In late February, a new lava flow was descending Arenal's N flank, filling gullies that had been formed by erosion. Summit area Strombolian activity was continuing, although explosions were not frequent. Explosions typically began with strong rumblings, followed a few seconds later by the ejection of bombs, blocks, and ash from the upper crater to a height of 2,000 m. The ash was carried by the prevailing winds toward the W, to a distance of 4 km from the crater. Blocks and bombs fell as far as 500 m from the crater. Some blocks reached 40 cm in diameter. The N and E flanks of the volcano are covered by trees, and falling tephra caused small fires that burned vegetation. Gas emission continued at the same rate as before.

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. and E. Fernández S., Univ. Nacional, Heredia.