OVSICORI-UNA reported that changes in morphology and the health of vegetation at Arenal were documented during approximately 14-20 September. Fine ashfall produced at the summit by an active lava flow impacted and burned vegetation along the upper and lower E and NE flanks. The impact on the vegetation was more severe near the summit. By mid September, additional material moving down the SW flank had filled the 6 June collapse scar and built an 800-m-high levee. Occasional incandescent blocks roll down the top of the levee. Material accumulated on both sides of the levee and at the distal end, creating a 200-m-wide fan. A mass of material also accumulated at the SW edge of the summit, causing blocks to occasionally roll down the N flank.

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.

Source: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)