The conical Volcan Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1657 meters (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, 84.703°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 most recent eruptive period began with a major explosive eruption in 1968. Continuous explosive eruption activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows (avalanche of hot gases, rocks and ashes) has occurred since then from vents at the summit and on the upper western flank. It's eruptions are of the strombolian type (name after the volcano Stromboli in Italy: A type of volcanic activity which produces frequent, moderate eruptions).

All hotels in the Arenal Costa Rica area are at a safe distance of the volcano, but hiking on the volcano beyond the warning signs is extremely dangerous. The toxic hot gases released during an eruption are traveling very fast, faster than you can run. The volcan Arenal is active everyday since 1968, but major eruptions forcing an evacuation is not common. The last one occurred September 5th. 2003.