OVSICORI-UNA reported that during June activity originating from Arenal's Crater C consisted of gas emissions, sporadic Strombolian eruptions, and occasional avalanches. Some of the Strombolian eruptions caused glass to vibrate in buildings located 4 km N. A lava flow that began in mid-January remained active on the S flank. Avalanches from edges of the lava flow and from the N and NE crater rim descended multiple flanks. Acid rain and small amounts of ejected pyroclastic material affected the NE, E, and SE flanks. Small explosions of gas and occasionally ash originated from a vent N of Crater C, while Crater D produced only fumarolic activity.

Geologic Summary. Conical Volcan Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1,657-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. The earliest known eruptions of Arenal took place about 7,000 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. Arenal's most recent eruptive period began with a major explosive eruption in 1968. Continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows has occurred since then from vents at the summit and on the upper western flank.