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Report on Irazu (Costa Rica) — October 1993


Bulletin of the Global Volcanism Network, vol. 18, no. 10 (October 1993)
Managing Editor: Edward Venzke.

Irazu (Costa Rica) Low seismicity; migrating fumaroles; lake level rises

Please cite this report as:

Global Volcanism Program, 1993. Report on Irazu (Costa Rica) (Venzke, E., ed.). Bulletin of the Global Volcanism Network, 18:10. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199310-345060


Costa Rica

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

All times are local (unless otherwise noted)

During October, Irazú exhibited low seismicity together with migrating subaqueous fumaroles and rising water in its crater lake. Over the 2-month interval from September to October the water level rose 40 cm. Scientists noted the following during October in Irazú's crater lake: an emerald-green color, a minimum pH of 5.6, and a typical water temperature range of 18.7-24.6°C. The maximum water temperature reached 92°C at bubbling sites within the NE part of the lake. Other sites with notable bubbling were situated in the N, NW, and E. Compared to the previous few months, steam emitted from the N fan decreased in vigor, discharged over a reduced area, and exhibited lower temperatures (less than 81.4°C in October). The seismometer, 5 km SW of the main crater, recorded little activity.

Geological Summary. 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 Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, ICE.