The following is from Boletin de Volcanologia No. 2 of the Volcanology Section, School of Geographical Sciences, National University of Costa Rica.

Lava extrusion and gas emission at Arenal continued through mid-October. A block lava flow descended NW from Crater C, near the summit at about 1,400 m elevation. By 15 July, this flow had bifurcated at about 1,100 m elevation, one arm moving N, the other W. Three months later, on 14 October, the front of the W arm had traveled 500-600 m from the point of bifurcation, to approximately 830 m elevation, and was about 15 m thick. The N arm had only advanced about 150 m because of damming by older flows. A large gas column from Crater C was also observed on 14 October.

William Melson notes that Crater C has merged with Crater D, the old summit crater, over about the past 5 years, forming a single much-enlarged summit crater breached on the NW. This crater is rimmed elsewhere by nearly vertical cliffs, up to an estimated 100 m high on the E side.

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; W. Melson, SI.