The lava flow that began to descend SW from Crater C in December remained active. The W lobe had reached 840 m elevation, the S lobe 820 m, covering a grassy area. Lava overflows continued to feed small avalanches. Strombolian explosions from Crater C were weaker and less frequent in January than in December. Fumarolic activity continued from Crater D.

Deflation has continued since deformation measurements began in 1982. The deflation is more evident on dry tilt stations nearest the active crater (at 1.8-3 km distance). Small-scale inflation averaging 9 µrads occurred between measurements in October and December 1992 on stations SW, W, NW, and NE of the summit. Although horizontal distance measurements generally contracted between November 1991 and January 1993, two of five distances expanded ~15 ppm between October and December 1992, coinciding with inflation detected by dry-tilt measurements. Geologists noted that these changes could have been influenced more by lava flows near the reflectors than by magma movements at depth.

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 Obadía, T. Marino, and R. Sáenz, OVSICORI; M. Martini, Univ di Firenze, Italy.