Strombolian and effusive activity accompanied by strong emissions of vapor and gas continued through July. Small block and ash flows were observed, principally on the NW flank where the active lava flows are situated (figure 29). The daily average number of volcanic earthquakes was 20 (figure 30), similar to the second half of June. A significant increase in harmonic tremor was noted.

Figure 29. Sketch of the profile of Arenal, showing the location of lava flows and pyroclastic deposits. Figure by G. Soto, from a photograph taken by Rafael Barquero on 28 June 1990.

Figure 30. Number of earthquakes recorded at Arenal by the Univ de Costa Rica, July 1990. Courtesy of R. Barquero and 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: R. Barquero and G. Soto, ICE.