Strombolian activity persisted from Crater C with continuous emission of lava and gases, and sporadic ejection of pyroclastic material. The stronger June/July activity produced occasional nuées ardentes that extended ~500 m from the crater on ... 13 July at 1630 (burning 1/2 hectare of forest on the S flank) and 17 July at 1430. Ash emission also increased in June, with fine ash generally carried to ~4-5 km W and NW from the crater by the prevailing winds. Lava flows continued to descend the N and NE flanks. The incipient lava flow seen 23 May during a helicopter overflight continued to advance NW toward the Río Tabacón. At the end of July, its front was at ~1,000 m elevation, roughly 1 km from the summit.

Activity declined sharply in August. Only 21 explosion earthquakes were recorded compared to 248 in July, and tremor weakened considerably.

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, OVSICORI; R. Barquero and Guillermo Alvarado, ICE; G. Soto and L. Morales, Univ de Costa Rica.