The active Crater C continued to emit gases and lava, and was the site of sporadic Strombolian eruptions. Crater D continued fumarolic degassing. The lava that began to exit from crater C at the end of December remained active along its two NW-flowing channels. These divided at 1,100 m elev, forming two arms that in April extended to 900 m and 750 m elev. The temperature and pH of hot and cold springs around the volcano remained stable during early April sampling.

Compared to previous months, the Strombolian eruptions that had accompanied strong rumbling have diminished in both number and magnitude. On days with good visibility observers on the W flank saw Arenal noiselessly send up small, ash-bearing eruptions that rose to 300 m above the rim of Crater C.

In April, low-frequency seismic events took place 621 times, and harmonic tremor lasted for 57 hours. Relative to previous months, there was a decrease in the duration of tremor with frequencies below 3 Hz.

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. Fernández, J. Barquero, V. Barboza, and W. Jiménez, OVSICORI; G. Soto, G. Alvarado, and F. Arias, ICE; H. Flores, Univ de Costa Rica.