The following is from the Institute of Volcanology, Boletín de Vulcanología, no. 10, March 1981.

Lava extrusion continued from the vent at 1,450 m elevation at the W end of the summit crater. Portions of multi-lobed flow 34 were still advancing in early 1981. One lobe partially covered the W flank explosion crater that had formed in 1968 and extruded lava until 1973. At about 1,000 m elevation, this lobe separated into three sub-lobes, all of which flowed generally westward, reaching an elevation of about 900 m by late February. Two other lobes had halted, one in December 1980 at 700 m elevation, the second in January 1981 at 750 m elevation.

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: J. Barquero H., Univ. Nacional, Heredia.