Report on Poas (Costa Rica) — January 1980
Scientific Event Alert Network Bulletin, vol. 5, no. 1 (January 1980)
Managing Editor: David Squires
Poas (Costa Rica) Small steam explosions continue; September-October activity described
Please cite this report as:
Global Volcanism Program, 1980. Report on Poas (Costa Rica). In: Squires, D (ed.), Scientific Event Alert Network Bulletin, 5:1. Smithsonian Institution. http://dx.doi.org/10.5479/si.GVP.SEAN198001-345040.
10.2°N, 84.233°W; summit elev. 2708 m
All times are local (unless otherwise noted)
Boletín de Vulcanología no. 6 of the Institute of Volcanology provided the following.
A resurgence of phreatic activity began on 8 September. Poás National Park personnel reported that a pine-tree-shaped eruption column containing tephra and a large quantity of water rose about 400 m, before falling back into the crater lake. Only fumarolic activity was observed 9-14 September. From 15 September until the end of the month, up to 10 small phreatic explosions occurred per day, ejecting material to only 15-50 m above the lake surface.
Geysering to heights of 150 m continued through October. Ejecta fell back into the lake or onto the beach at the E end of the crater. On 9 October, volcanologists measured an average lake water temperature of 60°C, and temperatures of 80-90°C on the central "dome." The lake level had risen. During the volcanologists' visit, ejections of water and tephra took place from the central part of the lake. Eruption columns rose a maximum of 300 m in November, and December activity was similar.
The following is from Guillermo Avila. Steam clouds produced by explosions from the crater lake reached heights of 100 m or less in January. The mottled brown color of the lake indicated the presence of active fumaroles on the lake bottom. Water samples from the lake are being chemically analyzed.
Further Reference. Francis, P.W., Thorpe, R.S., Brown, G.C., and Glasscock, J., 1980, Pyroclastic sulfur eruption at Poás volcano, Costa Rica: Nature, v. 283, p. 754-756.
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; G. Avila, ICE.