During July and August the lava flow on Arenal's N flank continued to extrude, descending to 760-800 m elevation. Near the end of August a second lobe running just W of the previous one became well developed. Blocks falling off the older flow's front continued to tumble E, burning vegetated areas. The small spatter cone near the vent (BGVN 21:06) disappeared.

Average daily ashfall 1.8 km W of the active vents (table 17) was higher during May and June than earlier in the year (>50 grams/m²), but then decreased somwhat in July.

Table 17. 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.

Eruptive activity during July appeared to decrease, but then in August appeared to again increase in terms of the amount and size of pyroclastic materials ejected. Some August eruptions vibrated the glass in houses 6.5 km E of the volcano and plumes from some of these eruptions rose up to 1 km above Crater C. On the morning of 29 August the volcano generated a small pyroclastic flow that traveled down the SE flank. During July, acidic rainfall most affected the NW, W, and SW flanks; during August, it affected the NE, E, and SE flanks. These acidic rains scalded the leaves of some plants.

Low-frequency earthquakes (table 18 and figure 79) often coincided with intervals of Strombolian eruptions. The days with the greatest numbers of eruptions were in the middle of the month; for example, 12, 14, and 18 July eruptions totalled 52, 55, and 59, respectively. July tremor (table 18 and figure 79), which was in the 2.1-3.4 Hz frequency range, took place for a total of 345 hours. For comparison, there were 420 hours in January 1996, the longest duration seen in the past two years; in 1994 many months had less than 100 hours of tremor. On seven days during July, tremor had durations of 15-18 hours; only one day had a duration of less than 5 hours.

Table 18. Average seismicity at Arenal, March-August 1996, as recorded 3.5 km E of the active vent (Fortuna station). Courtesy of ICE.

Figure 79. Arenal seismicity for 1994-August 1996 as registered at station VACR, 2.7 km NE of the active crater. There was no data in March 1994; five other months (June and December 1994 and December, February, and March 1995) had incomplete data. Consequently, the sums for these six months were scaled up and plotted assuming the rate of generated events during those months remained constant. Courtesy of OVSICORI-UNA.

Although prior to 1995 measurements along the distance network generally indicated deflation, from the end of 1995 through an unspecified date in 1996 surveys to two stations found 7 µrad of accumulated inflation. Other stations remained relatively stable. During July 1996 a continued contraction was seen amounting to 22 ppm/year on average.

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. Fernández, E. Duarte, V. Barboza, R. Van der Laat, E. Hernandez, M. Martinez, and R. Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; G.J. Soto and J.F. Arias, Oficina de Sismología y Vulcanología del Arenal y Miravalles: OSIVAM, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.