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Report on Arenal (Costa Rica) — August 1989


Scientific Event Alert Network Bulletin, vol. 14, no. 8 (August 1989)
Managing Editor: Lindsay McClelland.

Arenal (Costa Rica) Increased Strombolian activity; lava flows

Please cite this report as:

Global Volcanism Program, 1989. Report on Arenal (Costa Rica) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 14:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198908-345033


Costa Rica

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

All times are local (unless otherwise noted)

Strombolian activity remained moderate through August. Some of the August explosions were accompanied by detonations that rattled windows in the town of Fortuna . . . . Ash columns rose >1 km above the summit and were carried by the prevailing winds toward the W flank. Blocks and bombs fell as far as 800 m from the crater. Lava flows advanced N and NE from the crater, and the NE flank lava caused small fires as it moved through the forest. The permanent Univ Nacional seismic station continued to record harmonic tremor and explosions. Average values for tremor rose from 16.8 hours/day in May to 22.5 in June and 23 in July before dropping to 20 in August. Explosions, on the other hand, increased to a maximum of 18.5/day in August (from 2.5 in May, 3 in June and 5.2 in July) (figure 23). Eruption signals were of low to medium frequency (2.5-5 Hz) with amplitudes of 6-40 mm. Red Sismológica Nacional instruments recorded an average of 27 volcanic earthquakes daily in August, with a maximum of 38 on the 13th.

Figure (see Caption) Figure 23. Daily hours of tremor (solid bars) and number of recorded eruptions (striped bars), May-August 1989 at Arenal. Courtesy of OVSICORI.

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: J. Barquero, E. Fernández, and V. Barboza, OVSICORI; R. Barquero and G. Alvarado, ICE.