Strombolian activity, gas emission, and extrusion of lava flows persisted through December from Crater C. Fumarolic activity continued from Crater D. Both lobes of the November lava flow had stopped advancing, the SW lobe at 800 m elevation, covering a small grassy area, and the W lobe at 850 m. A new flow began to descend toward the SW in December, using the same channel. It divided at 1,100 m elevation, with one lobe moving more toward the S, to 1,000 m elevation, the other more toward the W to 1,050 m elevation. Lava overflows fed small avalanches.

Sporadic Strombolian activity ejected bombs, blocks, and ash. Impact craters 1.5 m in diameter and 40 cm deep were found on the W flank above 800 m elevation. Ash columns rose >1 km. Some of the explosions vibrated windows 4 km N of the volcano (in La Palma).

Plants colonizing the lava flows and the devastated zone continued to be affected by acid rain (pH 3.8-4.4) and ashfall. Some had burns on the tips and edges of their leaves, and the colors of others had faded.

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: E. Fernández and J. Barquero, OVSICORI.