Strombolian eruptions and lava output from Crater C continued in November with columns reaching as high as 1 km above the Crater. OVSICORI reported that during 1994 the following accumulated deflations took place: a) the W-flank leveling line, 7.8 µrad; b) the inclination network, 7.7 µrad; and c) the distance network, 28.6 and 18.5 ppm (SW- and S-flanks, respectively). ICE reported that seismicity for November 1994 was comparatively low (table 8).

Table 8. ICE reported seismicity for Arenal, fall 1994. Their seismometer sits 1.5 km from Crater C. * November seismicity extrapolated based on 15 days of data. Courtesy of G. Soto.

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 Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, Guillerma E. Alvarado, and Francisco (Chico) Arias, ICE.