The Arenal Volcano is an active andesitic stratovolcano in northwestern Costa Rica, around 90 km northwest of San José, in the province of Alajuela. Recognized as a volcano since the 19th century, it was known by foreign investigators as "Volcan Costa Rica, Rio Frio," "Canastos" Volcano and "Cerro de los Guatusos." Arenal is the youngest and most active of all the mountains in Costa Rica. Scientists have been able to date it back to more than 4000 years ago. The area remained largely unexplored until 1937, when a documented expedition took place to reach the summit.
Once locally known as Cerro Arenal (i.e. Mount Arenal) it was presumed extinct until July 29, 1968 when an earthquake caused it to erupt, after approximately 400 years of dormancy. The eruption wiped out the town of Arenal and killed 87 people. It lasted for several days, and during that time destroyed crops, property, livestock, and forests. 15 square kilometers were buried and the explosion affected over 232 square kilometers. The eruption caused three new and active craters to form. Before the eruption, there was a wide veriety of plant and wildlife on the mountain. It has been active since the explosion and can be reported to have minor eruptions every 5-10 minutes.
Arenal rises 1657 meters above sea level and overlooks Lake Arenal; both are part of Arenal Volcano National Park. The country's most active volcano, Arenal's eruptions are strombolian in type, being frequent but moderate and can be viewed without danger as long as the viewers are behind the safety perimeter. This constant activity and minimal endangerment makes Arenal a popular tourist destination, especially at night because the lava and incandescent rocks make a spectacular sight. The last major eruption of Arenal occurred on August 23, 2000. Most of the time, the volcano is surrounded by clouds near the tip, and it is touted as good luck to see the whole volcano without clouds. The volcano heats several hot springs nearby, including Tabacon, a popular tourist destination.
Arenal is one of seven historically active Costa Rican volcanoes, including Poás, Irazú, Miravalles, Orosí, Rincón de la Vieja, and Turrialba.
Information based on http://en.wikipedia.org/wiki/Arenal