Lava extrusion continued from the vent at 1,450 m elevation. The lava flow that had been active in September 1983 stopped advancing in October. During the same month, a new flow (the 43rd since 1968) began to emerge, moving NW before halting at 980 m elevation in November. Another flow (no. 44) started to advance NW in December, remaining active until February, and still another flow moved N between January and March. Extrusion of flow no. 46 started in March and it continued to travel westward late in the month. Rumblings, or sounds similar to those produced by jet aircraft, were often heard in the crater.

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. Fernández S., Univ. Nacional, Heredia.