Report on Poas (Costa Rica) — March 1979
Scientific Event Alert Network Bulletin, vol. 4, no. 3 (March 1979)
Managing Editor: David Squires.
Poas (Costa Rica) Small phreatic eruptions end
Please cite this report as:
Global Volcanism Program, 1979. Report on Poas (Costa Rica). In: Squires, D. (ed.), Scientific Event Alert Network Bulletin, 4:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN197903-345040.
10.2°N, 84.233°W; summit elev. 2708 m
All times are local (unless otherwise noted)
The following information is from Boletín de Vulcanología no. 3.
The period of mud and tephra ejection that began 22 September 1978 declined in November and ended in December. Eruption columns, containing ash and small lapilli (up to 0.4 cm), did not exceed 25 m in height during November, and only sporadic very small eruptions occurred in December.
The temperature of the crater lake water fell from 70°C on 4 October to 50°C on 7 December, and decreased further, to 40°C, in February. Fumarolic activity from the central dome continued, with vapor temperatures ranging from 60 to 90°C during both the December and February visits.
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: J. Barquero H., Univ. Nacional, Heredia.