During June, Crater C continued to emit gases, lava flows, and sporadic Strombolian-style eruptions. The lava flow that emerged in December, and took a more westerly course, stopped at an elevation of 800 m. The flow that emerged in late April followed the same channel as a previous flow and remained active to an elevation of 1,000 m. At ~1,300 m elev small blocky flows escaped the confines of bounding levees.

OVSICORI scientists reported that during June, Strombolian eruptions were both infrequent and small. ICE scientists watched ash plumes escape at rates averaging once each half-hour; these plumes rose up to 1,200 m above the crater. Though not erupting, Crater D maintained fumarolic activity.

During a 10-day period in June, a total of 227 earthquakes were recorded. The earthquakes mainly fell in the 1.2-2.6 Hz range. Tremor was also relatively rare; during these 10 days of recording, it remained below 2 Hz and totaled only 6 hours.

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, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, G. Alvarado, and F. Arias, ICE; H. Flores, Univ de Costa Rica.