Report on Poas (Costa Rica) — September 2007

Bulletin of the Global Volcanism Network, vol. 32, no. 9 (September 2007)
Managing Editor: Richard Wunderman

Poas (Costa Rica) Many fumaroles active; small phreatic emission from crater lake in December 2006

Please cite this report as:

Global Volcanism Program, 2007. Report on Poas (Costa Rica). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 32:9. Smithsonian Institution.

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Costa Rica

10.2°N, 84.233°W; summit elev. 2708 m

All times are local (unless otherwise noted)

The last Bulletin report on Poás provided information on the phreatic eruption on 25-26 September 2006 (BGVN 32:07). This report discusses continuing hydrothermal variations and one minor phreatic eruption (to 30 m above the crater lake) during October 2006 through September 2007. The pyroclastic cone continued fumarolic activity issuing columns of gases that reached 300 to 400 m above the crater. Information for this report were provided by the Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Crater lake. During the period October 2006 through September 2007, Laguna Caliente the active crater lake of Poás volcano, displayed a greenish-gray color with convection cells in the center and temperatures ranging from 41 to 58 °C. As depicted in table 12, between 5 and 12 October 2006, the temperature and level of the lake increased suddenly suggesting a greater heat flow into the lake. By late October, the temperature decreased slightly and the level of the lake began to fall. Overall, the lake level fell 6.45 m from September 2006 through August 2007, and then stabilized. Often gases originating from the bottom of the lake produced a ring of dark gray material 80 m in diameter floating at the center of the lake. In the NE wall at the lake level, fumaroles produced yellowish particles that floated on the lake.

Table 12. For Poás, the temperature of Laguna Caliente and changes in lake level from September 2006 through September 2007. Courtesy OVSICORI-UNA.

[Skip text table]
    Date              Lake          Relative elevation
                   temperature    change in lake surface

    Sep 2006         41°C                  --
    05 Oct 2006      46°C                  --
    12 Oct 2006      55°C              + 0.43 m
    27 Oct 2006      53°C              - 0.46 m
    Nov 2006         48°C              - 0.33.m
    Dec 2006         48°C                  --
    Jan 2007         --                    --
    Feb 2007         48°C              - 0.70 m
    Mar 2007         49°C              - 1.11 m
    Apr 2007         51°C              - 1.18 m
    May 2007         27°C              - 1.45 m
    Jun 2007         58°C              - 1.58 m
    Jul 2007         57°C              - 0.15 m
    Aug 2007         58°C              - 0.59 m
    Sep 2007         56°C              + 0.08 m

Small phreatic eruption. According to reports of a park ranger, a phreatic eruption occurred at 1230 on 16 December 2006. The eruption reached a height of 30 m and the erupted material fell back into the lake.

Fumaroles. In October 2006 the fumaroles of the N terrace emitted columns of gases and deposits of sulfur forming a small dome. One fumarole produced a whistling noise and had temperature of 144°C. By February 2007, the fumarole cooled to 124°C but continued building a small sulfur cone (figure 82). Sulfur depositions continued through September 2007 when the fumarole closed. Table 13 depicts fumarole temperatures through September 2007.

Figure (see Caption) Figure 82. Fumarolic activity at Poás formed a sulfur cone during late 2006 and into 2007. This shot shows the upper part of the sulfur cone in the left foreground. Parts of the steaming pyroclastic cone and adjacent crater lake appear in the background. Courtesy of OVSICORI-UNA.

Table 13. Temperature of N terrace fumarole at Poás depositing sulfur during October 2006 through September 2007. Courtesy OVSICORI-UNA.

[Skip text table]
    Date        Temperature

    Oct 2006      144°C
    Nov 2006      143°C
    Dec 2006       --
    Jan 2007       --
    Feb 2007      124°C
    Mar 2007      118°C
    Apr 2007      116°C
    May 2007      110°C
    Jun 2007      117°C
    Jul 2007      108°C
    Aug 2007      108°C
    Sep 2007       67°C

On the SE and NW walls, hot springs with gas temperatures between 89°C and 94°C also deposited sulfur. By March 2007, the hot springs had largely dried and only two gave off very low volume emissions with a temperature of ~ 55°C. Throughout the reporting period, cracks on the intermediate terrace and on the NE edge of the crater widened with new points of gases appearing and deposition of sulfurous material. By February 2007, emission levels began to diminish, and they continued diminishing through September. Fumarole gas temperatures in this area remained steady at near 94° C.

Geologic Background. The broad, well-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, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 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 the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: E. Fernández, E. Duarte, W. Sáenz, V. Barboza, M. Martinez, E. Malavassi, and R. Sáenz, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL:, Email:,