Two lobes of the lava flow active since November continued to extend down the W flank in May, with an estimated total volume of 3 x 10⁶ m³ of lava. The northernmost lobe divided into several fronts; the longest reached to ~800 m elevation, while the most active front became channeled in a valley at ~855 m elevation on 14 May. A lava temperature of 820°C was measured at the front using an infrared thermometer. The southern lobe continued to travel along a more gentle slope to ~700 m elevation, covering and burning roughly 100 m² of forest and grasslands. Summit incandescence, visible at night, suggested to scientists that a lava lake was feeding the active lava flow. Small pyroclastic flows occurred sporadically. One observed at 0723 on 13 May flowed down the W flank to 1,200 m elevation.

Explosive activity increased in number and magnitude from preceding months, especially since 26 May, when new explosions produced ash columns >1 km high and bombs fell to 1,000 m elevation. Between 23 April and 12 May, 80 g/m² ash had accumulated 1.8 km W of the crater (at 740 m elevation). Samples were composed of very fine ash (40%), and fine and medium-sized scoria fragments and plagioclase crystals (60%). Volcanic earthquakes averaged 10/day in May (compared to 6 and 15 daily in April and March, respectively), with maxima of 20-24 on 15, 23, and 28 May. The month's highest levels of tremor were recorded on 7, 12, 14, 17, and 22 May.

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: G. Soto, R. Barquero, and G. Alvarado, ICE; M. Fernández, Univ de Costa Rica; E. Fernández, J. Barquero, and V. Barboza, OVSICORI.