Arenal's eruption continued, as lava descended from Crater C towards the N and NE; a small lava flow moved S in April. Occasional tephra eruptions were observed, some of which ejected considerable ash in March. On 11 March at 0954, a nuée ardente descended in two lobes, one toward the SW and the larger toward the S, reaching 800 m from Crater C. Ash was carried W by the wind, to 12 km from the crater. Flank vegetation was covered with ash, some of which remained on trees a month later. On 28 March at 1105, another nuée ardente was generated, again with two lobes to the NW and SW. The larger (NW) lobe advanced 500 m. Ash fell 10 km W of the crater. During a period of moderate explosive activity 3-9 April, there were 6-13 explosions/day. Activity declined considerably from 10 April, and on the 17th and 18th, for example, no explosions were seen or heard. At the end of the month, a slight increase was detected in volcanic noise or explosion shocks.

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 and E. Fernández, OVSICORI; G. Soto, Univ de Costa Rica; G. Alvarado, ICE.