The following is from the Institute of Volcanology, Boletín de Vulcanología, no. 6.

During a late December visit to Arenal by Guillermo Avila and Jorge Barquero H., the SW-flank lava flow continued to advance at 20-25 m/day, more slowly than when the flow was higher, on steeper slopes. The flow, originating from Crater C, was channeled and composed of blocks ~0.5 m across. Vegetation surrounding the flow front was ignited by the heat of the lava. Since the last ascent of Arenal on 20 September, a new lobe had separated from the main flow and was moving downslope to the S.

Whitish vapor ejections from Crater C remained frequent, accompanied by loud noises and a very strong sulfurous odor. The strong fumarolic emanations forced Avila and Barquero to wear gas masks. A red glow was visible in the summit area at night. Major fissures in the upper wall of the summit crater threaten collapse of part of this wall into the vent area.

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: G. Avila, ICE; J. Barquero H., Univ. Nacional, Heredia.