Fumarolic activity continued during November, with the greatest activity concentrated in the SE and N parts of the yellow-green crater lake. Fumaroles created mud pots, and small sprays of mud and sulfur constructed cones within the lake. The temperature of the lake oscillated between 61 and 85°C (maximum 80°C in October), and lake level was relatively high due to heavy rainfall. Hot springs at the periphery of the lake had temperatures of 40°C, cold springs at the edge of the crater were 16°C, and fumaroles on the 1953-55 dome were 90°C.

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 and R. Barquero, ICE.