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Report on Irazu (Costa Rica) — 26 August-1 September 2020


Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
26 August-1 September 2020
Managing Editor: Sally Kuhn Sennert

Please cite this report as:

Global Volcanism Program, 2020. Report on Irazu (Costa Rica). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 26 August-1 September 2020. Smithsonian Institution and US Geological Survey.

Weekly Report (26 August-1 September 2020)


Costa Rica

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

All times are local (unless otherwise noted)

OVSICORI-UNA reported that Irazú’s seismic network recorded land movement 1.2 km SW of the SW crater rim in a high area used for radio and television antennas. Landslides in the area had been observed since 2014, but activity accelerated in the previous months. The number of events indicating landslides began to exponentially increase particularly after 20 August. The rate of movement had increased to 20 m/year horizontally and about 25 m/year vertically by 24 August; movement of more than 20 cm was recorded during 23-24 August. A large block collapsed to the NW, into the Rio Sucio drainage, during the morning of 26 August.

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.

Source: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)