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Report on Arenal (Costa Rica) — October 1976


Natural Science Event Bulletin, vol. 1, no. 13 (October 1976)
Managing Editor: David Squires.

Arenal (Costa Rica) Thick block flows advancing NW from the summit crater

Please cite this report as:

Global Volcanism Program, 1976. Report on Arenal (Costa Rica) (Squires, D., ed.). Natural Science Event Bulletin, 1:13. Smithsonian Institution. https://doi.org/10.5479/si.GVP.NSEB197610-345033


Costa Rica

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

All times are local (unless otherwise noted)

Arenal Volcano was very active during observations 12-22 October. Thick block flows were being emitted from the summit crater (D in figure 1) which has been greatly enlarged and now opens to the W. The flows were advancing NW. Avalanches from the flow fronts near the crater were audible about once per minute. Since the initial explosions of July-August 1968, a very large lava "delta" of basaltic andesite (about 0.1 km6 in volume) has been built, covering most of the W slope of the volcano. An additional hot avalanche was emitted in June 1975.

Figure (see Caption) Figure 1. Cross-section along N85°E through Arenal, showing locations of craters (no vertical exaggeration). After Melson and Sáenz (1973).

The activity has sequentially migrated from the lower new explosion crater (A in figure 1) to the middle slope craters (B and C), and is presently from the summit crater (D). Arenal's activity presents an excellent opportunity to observe the eruption and dynamics of thick block flows.

Reference. Melson, W.G., and Saenz, R., 1973, Volume, energy, and cyclicity of eruptions of Arenal Volcano, Costa Rica: BV, v. 37, p. 416-437.

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: W. Melson, SI.