Report on Poas (Costa Rica) — March 1992
Bulletin of the Global Volcanism Network, vol. 17, no. 3 (March 1992)
Managing Editor: Lindsay McClelland.
Poas (Costa Rica) Small phreatic eruptions; acid rain and mist cause damage on flanks; continued seismicity
Please cite this report as:
Global Volcanism Program, 1992. Report on Poas (Costa Rica). In: McClelland, L. (ed.), Bulletin of the Global Volcanism Network, 17:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199203-345040.
10.2°N, 84.233°W; summit elev. 2708 m
All times are local (unless otherwise noted)
Small phreatic explosions, observed in the crater lake in February, were concentrated in March at mud pots exposed by the continued decline in lake water level. Lake temperature measurements oscillated between 65 and 75°C. Gas emission continued, producing plumes that rose to >1 km and extended W and SW. Residents on the W and SW flanks reported that acid rain and mist affected vision, caused skin rashes, and damaged agricultural areas. On average, 237 earthquakes were recorded daily in March (at station POA2, 2.7 km SW of the crater), with a maximum of 398 on 4 March. A M 2.5 earthquake was recorded at 0652 on 3 March, centered 3.5 W of the crater at 7 km depth.
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, OVSICORI.