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Report on Poas (Costa Rica) — 29 January-4 February 2025


Poas

Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 29 January-4 February 2025
Managing Editor: Sally Sennert.

Please cite this report as:

Global Volcanism Program, 2025. Report on Poas (Costa Rica) (Sennert, S, ed.). Weekly Volcanic Activity Report, 29 January-4 February 2025. Smithsonian Institution and US Geological Survey.

Weekly Report (29 January-4 February 2025)

Poas

Costa Rica

10.2°N, 84.233°W; summit elev. 2697 m

All times are local (unless otherwise noted)


The Observatorio Vulcanológico y Sismológico de Costa Rica-Universidad Nacional (OVSICORI-UNA) reported that small eruptions at Poás ejecting water and steam were recorded by seismic and infrasound sensors at 0038 and 0450 on 28 January and at 0643 on 29 January. The lake water was milky-gray in color and intense gas-and-steam emissions rose from the water’s surface due to fumarolic activity on the crater floor and convecting water. Gas monitoring instruments on the W crater rim detected significant amounts of hydrogen sulfide and sulfur dioxide. The report noted that during January webcams recorded several bubbles from Boca A (SW part of the lake) on the surface of the lake and upwelling of bubbles from Boca C (central part of lake). Volcanologists directly observed these bubbles at 1055 and 1206 on 29 January; sulfur dioxide emissions averaged 384 tons per day. Gas emissions were lower on 30 January and two vigorous convection cells in the lake over both Boca A and Boca C were visible during 30-31 January. During 2-4 February gas emissions fluctuated between low and high values and bubbles or convection cells were active over both vents.

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)