Report on Poas (Costa Rica) — 27 July-2 August 2011
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 27 July-2 August 2011
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2011. Report on Poas (Costa Rica). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 27 July-2 August 2011. Smithsonian Institution and US Geological Survey.
10.2°N, 84.233°W; summit elev. 2708 m
All times are local (unless otherwise noted)
OVSICORI-UNA reported that on 23 July a group of observatory scientists visited Poás to document changes that had occurred there during the previous weeks. They noted that the subtle, semicircular scarp observed a few months earlier had rapidly progressed to a sharp scarp on the SE shore of Laguna Caliente. The 60-m-wide, 2.5-m-high scarp degassed and geyser activity was observed on the W end, next to the steaming lake. In an area about 40 m above the surface of the lake where there were fractured rocks and vigorous gas venting, incandescence emanated from the lava dome and a temperature of 670 degrees Celsius was measured.
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.