During July 2000 through August 2001, the level of the crater lake fluctuated ten's of centimeters. The color of the lake was generally blue, with sulfur particles floating on the surface. The crater-lake temperature varied between 24 and 35°C. Bubbling continued in the S, SW, NE, and central parts of the lake. A convection cell was observed in the central part of the lake during August-October 2000. During the reporting interval, the following areas showed movement toward the crater lake; the W wall, E, NW, and SW terrace, and NE, N, and NW sides of the pyroclastic cone.

Most fumarolic activity was concentrated in the pyroclastic cone area, with gas columns reaching heights of 300-500 m on the crater floor and blowing chiefly towards the W and SW flanks during July through May 2001. During May 2001, the points of greater gas pressure were in the N wall of the dome. During July 2001, a fumarole appeared in the NE wall with sulfur deposition.

Temperatures of fumaroles ranged between 87 and 111°C, and the typically reported-on access points were 92-94°C. During September 2000, relatively new fumaroles at the NE terrace (94°C) continued to deposit sulfurous sublimates that began accumulating during the previous month. During March 2001 fumarolic activity remained vigorous in the dome and towards the NE and E on the foot of the walls. Thermal sources of these fumaroles were 92-94°C. During May 2001, new fumaroles continued to appear on the floor and the E and NE walls, with sulfur deposition and increasing gas emissions. During May 2001, the fumarole of the N terrace had a temperature of 110°C. During June 2001, vigorous steaming from fumaroles in the area of the lake formed some plumes 100 m tall.

On 28 June, an M 4 earthquake registered by instruments in the Central Valley and Puntarenas area was centered 100 km beneath the volcano. The earthquake was attributed to regional subduction tectonics, but influenced the volcano's seismic and fumarolic activity. On 29 August an M 3 earthquake was registered at a depth of 5.5 km and located 1.7 km SW of the active crater.

The geodetic network lacked significant evidence of deformation during July-August 2000. The 35 hours of low-frequency tremor registered during March 2001 mainly occurred during 1-3 March. These medium and high frequency earthquakes continued to be associated with the appearance of new fumaroles within the main crater and the pyroclastic cone. A summary of earthquakes at Poás during July 2000 to August 2001 is shown in table 11.

Table 11. Summary of seismicity at Poás during July 2000 to August 2001. All columns represent cumulative monthly totals except for the first data column, which shows daily averages of reported low-frequency earthquakes (the predominant type registered). In cases where the seismometer failed to work for a portion of a month, the monthly sum was scaled up and presented assuming the rate of generated events remained constant. Missing months indicate that no data were available. LF indicates low-frequency earthquakes. Paired AB-type earthquakes arrived closely spaced in time. Courtesy of OVSICORI-UNA.

During July and August 2001 modest portions of the crater wall were unstable. During August the collapse of a portion of the E wall mobilized an unusual amount of material towards the bottom of the crater (figure 73). The collapse has been associated with the cracks and permanent fumarolic action weakening the E part of the crater.

Figure 73. Photo of a collapsed portion of the E wall at Poás, August 2001. Lines show where the rock detached from the sub-vertical wall and where the loosened rock came to rest. The affected part of the wall was cracked and fumarolically altered. Courtesy OVSICORI-UNA.

General References. Casertano, L., Borgia, A., Cigolini, C., Morales, L.D., Montero, W., Gómez, M., and Fernández, J.F., 1985, Investigaciones geofísicas y caracteristicas geoquímicas de las aguas hidrotermales: Volcán Poás, Costa Rica: Geofísica Internacional, v. 24, p. 315-332.

Prosser, J., 1985, Geology and medium-term temporal magmatic variation found at the summit region of Poás volcano, Costa Rica: Boletín de Vulcanología, n. 15, p. 21-39.

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: Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.