Lava flows that began earlier this year remained active, and gas emissions continued from Crater C in June. Sporadic Strombolian activity sent ash columns >500 m above the crater rim; some ash fell up to 1 km away. Ash was carried by the wind to the NW, W, and SW. Ballistic bombs and blocks fell to 1,100 m elevation. Several pyroclastic flows produced by lava-front collapses have been seen. The largest observed pyroclastic flow, at 1434 on 24 June, produced an ash cloud about 1 km high and traveled about 750 m while descending from 1,400 to 1,000 m elevation on the SW flank. Based on morphological changes of the cone, other rockslides are believed to have occurred high on the NW flank at ~1,300 m elevation. The lava flow on the SW flank that began in March has stopped with both the W and SW lobes at ~1,000 m elevation. The W flank lava flow that began in May descended to 1,000 m elevation. The lava flow that started in mid-April on the SW flank remained active as its W lobe reached 1,200 m elevation and its SW lobe descended to 1,250 m. Summit fumaroles also remained active.

The seismic station 2.7 km NE of the active crater (VACR, operated by OVSICORI) recorded 790 seismic events (1.4-3.0 Hz), an average of 26 events/day. The highest daily totals were 65 events on 16 June and 64 on 22 June. Most of the seismicity was associated with explosive gas emissions that sounded like jet engines or trains. The VACR station also registered >195 hours of tremor with frequencies of 1.3-2.8 Hz. The highest daily totals were on 24-26 June, when >57 hours of tremor were recorded. ICE geologists saw or heard an average of 10 explosions/day in late June. Tremor was recorded for several hours/day in June by portable seismometers operated by ICE.

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, and V. Barboza, OVSICORI; G. Soto, ICE.