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Report on Poas (Costa Rica) — October 1988


Scientific Event Alert Network Bulletin, vol. 13, no. 10 (October 1988)
Managing Editor: Lindsay McClelland.

Poas (Costa Rica) Crater lake activity continues

Please cite this report as:

Global Volcanism Program, 1988. Report on Poas (Costa Rica) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 13:10. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198810-345040


Costa Rica

10.2°N, 84.233°W; summit elev. 2697 m

All times are local (unless otherwise noted)

Geologists visited Poás on 2 November and observed activity similar to that of previous months. Mud plumes rose 2-3 m from three sites near the center of the crater lake. Intense convective bubbling occurred at multiple points in the lake. No explosions were observed. Since September, the level of the lake had dropped ~1 m and its diameter had decreased, leaving a growing sedimentary terrace. Fumarolic activity continued on the summit and N edge of of the 1953-55 [dome]. Several tens of volcanic earthquakes were recorded daily, predominantly B-type events.

Acid vapor produced by the activity was blown W by strong dry-season trade winds, away from the overlook area. Plants inside the crater, around the overlook, and SE toward Lake Botos sprouted leaves without acid burns, in contrast to earlier leaves that had burned edges. The pH of drinking water in the Visitors Center, supplied from Lake Botos, had risen since September to 5.0-5.5, indicating lower ambient acidity.

Park guards have experienced coughing, sneezing, itching of eyes and skin, and stomach problems because of the acid. The effects are transitory and have not caused them permanent damage (Dra. Y. Soto, personal communication). According to the park guards, birds that had been absent from the summit area in recent months have returned and can be heard singing, demonstrating a decrease in ambient acidity. Dead birds had been seen in the crater near the 1953-55 [dome] in January.

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, Univ de Costa Rica (UCR), San José.