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Report on Irazu (Costa Rica) — January 1993

Bulletin of the Global Volcanism Network, vol. 18, no. 1 (January 1993)
Managing Editor: Lindsay McClelland.

Irazu (Costa Rica) Period of inflation has ended; fumarole gas analyses reported

Please cite this report as:

Global Volcanism Program, 1993. Report on Irazu (Costa Rica). In: McClelland, L (ed.), Bulletin of the Global Volcanism Network, 18:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199301-345060.

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Costa Rica

9.979°N, 83.852°W; summit elev. 3432 m

All times are local (unless otherwise noted)

The pulse of inflation (50 microradians/year) that began in August 1991 appears to have ended, with both leveling and dry-tilt measurements in the summit area showing constant deflation. Data from the geodesic net in the crater area, measured in April 1991 and March 1992, show a mean horizontal expansion of 13 ± 2.5 mm radial to the active crater. Four reoccupations of a sector of the geodesic net between January 1992 and January 1993 did not show significant changes in linear deformation. Areal dilatation, which had increased 48 ppm between April 1991 and September 1992, declined 10 ppm by January 1993, consistent with deflation of the summit dry-tilt net.

Gases were collected from a fumarole on the NE side of the crater lake by Marino Martini, Franco Prati, and Riccardo Balsotti on 21 November 1992. Chemical characteristics (table 4) and the apparent equilibrium temperature of 143°C fall within the range observed for most quiescent volcanic systems.

Table 4. Analysis of gases collected at Poás and Irazú, November 1992. Percentages shown are for dry gas. Poás: 19 November 1992; sample included 88.46 volume % water; temperature 118°C. Irazú: 21 November 1992; sample included 89.54 volume % water; temperature 93°C. Courtesy of M. Martini.

Gas Irazú Poás
CO2 98.91 42.40
SO2 -- 34.16
H2S 0.80 11.62
HCl 0.37 11.27
HF 0.0064 0.26
H2 0.87 0.26
CO 0.0001 0.0003
N2 1.03 0.027
B 0.0041 0.0056

Geologic Background. Irazú, one of Costa Rica's most active volcanoes, rises immediately E of the capital city of San José. The massive volcano covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad flat-topped summit crater complex. At least 10 satellitic cones are located on its S flank. No lava flows have been identified since the eruption of the massive Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the historically active crater, which contains a small lake of variable size and color. Although eruptions may have occurred around the time of the Spanish conquest, the first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas.

Information Contacts: E. Fernández, J. Barquero, R. Van der Laat, F. de Obadía, T. Marino, and R. Sáenz, OVSICORI; M. Martini, Univ di Firenze, Italy.