Report on Poas (Costa Rica) — September 1992
Bulletin of the Global Volcanism Network, vol. 17, no. 9 (September 1992)
Managing Editor: Lindsay McClelland.
Poas (Costa Rica) Degassing continues in and around crater lake
Please cite this report as:
Global Volcanism Program, 1992. Report on Poas (Costa Rica). In: McClelland, L (ed.), Bulletin of the Global Volcanism Network, 17:9. Smithsonian Institution. https://dx.doi.org/10.5479/si.GVP.BGVN199209-345040.
10.2°N, 84.233°W; summit elev. 2708 m
All times are local (unless otherwise noted)
The level of the crater lake was 30 cm lower in September than in August. Lake temperature dropped slightly to about 70°C, pH rose to 1.8, and its color remained green. Degassing continued from fumaroles in the N part of the crater. Active cones on the E side ejected sulfur. The maximum temperature measured at fumaroles on the 1953-55 dome was 80.5°C.
The total of 4,905 low-frequency events recorded by UNA was similar to August, with the daily average remaining at 163. An additional 37 medium- to high-frequency events were recorded, plus 5 hours of low-frequency tremor at irregular intervals.
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; G. Soto and R. Barquero, ICE.