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Report on Poas (Costa Rica) — July 1988


Scientific Event Alert Network Bulletin, vol. 13, no. 7 (July 1988)
Managing Editor: Lindsay McClelland.

Poas (Costa Rica) Phreatic explosions subside to convective bubbling

Please cite this report as:

Global Volcanism Program, 1988. Report on Poas (Costa Rica) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 13:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198807-345040


Costa Rica

10.2°N, 84.233°W; summit elev. 2697 m

All times are local (unless otherwise noted)

Since mid-April, the level of the crater lake has dropped ~1-1.5 m, reducing its diameter by 20 m and revealing terraces of thixotropic sediment. The lake was bright yellow, presumably from suspended sulfur. Six zones of continuous convective bubbling lifted plumes of dark to light brownish-gray mud 1-10 m above the lake surface. This activity, concentrated on the N side of the lake, has declined from the strong phreatic explosions that ejected acidic mud and sulfur during April. Bombs and lapilli composed of relatively pure crystalline sulfur, ejected during explosions between April and June, were found on the southern terraces. Geologists noted that the change from periodic explosions to continuous convective bubbling may be due to a decrease in hydrostatic pressure with lowering lake-level rather than an increase of energy into the lake. The eroded 1953-55 [dome] slipped slightly towards the lake along fractures, but fumarolic activity was unchanged.

Geological Summary. The broad 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 are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 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 an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: G. Soto and Héctor Flores, UCR; W. Melson, SI.