In August, Crater C continued its regular emission of gases, lava, and sporadic Strombolian eruptions; in addition, there were occasional lava avalanches. Lava that began to be emitted in May 1995 followed a course toward the SW and flowed down as far as 720 m elevation. Lava emitted a month earlier branched into two arms at ~1,300 m elevation. Explosions sent columns to 1 km above the active vent. These columns were typically blown to the NW, W, and SW; however, some minor local ashfalls also took place on the N and NE as well.

During August, OVSICORI-UNA reported comparative highs in both the number of seismic events (925) and the duration of tremor (348 hours). No seismic data are currently available for July 1995. Comparing the available record starting in January 1994, greater numbers of seismic events took place only in February and March 1994 (956 and 1,011 events) and in June 1995 (1,027 events). For the same interval, tremor prevailed longer only in May and June 1995 (419 and 402 hours, respectively).

A May explosion of >1.2 minutes duration and a plot of the available 1994-95 seismic record were both discussed in BGVN 20:04. The volcano's first chronicled eruption occurred in 1968 and many basaltic andesite discharges have followed since then.

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. Fernandez, E. Duarte, R. Saenz, W. Jimenez, and V. Barboza, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.