Fluctuations in the intensity and frequency of explosive activity were reported by OVSICORI-UNA. Activity during April increased above that of the previous several months but diminished during May. The April increase was accompanied by a corresponding rise in the amount of pyroclastic material produced; columns ascended over 1 km above Crater C in April and somewhat lower in May; these were commonly blown towards the NW, W, and SW. Ashfall measured at the ICE station 1.8 km W of the vent was higher during March-May than earlier in the year (table 14).

Table 14. Ash collected 1.8 km W of Arenal's active vent. Courtesy of Gerardo Soto, ICE.

During April and May, bombs and blocks fell to 1,200 m elevation. New pyroclastic-flow deposits were noted in April. Early April pyroclastic deposits descended the SW flank (to 1,000 m elevation) and those of late April descended the NW flank (to 1,250 m elevation). Light ash fell towards the N and NE in May.

Lava flows emitted in the previous month divided into two arms that both trended about NW. A new, NE-trending flow began during April and by the end of the month its front reached 1,200 m elevation. Sporadic avalanches fell off this front and sometimes reached into forested land. During May, continued descent of the flows to as low as 750 m elevation led to avalanches off their fronts producing small fires in the woods. Accumulating tephra and lava have caused Crater C's floor to rise an average of 5.4 m/year since 1987.

OVSICORI-UNA reported a progressive seismic buildup during April; over the course of the month the number of local earthquakes increased 4- to 6-fold peaking on the 27th. Station VACR (2.7 km NE of the Crater C) registered rather typical numbers of earthquakes for both April and May: 798 and 828 events, respectively. Many of these earthquakes were associated with Strombolian eruptions that took place on 20-28 April.

The number of hours of tremor during April, 434, was the highest measured in more than two years. While there occurred a progressive buildup in the number of earthquakes during April (ending on the 27th), tremor during the same interval fluctuated strongly, with daily totals between about 6 and 23 hours. May tremor totalled 325 hours. Results for monthly earthquakes and tremor obtained by ICE are smaller but also show relative increases (table 15).

Table 15. Average seismicity at Arenal, as recorded in Fortuna station, 3.5 km E of active vent. Courtesy of ICE.

Deformation studies carried out during April and May indicated no significant changes in that time interval. By the end of April 1996 the distance network had indicated a contraction of 22.4 ppm/year during the last two years.

OVSICORI-UNA and a team of seven visiting scientists reported that on 1-9 March Arenal's summit was almost continuously visible due to abnormally clear weather. Two gas plumes were observed, the largest being associated with the continuing Strombolian activity. This plume had extremely variable output and was often ash laden. The smaller plume, which was emitted at a more-or-less constant rate (even during the Strombolian explosions), carried no ash. The separate plumes were thought to signify the existence of two or more summit vents.

The Strombolian activity remained vigorous and variable, with large bombs being regularly thrown over the crater rim, making access to points on the edifice above 1000 m extremely hazardous. The ash column sometimes collapsed, resulting in pyroclastic surges, some of which were witnessed. Ash fallout from the plume was observed to vary from a wet, fine powder to dry particles up to 0.5 mm in diameter. Ash occasionally fell on the lower western flanks of the volcano.

The two lava flows referred to above were active when observed by visiting scientists. One flow was more vigorous; it issued from a steeply leveed channel aligned westwards from the summit for 200 m before diverging northwestwards.

A survey of lava flows erupted during 1995 showed that the westward flow had halted at 750 m and was composed of Arenal's typical basaltic andesite. The visiting scientists saw one anomalously hot area at 850 m elevation on the N levee that was distinguished by escaping steam. The levee on the flow's opposite side had completely collapsed. The flow was beginning to be vegetated by moss and ferns. The westward flow, which halted at 850 m in November 1995, contained vesiculated lava as well as the usual basaltic andesite mixed with blocks of ash. Flow thickness at the front of the surveyed flow that lies to the NW was around 100 m.

SO₂ fluxes were also measured by COSPEC as a follow-up to measurements made at the same time last year. Six days of flux data during 29 February-8 March were collected, the result of more than 40 measurements. Daily averages were 110, 194, 111, 130, 259, and 171 metric tons/day (t/d); the mean for the period was 163 ± 53 t/d (1 sigma). The flux appeared to be small and variable, though less so than at the same time last year (BGVN 20:04). The highest SO₂ flux was associated with mild explosive eruptions. Also evident in the fluxes in some instances were both a strong post-eruption decrease and a possible gradual pre-eruption increase.

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: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Hazel Rymer and Mark Davies, Dept. of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom; John Stix, Dora Knez, Glyn Williams-Jones, and Alexandre Beaulieu, Dept. de Geologie, Universite de Montreal, Montreal, Quebec, H3C 3J7, Canada; Nicki Stevens, Dept. of Geography, University of Reading, Reading RG2 2AB, United Kingdom; Gerardo J. Soto, Oficina de Sismología y Vulcanología, Departamento de Geología, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.