Activity at Arenal since May 1999 (BGVN 24:06) has been comparatively quiet, a condition which has generally prevailed since an energetic outburst in May 1998 (BGVN 23:06), continued during July 1999 through January 2000. However, this relative quiet was broken in October 1999 by an anomalous pyroclastic flow, discussed below. Elevated activity occurred in November as well. In addition to lava flows traveling N and NE during this interval, in July a new, N-directed lava flow was emitted. Another during September traveled NE spawning occasional rockfalls off its front. Few eruptions produced plumes rising more than a kilometer above crater C, and crater D remained fumarolic in nature. Cold avalanches continued to occur down local valleys (including the Calle de Arenas, Manolo, Guillermina, and Agua Caliente). During June-August electronic distance measuring disclosed that the survey points underwent an average of 2 cm expansion.

A pyroclastic flow at 1721 on 26 October descended the W flank as far as the ~900 m contour and left an eroded swath. It was smaller and its path differed from the May 1998 pyroclastic flow down the N-NW flank along the Tabacón river (BGVN 23:04). In the 26 October event, fine tephra accumulated in an area ~200 m wide. This pyroclastic flow may have consisted of a series of several distinct events. The column height was ~1 km above the vent.

The 26 October event's precursors included heightened explosive activity and increased seismicity beginning on 8 October, and a decrease in tremor. A precursory seismic swarm may have also been related. Although high-frequency earthquake swarms are generally rare at Arenal, a modest one began in August 1999 and reached a maximum on 17 September (5 events per day). The swarm ceased after 1 October; it consisted of 55 registered events with 13 of these located. The earthquakes comprising the swarm had amplitudes below Mc 2.3. Hypocenters for the located earthquakes typically occurred at depths of 2-4 km. After the pyroclastic flow, the energy transmitted in explosions and the amplitude of tremors dropped considerably.

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. Fernandez, E. Duarte, V. Barboza, R. Sáenz, E. Malavassi, R. Van der Laat, T. Marino, J. Barquero, and E. Hernández, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; M. Martinez and J. Valdez; Laboratorio de Quimica de la Atmosfera, Universidad Nacional; R. Barquero, Instituto Costarricense de Electricidad (I.C.E.), Departamento de Geologia, Apdo. 10032-1000, San José, Costa Rica; I. Arroyo and G. Alvarado, Observatorio Sismológico Vulcanológico Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.