Arenal remained active, with emission of lava from a summit crater (C) toward the N, NW, SW, and S. Vapor emission was continuous. During March, there was a small increase in activity, principally Strombolian eruptions. Ash was ejected to 1 km height and carried W by the prevailing wind. The largest material, blocks and bombs, fell on the flanks to 600 m from the crater. Numerous strong explosion earthquakes were recorded, and periods of harmonic tremor lasting up to 20 minutes were associated with the explosive episodes (table 3). Electronic tiltmeters on the E and W flanks have not shown significant changes this year.

Table 3. Daily number of seismic events and tremor duration at Arenal, 15-26 March 1987, recorded by a seismic station just E of the volcano.

On 18 March at 1005, a large 10-minute ash eruption occurred, and a small nuée ardente moved 500 m down the SW flank. The first material that fell (at the volcano's W base) was very fine ash that originated from the nuée ardente. Fifteen minutes after the start of the eruption, deposition of coarser-grained ash began, lasting 6 minutes. The ash column was carried some 10 km W by winds. The eruption was accompanied by a seismic event that lasted 2 minutes 42 seconds. Explosions continued about once an hour. On 22 March, the wind direction changed to the E, causing 2 days of ashfall in the town of La Fortuna . . . .

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, M. Protti, and E. Fernández, OVSICORI.