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Report on Arenal (Costa Rica) — July 1986


Scientific Event Alert Network Bulletin, vol. 11, no. 7 (July 1986)
Managing Editor: Lindsay McClelland.

Arenal (Costa Rica) Increased seismicity with gas/ash emissions & lava flows

Please cite this report as:

Global Volcanism Program, 1986. Report on Arenal (Costa Rica) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 11:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198607-345033


Costa Rica

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

All times are local (unless otherwise noted)

"Continuous eruptive activity at Arenal started in 1968 with a strong explosive phase and nuées ardentes, preceded by a seismic swarm lasting several days. The volcano then entered into an effusive phase with more or less continuous block lava flows and some small explosions. This activity was interrupted in 1975 by a new explosive phase that included hot avalanches. From 1976 through 1983 activity was mainly effusive, with numerous lava flows. In 1984, another moderate explosive phase lasted for 6 months. Normal effusive Strombolian activity occurred in 1985, with a moderate increase in October and November.

"This year, a moderate increase in Arenal's activity started 1 May, with a maximum of 25 volcanic earthquakes (B-type)/day associated with gas and ash eruptions, and with block lava flows. Countable events totalled 188 in May (figure 7). The maximum trace amplitudes of the earthquakes showed a rather important increase relative to previous months (figure 7), indicating an increase in the energy liberated by the volcano, perhaps due to the beginning of the rainy season. Activity was intermittent through mid-July; the number of events in June was 144 with a maximum of 16/day. Maximum daily amplitudes of volcanic tremor showed clear increases prior to days with a greater number of volcanic earthquakes (figure 7). So far the activity can be considered normal within the present active period."

Figure (see Caption) Figure 7. Seismicity at Arenal recorded at the FOR station during 1 May-15 July 1986. Daily number of B-type volcanic earthquakes (top), maximum daily amplitude of volcanic earthquakes (center), and maximum daily amplitude of volcanic tremor (bottom). Courtesy of R. Barquero.

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, ICE.