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Report on Arenal (Costa Rica) — July 1995


Bulletin of the Global Volcanism Network, vol. 20, no. 7 (July 1995)
Managing Editor: Richard Wunderman.

Arenal (Costa Rica) Crater C continues to emit lavas and Strombolian eruptions

Please cite this report as:

Global Volcanism Program, 1995. Report on Arenal (Costa Rica) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 20:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199507-345033


Costa Rica

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

All times are local (unless otherwise noted)

During July, craters C and D continued to emit gases, and Crater C generated lava flows and sporadic Strombolian eruptions. The lava flow that started in May continued to move, but during June its SW arm only advanced ~180 m. During June this arm had a 52-m width and terminated at the 760-m elevation with the final 100 m of its length inclined 10-12° downward. A newer flow began to be extruded into a previously active channel and reached ~1,100 m elevation; the front of this flow produced small avalanches. Distance measurements revealed a transitory expansion of the edifice beginning in March 1995 and attaining an average maximum of ~20 ppm on about 25 May. Measurements in early July showed a return to the previous tendency of contraction. Ashfall was again measured W of the crater (table 12).

Table 12. Ash collected 1.8 km W of Arenal's active vent. Courtesy of Gerardo Soto, ICE.

Collection Interval Avg daily ashfall (grams/m2) Ash % 300+µ Ash % less than 300µ
21 Apr-23 May 1995 37.6 51.1 48.9
23 May-29 Jun 1995 15.4 51.2 48.8
29 Jun-27 Jul 1995 17.5 19.3 80.7

Arenal's recent lavas have had basaltic andesite compositions. The volcano lies directly adjacent to Lake Arenal, a dammed reservoir that generates hydroelectric power.

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. Fernandez, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez and R. Saenz, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apdo. 86-3000, Heredia, Costa Rica; Mauricio Mora, Escuela Centroamericana de Geologia, Universidad de Costa Rica; Gerardo J. Soto, Observatorio Sismologico y Vulcanologico del Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apdo. 10032-1000, San José, Costa Rica.