The following is from the Institute of Volcanology, Boletín de Vulcanología, no. 4.

A team from the Institute of Volcanology climbed Arenal via the W and N flanks on 18 and 19 May. A new block lava flow from the summit crater was moving slowly down the W flank, where its front had reached 1,300 m elevation (~330 m below the summit). The NE-flank flow described in SEAN 04:03 had stopped at 1,000 m elevation, ~100 m below its elevation of early December 1978. Glow had been visible over the NE flank in March and April. Strong fumarolic activity obscured the summit crater. Within about 70 m of the summit on the N flank, numerous fumaroles emitted vapors ranging in temperature from 55 to 95°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.