Lava production, explosive activity, and gas emission continued from Crater C. The lava flow that had been descending the E and ESE flanks since mid-January has stopped, with its front at 610 m elevation. The lava channel remained obstructed near 1,100 m elevation. Levees had been pushed away and broken on both sides of the channel. Residents of the area reported that some of the resulting block overflows caused small pyroclastic flows that reached ~800 m elevation, producing ash columns several hundred meters high. At the beginning of October, a new lava flow began to descend to the SW, using the same channel to 1,200 m elevation, where it bifurcated. One lobe (to the W) reached 950 m elevation, the other (to the SW) extended to 1,000 m elevation. The lava flow that had been moving SW in September, covering a grassy area, stopped at 700 m elevation.

Intermittent Strombolian eruptions continued, some producing ash columns that rose more than a kilometer above Crater C. Activity was vigorous during the first half of the month, when ashfall on a fixed sampling point 1.8 km W of the active crater (at 735 m elevation) was at the highest rate of the year (see table 5). Blocks and bombs fell to 1,000 m elevation. The explosions vibrated windows in [La Fortuna]. Pyroclastic flows were occasionally generated, and probably produced the pyroclastic-flow deposits found on the W, SW, and S flanks.

Seismicity recorded at a station (Fortuna) 4 km E of the active crater showed a moderate increase during the first 2 weeks of the month, coinciding with the vigorous explosive activity. The 80 events recorded on 9 October were the most on a single day in 1992. Fewer earthquakes occurred during the second half of October, with a mean of 25 events daily and a maximum of 46 (on the 31st). High-energy tremor accompanied the increased activity, especially during the first and third weeks of the month.

Floods during the intense rainy period in September and October caused some small erosive changes in the depositional fans of the Chato and Calle de Arena rivers (E of the volcano), carried sand into Laguna Cedeño (N side), and deposited sand and blocks in the Agua Caliente river (to the SW).

Species repopulating the lava flows and the devastated area continued to be affected by acid rain. Some plants had burns on their edges and tips, and others had discolored leaves.

Two persons were injured on the W flank during the last week in September while making separate ascents to the summit. The first was a local guide, overtaken by a landslide of unstable material near Crater D, who received head injuries that required several days of hospitalization. The second was an English tourist, who suffered a fractured tibia and fibula when he fell into a canyon after the collapse of the wall on which he was walking.

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: G. Soto and R. Barquero, ICE; E. Fernández, J. Barquero, and V. Barboza, OVSICORI.