The lava flow first emitted in April 1995 trended W and branched into two arms at 1,150 m elevation. During May one of these branches progressed to the 1,050-m elevation, and the more SW-directed flow progressed to 950 m elevation. During June, these same two branches descended to the 1,000- and the 800-m elevations, respectively. In June, the lower flow measured 23-25 m thick, and 50-m wide.

During May, there were increases in the number of eruptions, their sound intensity, and the amount of ash in eruptive columns; in both May and June some ash column heights ascended to over 1 km above Crater C. Fumarolic activity continued at Crater D during May and June.

ICE reported that from late April through most of June the amount of ash collected 1.8 km W of the active vent remained relatively high, 15-38 grams/m² (table 11). Shifting wind directions brought ash to the village of La Fortuna, 6.5 km E of Arenal. Ashfall was reported in Arenal's NW, W, and SW sectors, and infrequently in the S sector.

Table 11. Ash collected 1.8 km W of Arenal's active vent; note the corrected grain size of 300 µm (rather than 250 µm) also applies to tabled data in previous reports. Courtesy of Gerardo Soto, ICE.

Seismic activity in May consisted of 866 events (low frequency-3.5 Hz range), mainly associated with Strombolian eruptions. Some events were sufficiently large to be detected 30 km SW of Crater C (station JTS). On the most seismically active day of the month, 7 May, there were 50 events. June seismic activity consisted of 1,027 events.

Tremor took place during May for 419 hours, and during June for 402 hours. The tremor signal was centered between 2 and 3.2 Hz, with amplitudes in May reaching over 100 mm, and in June, typically in the 50-80 mm range. The relatively large tremor in May was also registered at the more distant station JTS.During April and May the leveling network continued to show an average deflation of 15 microrad, a continuation of the tilt direction and magnitude witnessed in previous years. Surveys of the distance measuring network in 1994 and principally in 1995 registered a contraction of 15 ppm/year. A local reversal of this trend was seen between 17 and 25 May 1995 when one of four distances measured on the S flank revealed a 23 ppm expansion.

Arenal's first historical eruption, in 1968, began an unbroken sequence of Strombolian explosions and basaltic andesite discharges from multiple vents. The volcano has been watched by many tourists from a mountain lodge 2.8 km S of the vent that enables visitors to hear, to see, and occasionally to smell its dynamism.

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, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez and R. Saenz, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora, Escuela Centroamericana de Geologia, Universidad de Costa Rica; J.F. Arias, L.A. Madrigal, and G.J. Soto, Oficina de Sismologia y Vulcanologia del Arenal y Miravalles: OSIVAM, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.