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Report on Arenal (Costa Rica) — May 1991


Bulletin of the Global Volcanism Network, vol. 16, no. 5 (May 1991)
Managing Editor: Lindsay McClelland.

Arenal (Costa Rica) Strombolian activity and seismicity increase, then decline; block lava flows on S and SW flanks

Please cite this report as:

Global Volcanism Program, 1991. Report on Arenal (Costa Rica) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199105-345033


Costa Rica

10.463°N, 84.703°W; summit elev. 1670 m

All times are local (unless otherwise noted)

On 20 April, seismic explosion signals became moderately more frequent, and seismicity increased to >40 recorded earthquakes/day (RSN network). Seismicity was similar in May, with a daily average of 20 recorded earthquakes and a maximum of 43 (Univ Nacional network). Strombolian explosive activity was stronger, more voluminous, and more frequent, especially on 19-26 May when explosions vibrated windows and were heard 34 km SE (in Quesada). Several explosions were recorded at a seismic station 98 km away (Juan Diaz). Plumes rose to 1 km height above Crater C, depositing ash to Chambacú (17.5 km NE) and La Palma (4 km N). After 26 May, seismic and eruptive activity returned to normal levels. Gas emission continued with periodic, smaller explosions; plumes were carried predominantly to the NE, W, and SW. Block lava flows continued down the SW and S flanks, reaching 700 m elevation by the end of April.

Geological Summary. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: E. Fernández, J. Brenes, V. Barboza, and T. Marino, OVSICORI; R. Barquero, ICE.