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Report on Poas (Costa Rica) — 5 September-11 September 2007


Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 5 September-11 September 2007
Managing Editor: Sally Sennert.

Please cite this report as:

Global Volcanism Program, 2007. Report on Poas (Costa Rica) (Sennert, S, ed.). Weekly Volcanic Activity Report, 5 September-11 September 2007. Smithsonian Institution and US Geological Survey.

Weekly Report (5 September-11 September 2007)


Costa Rica

10.2°N, 84.233°W; summit elev. 2697 m

All times are local (unless otherwise noted)

OVSICORI-UNA reported that during August, Laguna Caliente, a summit lake of Poás, was greenish-gray in color, exhibited convection cells in the lake's center, and contained dark gray floating particles. Subaqueous fumaroles at the center of the lake released gases that spread and covered the entire surface of the lake. The level of the lake had dropped 59 cm with respect to its level in July, and it had a temperature of 58 degrees Celsius. Fumarolic activity from a pyroclastic cone produced gas plumes that rose 400 m above the floor of the crater. Points of gas discharge were noted from the N and NW crater walls, the terrace, and the NE edge of the crater. Fumaroles in contact with the lake and the NE wall produced sulfur particles that floated in the lake. Emissions from the SE and NE crater walls were very low in volume.

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.

Source: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)