Heavy rains caused the nearly dry crater lake to rise 1.8 m with respect to the level in September, filling it enough so that the diameter reached about 180 m. A pan-like structure on the crater floor became covered by silt and pale-green 60°C lake water. In October, a zone of boiling water was located at a site in the NW quadrant of the crater, outside the lake. The zone produced tiny (1- to 2-m high) phreatic eruptions and modest (<100-m high) gas columns. Fumaroles on the dome appeared unchanged. During October, low-frequency seismic events at Poás totaled 3,630 (see table 6).

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: E. Fernández, J. Barquero, and V. Barboza Moreira, OVSICORI-UNA; G. Soto and F. Arias, ICE; M. Mora, UCR.