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Report on Arenal (Costa Rica) — February 1987


Scientific Event Alert Network Bulletin, vol. 12, no. 2 (February 1987)
Managing Editor: Lindsay McClelland.

Arenal (Costa Rica) Tephra eruptions and lava flows; seismicity declines

Please cite this report as:

Global Volcanism Program, 1987. Report on Arenal (Costa Rica) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 12:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198702-345033


Costa Rica

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

All times are local (unless otherwise noted)

A moderate increase in seismic activity . . . continued until August, when seismicity began a gradual decline that persisted through the end of 1986. The number of explosive volcanic earthquakes (C-type) peaked during July with 261 events recorded at a rate of 8/day. Totals of 160 events were recorded in August, 89 in September, 100 in October, 85 in November, and 55 in December (figure 8, top). These earthquakes were associated with moderate eruptions of gas, ash, and bombs, and small block lava flows from the upper crater. Tremor activity declined after increases in May, July, and August (figure 8, bottom).

Figure (see Caption) Figure 8. Seismicity at Arenal recorded at station FOR during July-December 1986. Daily number of volcanic earthquakes (top) and maximum daily amplitude of tremor (bottom). Courtesy of ICE.

A small volcanic debris flow, probably of low temperature, was generated in 1986 (possibly on 16 March). A new well-defined lava flow was observed in January 1987 on the NW flank. The flow descended to ~1,000 m elevation before stopping temporarily by the beginning of February. In early February, Strombolian explosions occurred every 2 hours. Continuous eruptive activity started in 1968 with strong explosions and pyroclastic flows. More than 60 lava flows have emerged from the summit crater since then.

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: R. Barquero and Guillermo Alvarado, ICE.