In September, continuous gas and lava emission were accompanied by sporadic block and ash ejections from Crater C (300 m W of the summit). Blocks landed as much as 1.2 km away (at 850 m elevation), and impact craters on the W flank reached 50 cm in diameter. Explosion columns rose to 1 km height and were carried by wind predominantly to the NW, W, and SW. Fumarolic activity was visible in the summit crater. Vegetation on the NE, E, and SE flanks continued to be affected by acid rain. Resulting erosion problems included small debris flows, especially on the NE flank (Quebrada Guillermina), E flank (Quebrada Calle de Arenas), and SW flank (Río Agua Caliente).

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, E. Fernández, V. Barboza, and J. Brenes, OVSICORI.