Small phreatic explosions from the crater lake resumed in September after about 9 months of quiet. Water and tephra formed frequent mushroom- or pine-tree-shaped clouds that rose 15-300 m. Tephra fall was confined to the crater lake. Vapor emission was continuous, occurring in about equal quantities from the lake and the N wall of the central "dome" [the eroded cone from the 1953-55 eruption at the S end of the lake].

On 29 November, personnel from the Volcanology Project, National University, descended into the crater. Explosions, accompanied by strong noise, occurred at 20-minute intervals. The mean lake temperature had risen to 65°C, from 30-40°C in August. Some morphological changes had taken place, for which the Volcanology Project personnel suggested two causes: the force of the explosions had dislodged material from the crater walls, especially at the S coast of the lake, and tephra fall into the lake produced wave surges, which caused landslides when they struck the crater walls. The level of the lake had risen about 2 m because of heavy rainfall.

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: J. Barquero H., Univ. Nacional, Heredia.