The following is from the Institute of Volcanology, Boletín de Vulcanología, no. 7.

The block lava flow [noted on 20 September 1979] continued to advance down the SW flank through the end of February. An inspection of the N flank on 23 January revealed no new lava flows on that side of the volcano. The rate of extrusion from Crater C had diminished in late January to the extent that it was difficult to see blocks moving in the central channel of the SW-flank flow. However, the extrusion rate increased in February, and lava overflowed its channel at about 1,200 m elevation. By 28 February, the new lobe had reached an elevation of ~1,000 m, with a front 50 m wide and 20 m high, partially covering an older flow. Emission of white vapor from Crater C also continued, accompanied by loud noise.

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.