November activity remained similar to that of previous months, with some changes in the hot crater lake. The hot areas in the N and SE parts of the lake remained as boiling mud springs. In the middle of the lake, mud plumes intermittently rose about 2 m, persisting for about 15 seconds. At the end of the month, the cones and castle-like features of mud were no longer visible, probably destroyed by the surges associated with the plumes. Activity on the remnants of the 1953-55 [dome] remained stable. Temperatures of about 66°C were measured on the top of the [dome], and about 87°C on its N flank.

Volcanic microseismicity totaled 8,366 events in 28 days, for a daily mean of 299. The largest number of events, 417, occurred on the 26th, and the smallest, about 200, on the 19th. Small increases have been recorded in each month since June. All of the seismicity was of low frequency; no A-type events were detected.

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, Mario Fernández, and Héctor Flores, Univ de Costa Rica.