Small phreatic explosions, observed in the crater lake in February, were concentrated in March at mud pots exposed by the continued decline in lake water level. Lake temperature measurements oscillated between 65 and 75°C. Gas emission continued, producing plumes that rose to >1 km and extended W and SW. Residents on the W and SW flanks reported that acid rain and mist affected vision, caused skin rashes, and damaged agricultural areas. On average, 237 earthquakes were recorded daily in March (at station POA2, 2.7 km SW of the crater), with a maximum of 398 on 4 March. A M 2.5 earthquake was recorded at 0652 on 3 March, centered 3.5 W of the crater at 7 km depth.

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, OVSICORI.