In June 1979 and again in April 1980, the active vent was obstructed by a lava dome. Lava extruded from the dome flowed down the flanks, and gases (table 2) with temperatures between 930 and 954°C escaped at low pressure from radial fissures in the dome. Beginning in March 1981, violent degassing of the upper part of the conduit ejected large vapor columns, accompanied by strong detonations heard in La Fortuna, 6 km to the E. The vapor columns occasionally included fine tephra and bombs, which fell as far as 100 m from the crater rim. Incandescent tephra was sometimes observed at night.

Table 2. Analysis of the anhydrous component (H₂O = 94%) of one of 60 samples collected from fissures in Arenal's dome between June 1979 and October 1980. Courtesy of J.L. Cheminee.

In May 1981 the dome deflated, reopening the active vent's central conduit. At about the same time a multi-lobed flow, fed by the dome for the past several months, stopped advancing down the W flank. Later in May, block lava overflowed the S wall of the vent and began to move downslope at 50 m/hour in a channel 20 m wide. The flow split into two parallel lobes at 1,200 m elevation. The front of one lobe halted at 500 m elevation on the S flank, but the second lobe continued down the SW flank. Vapor emission continued to kill vegetation on the N and E flanks, resulting in increased erosion that carved large gullies.

An ascent to the crater area 25 June revealed a new flow (the 36th since 1968) descending the W flank. Continuous vigorous emission of gas and fresh tephra (basaltic andesite with plagioclase phenocrysts) was punctuated 1-3 times per day by stronger explosions that ejected bombs and blocks to roughly 200 m above the crater rim. The stronger explosions, which typically occurred after periods of decreased degassing but without immediate warning, kept geologists from getting close to the crater. A network of five radon-monitoring stations were installed on the volcano, in cooperation with Michel Monnin, Univ. of Clermont-Ferrand.

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 H., E. Malavassi R., Univ. Nacional, Heredia; J. Cheminée, IPG, Paris; H. Delorme, Univ. de Paris; G. Avila, F. Guendel, ICE.