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Report on Arenal (Costa Rica) — January 1990


Bulletin of the Global Volcanism Network, vol. 15, no. 1 (January 1990)
Managing Editor: Lindsay McClelland.

Arenal (Costa Rica) Continuous tephra emission; blocks and bombs to 1,200 m from the crater; small nuées ardentes; flank lava flows

Please cite this report as:

Global Volcanism Program, 1990. Report on Arenal (Costa Rica) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 15:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199001-345033


Costa Rica

10.463°N, 84.703°W; summit elev. 1670 m

All times are local (unless otherwise noted)

Continuous gas and tephra emission was accompanied by sporadic Strombolian eruptions. Blocks and bombs fell as much as 1,200 m from the active crater (C) to 880 m elevation. The activity occasionally produced small nuées ardentes, such as one observed on 9 November at 0230 that traveled some 400 m down the S flank from Crater C. Lava flows moved down the NE, N, and NW flanks. Some of the flows covered and burned vegetation on parts of the NE and N flanks. Fumarolic activity continued from the summit crater (D).

Winds carried ash columns toward the NW, W, and SW flanks, the area most affected by gases. During November, the mean pH of rain in this area was 3.8. Vegetation on the E flank continued to be affected by gases, acid rain, and falling blocks. The combination of vegetation damage, steep slopes, poorly consolidated material, and heavy precipitation has caused strong erosion, producing small cold avalanches in the canyons Calle de Arenas, Guillermina, and Río Agua Caliente.

During observations by geologists 9-10 January, loud explosions (Type 1 in 14:06) were heard, on average, about every 1.5 hours. Ash columns associated with five of the explosions were seen through breaks in the cloud cover. The loudest explosion during the period, on 10 January at 1720, was associated with the largest observed ash column, which rose ~1 km and was videotaped from the N side of Lake Arenal, ~20 km from the volcano. On 9 January at 2240, another Type 1 eruption ejected incandescent blocks and bombs onto the upper 2/3 of the N flank. A lava flow from the summit area descended the N flank, spalling blocks from its surface and levees.

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, J. Barquero, and J. Bravo, OVSICORI; S. Halsor, Wilkes Univ, USA; C. Chesner, Eastern Illinois Univ, USA.