Costa Rica

Arenal Volcano

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Volcanic Activity of Arenal Volcano

[Arenal Volcano]

Prior to 1968

Prior to 1965, no research had been conducted on Arenal volcano, and it was not even mentioned in the Catalogue of Active Volcanoes of the World. However, based on tephrachonology and radiocarbon dating, Arenal has previously erupted in 1750 ± 50 A.D., 1525 ± 20 A.D., 1080 ± 50 A.D., 220 ± 75 B.C., and 900 ± 150 B.C.. Arenal formed from the base of Chato’s edifice, with pyroclastic and lava flows being deposited mainly to the northeast between the Monterrey Hills to the north and Chato to the southeast. To the west, Lago Arenal was formed by the damming effect of the volcanic deposits on the local drainage system. Since the construction of a hydroelectric dam, the area of the lake has increased dramatically to within 10 km of the summit of Arenal.

During and After 1968

Premonitory manifestations of volcanic activity began in 1965 with the release of colourless gas on the northeast flank of Arenal, the drying out of the Cedeño lagoon, formation of a new hot spring, and an increase in the temperature of Rio Tabacón.

On the morning of July 29, 1968, after 10 hours of intense seismic activity, Arenal erupted explosively and continued to erupt over a period of three days, killing 78 people and devastating 12 square kilometres. The activity started with a lethal lateral blast that levelled the densely forested western flank and destroyed the village of Pueblo Nuevo, 6 km west of the summit. The initial blast was followed by Plinian eruption columns, pyroclastic flows, and ballistically ejected blocks and bombs. Three new craters (A, B, C) were formed during this time, with an approximately east-west orientation on the western flank of the volcano. The largest, crater C (1100 m), was the source of all the major explosions. These events were followed by three days of relative calm consisting of minor ash and fumarolic activity. A fumarolic phase began August 10 and continued to September 14. Activity was noted at all of the craters, with the most intense activity at crater C. From September 14 to September 19, renewed explosions consisting of low-energy, low-volume ejection of scoriaceous to pumiceous andesite were observed.

This was followed by a period of lava effusion from crater A (Sept. 19, 1968 to the end of 1973) consisting of blocky lava which descended into the Quebrada Tabacón valley. Another explosive phase started on June 17, 1975, with the emplacement of a blocky ash flow along the Rio Tabacón valley. This deposit resulted from the formation of nuées ardentes produced by avalanching of a lava flow being extruded from crater C. This eruptive phase was followed by upslope migration of crater C to within close proximity of crater D. Strong fumarolic activity and extrusion of blocky aa lava followed, continuing until June 1984. This date marked an increase in activity at Arenal, with the beginning of eruptions with ash and large pyroclastics. This activity has led into a Strombolian eruptive phase which has produced basaltic-andesite tephra and lava and continues to date.