The following is a report from Dartmouth College geologists.

"A blocky lava flow continued to descend the upper slopes. On 17 November, a short, very slow-moving flow descended barely a hundred meters from the summit on the W side. This flow was much like others of recent months, slow-moving and not far-reaching, and the summit elevation had been built up as a result. By 27 November, a new flow had moved 250 m down the NE slope toward the head of the Río Tabacón, where a small nuée ardente eruption killed one and injured another in 1975. This flow was moving at a velocity more like that observed in November, 1981 (~1 m/hour).

"Numerous loud gas explosions could be heard during several days of observation. SO₂ output, as determined by COSPEC, was extremely small considering the active state of the volcano. [Total SO₂ output was about a quarter of the 200 ± 30 metric tons/day reported by Casadevall and others (1984) for Arenal in February 1982.] Large areas of the volcano's lower slopes were covered by a thin haze of blue gas suspected of being HCl-rich."

Reference. Casadevall, T., Rose, W.I. Jr., Fuller, W., Hunt, W., Hart, M., Moyers, J., Woods, D., Chuan, R., and Friend, J., 1984, Sulfur dioxide and particles in quiescent volcanic plumes from Poás, Arenal, and Colima volcanoes, Costa Rica and México: JGR, v. 89, no. D6, p. 9633-9641.

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. Stoiber, S. Williams, H. Naslund, C. Connor, J. Prosser, and J. Gemmell, Dartmouth College; E. Malavassi R. and J. Barquero H., Univ. Nacional, Heredia.