The following is from the Institute of Volcanology, Boletín de Vulcanología, no. 5.

A team from the Institute of Volcanology climbed the W flank on 20 September. The active block lava flow observed during the previous ascent on 18-19 May had stopped when its front reached an elevation of ~1,130 m on the W flank. A new block flow from the same crater (Crater C, at the W end of the elliptical summit crater area) had moved to about 900 m elevation on the SW flank. From there, a small lobe flowed W, reaching 700 m elevation by 20 September, with an approximate thickness of 20 m at its front.

During the team's ascent, a series of white vapor ejections occurred from Crater C, producing clouds that rose about 200 m above the summit. Each ejection was accompanied by loud noise. Collapse appeared to have taken place in the summit crater's W wall, which separates it from the active Crater C. Collapse debris fell above Crater C.

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.