolic and phreatic activity from N crater lake

Please cite this report as:

Global Volcanism Program, 1994. Report on Poas (Costa Rica). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 19:3. Smithsonian Institution.

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Costa Rica

10.2°N, 84.233°W; summit elev. 2708 m

All times are local (unless otherwise noted)

Escaping gases in the 200-m-diameter, northernmost crater lake at Poás continued to bubble, gush, and geyser, and they produced weak phreatic eruptions through the lake surface. In March, subaqueous fumaroles in the SE emitted small bubbles, but those in the lake center produced phreatic eruptions that drove through the lake surface and reached several meters in height. The lake was dark green in color and 50.5°C; its level had subsided 60 cm with respect to the level in January, leaving a yellow strandline along the banks. A gas cloud or plume frequently rose 500 m above the lake surface, damaging vegetation at several locations near the active crater.

The seismic station adjacent the active crater (POA2) registered 7,118 low-frequency events and 114 moderate-frequency events during March, the most active month so far this year. On the most seismically active day of the month, 16 March, 436 seismic events took place.

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, G. Alvarado, and F. Arias, ICE; E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI.