"Dartmouth geologists visited the volcano 3-4 December. Lava flows had reached Arenal's lower flanks earlier in the eruption, but lava now being extruded was restricted to the summit. Eruption noises were heard at approximately half-hour intervals, some as far as 5 km from the summit. They were accompanied by sounds of ejecta falling on the upper slopes. Light ashfall was observed 2 km from the summit. A local farmer reported that eruptions (perhaps mud pots?) had occurred in a small lake near La Fortuna, 6 km E of Arenal, but the report was not substantiated."

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: B. Barreiro, R. Naslund, R. Stoiber, and P. Turner, Dartmouth College.