At . . . Crater C, July marked another month of continued gas and lava emissions, and sporadic Strombolian eruptions. During July, the lava flow that began at the end of April continued to erupt and flow down an established channel. During 23 days in July, seismic station VACR (2.7 km NE of crater C) recorded an average of 18 events/day. These were interspersed with days having very low seismicity and tremor. Beginning on 23 July, Strombolian-type eruptions became common, and during 23-30 July they were seen 52 times. In some cases these eruptions were accompanied by sounds similar to a jet or steam engine. On 28 July tremor reached an amplitude of 27 mm at a frequency below 2.5 Hz.

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, J. Barquero, V. Barboza, R. Van der Laat, F. de Obaldia, and T. Marino, OVSICORI; G. Soto, G. Alvarado, and F. Arias, ICE; M. Mora, C. Ramirez, and G. Peraldo, Univ de Costa Rica.