Report on Poas (Costa Rica) — 2 February-8 February 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 2 February-8 February 2005
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2005. Report on Poas (Costa Rica). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 2 February-8 February 2005. Smithsonian Institution and US Geological Survey.
10.2°N, 84.233°W; summit elev. 2708 m
All times are local (unless otherwise noted)
A field team from OVSICORI-UNA visited Poás on 25 January and found that the level of the volcano's hot acidic crater lake had risen in comparison to the previous month. Intense and sustained rainfall during the previous months caused the water level to increase by ~4 m. The area of the lake increased by ~20%. Flooding occurred in relatively flat areas to the N, E, and SE. Water reached about 150 m towards the SE of the lake. Scattered fumaroles and hot spots at the N base of the lava dome were flooded. Increased steaming was visible from the National Park. The lake temperature remained at 22 degrees C, with hot spots near the rim reaching up to 80 degrees C. OVSICORI-UNA noted that in the past an increase in lake level during a rainy period has been followed by a decrease during the drier months of February to April.
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.