This report covers late 1998 through March 1999. During this time, the terrace SE of the active crater lake continued to slide. The active crater lake, the northernmost of the two summit lakes, remained sky-blue to green-turquoise in color. The pyroclastic cone within the active crater remained the principal source of fumarolic outgassing, sometimes giving off plumes that rose 500 m and during November occasionally reaching 500-600 m. OVSICORI-UNA noted that fumaroles alongside the cone typically had temperatures up to 93°C. They found identical maximum temperatures at fumaroles along the crater walls and on the terrace N of the active crater lake. However, in September 1998, Wendy Perez Fernandez reported hotter temperatures (elevated by 19°C) and greater than usual fumarolic vigor.

Seismicity during January 1998-March 1999 was dominated on the OVSICORI-UNA system by low-frequency events. They occurred in largest number during early 1998 with the highest number recorded (during February 1998) consisting of 2,718 events. A substantial decrease in low-frequency events occurred during late 1998 and early 1999 with the lowest number recorded (during January 1999) consisting of 381 events. Tremor duration followed roughly similar patterns: the maxima (during February 1998) consisted of 55 hours. Tremor was absent for the four months after November 1998.

Scientists measured the comparatively clear waters of the southernmost, less-active lake (Laguna Botos). During January 1995 through 21 September 1998 they recorded rises in both temperature (from ~14°C in 1995 to ~28°C in 1998) and pH (from ~4 in 1995 to ~6 in 1998). Sulfate ion, although highly variable, also tended to climb during the four-year interval. Chloride ion concentrations decreased (from ~4 down to ~1 ppm).

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, V. Barboza, M. Martinez, E. Duarte, R. Sáenz, E. Malavassi, R. Van der Laat, E. Hernández, and T. Marino, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA).