Info: The conical Volcano Arenal is the youngest volcano in Costa Rica and the most active. It is a stratovolcano (also known as composite volcano) like Mt. Fuji in Japan, Mt. Mayon in the Philippines, and Mt. Aqua in Guatemala. The 1657 meters (5,437 Feet, 1 meter = 3.28 feet) high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal was constructed by successive eruptions to the NW (10.463°N (10°27'48"N), 84.703°W (84°42'12"W)) of the older Chato volcano, which contains a 500 meters wide summit crater. The earliest known eruptions of Arenal took place about 7,000 years ago, and the two volcanoes were active concurrently until the activity of Chato ended about 3,500 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. Volcan Arenal actual eruptive period began with a major explosive Arenal eruption in 1968. Continuous daily explosive eruption activity accompanied by slow lava effusion and the occasional emission of pyro-clastic flows (avalanche of hot gases, rocks and ashes that can travel @ 80 km per hour) has occurred since then from vents at the summit and on the upper western flank. Arenal eruptions are of the strombolian type (name after the volcano Stromboli in Italy: A type of volcanic activity which produces frequent, moderate eruptions.
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I took this photo of the Arenal Volcano erupting the first evening as we drove into the hotel. We thought at first it was just clouds but the gate man said as we entered, "The volcano is erupting again!" So we of course, agreed!