Continuing activity in September consisted of Strombolian eruptions and lava output from Crater C and fumarolic activity from Crater D. Two lobes of lava continued to progress toward the Tabacón valley (figure 70). ICE workers suggested that at elevations below 800 m the estimated velocities of lava flows have averaged roughly 2.5 m/day. In some of the steeper upslope reaches flows may have averaged as much as roughly 50 m/day, but velocities were more typically 10-20 m/day. These values are approximate, because field work is hampered by hazards associated with sudden collapse of lava-flow fronts.

Figure 70. Rough field sketch of Arenal, from the ash-sampling locality mentioned in the text (1.8 km W of the summit); view is toward the E. Courtesy of G. Soto, ICE.

In the summit crater, vents active for the past several months had built two small cones. The northernmost cone extruded lava during the past several months. The southerly cone appears to be mainly composed of pyroclastic materials. Toward the crater's center there was a third vent. Summit fumaroles remained vigorous and occasional explosions took place (table 6); at night a red glow still prevailed over the crater area suggesting ponded lava remains molten there. Seismicity reported by ICE appears in table 7; their mid-October sampling found that both pH values and water temperatures remained unchanged.

Table 6. Ash collected downwind at a spot 1.8 km W of Arenal's crater. "Collection Interval" refers to the time period in 1994 when the ash sample accumulated (also shown as "Days," the number of days), but the mass/area value is a computed daily average. Courtesy of G. Soto, ICE.

Table 7. Number of seismic events and tremor duration at Arenal. October values are extrapolated from 20 days of observations. Courtesy of ICE.

OVSICORI-UNA reported that September seismic events often accompanied gas- and ash-bearing eruptions. During September seismic events in the frequency range 1.2-2.5 Hz totaled 657; tremor duration totaled 55 hours.

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, and V. Barboza, OVSICORI; G. Soto and F. Arias, ICE; M. Mora, Univ de Costa Rica.