Arenal Volcano (Spanish: Volcán Arenal) is an active andesitic stratovolcano in north-western Costa Rica around 90 km (56 mi) northwest of San José, in the province of Alajuela, canton of San Carlos, and district of La Fortuna. The Arenal volcano measures at least 1,633 metres (5,358 ft) high. It is conically shaped with a crater 140 metres (460 ft) in diameter. Geologically, Arenal is considered a young volcano and it is estimated to be less than 7,500 years old. It is also known as "Pan de Azúcar", "Canaste", "Volcan Costa Rica", "Volcan Río Frío" or "Guatusos Peak".

The volcano was dormant for hundreds of years and exhibited 2 craters at its summit, with minor fumaroles activity, covered by dense vegetation. In 1968 it erupted unexpectedly, destroying the small town of Tabacón. Due to the eruption three more craters were created on the western flanks but only one of them exists today. Arenal's eruption from 1968 to 2010 is the tenth longest duration volcanic eruption on Earth since 1750. Since 2010, Arenal has been dormant.

Arenal is one of seven historically active Costa Rican volcanoes along with Poás, Irazú, Miravalles, Orosí, Rincón de la Vieja complex, and Turrialba. It was Costa Rica's most active volcano until 2010, and one of the ten most active volcanoes in the world. It has been studied by seismologists for many years. The volcano is located at the center of Arenal Volcano National Park in the northern zone of the country, 15 kilometres (9.3 mi) southwest of the La Fortuna district in San Carlos (canton), Costa Rica.

Arenal Volcano area is an important watershed for the Arenal Lake Reservoir. The reservoir's water is used for hydroelectric power. It is also connected to the national system.

Arenal has several eruptive vents. Chato is a dormant stratovolcanic cone. It is believed that Chato first erupted 38,000 years ago during the Pleistocene period and last erupted about 3,500 years ago. Chatito is a lava dome with an elevation of 1,100 metres (3,609 ft). Espina is another lava dome.

Arenal is the youngest of all the volcanoes in Costa Rica. Scientists have been able to date its activity back more than 7000 years. The area remained largely unexplored until 1937, when a documented expedition took place to reach the summit. It was eruptive from 1968 until 2010.

On Monday, July 29, 1968, at 7:30 am, the Arenal Volcano suddenly and violently erupted. The eruptions continued unabated for several days, burying over 15 square kilometers (5.8 sq mi) under rocks, lava and ash. When it was finally over, the eruptions had killed 87 people and buried 3 small villages – Tabacón, Pueblo Nuevo and San Luís – and affected more than 232 square kilometers (90 sq mi) of land. Crops were spoiled, property was ruined, and large numbers of livestock were killed.

At the height of its ferocious activity, the volcano flung giant rocks – some weighing several tons – more than a kilometer (half a mile) away at a rate of 600 meters per second (2,000 ft/s; 1,300 mph). These explosions would go on to form three new active craters.

As the three towns were destroyed on the western side of the volcano, a town by the name of El Borio on the east side was untouched and unharmed. It is a popular myth that after the volcano ceased to erupt, El Borio was renamed La Fortuna, which means "The Fortune", referring to its luckiness that the volcano erupted to the west and not the east. However, the town was renamed La Fortuna by its residents before the 1968 eruption. "The Fortune" refers to the flat, fertile lands in the area, which are unlike the rough, mountainous terrain which surrounds most of the Arenal volcano.

Between the 17th and the 21st of June 1975 several avalanches went down from one of the craters. The vegetation along Tabacon River was destroyed and a great amount of material was deposited on the riverbed. Four strong explosions also blew large amounts of ash into the sky. The ash was spread within a distance of 26 kilometers (16 mi).

At this time the volcano started to produce regular lava flows, accompanied by intermittent explosions of gases. This was the regular activity of the volcano until May 5, 1998.

The Arenal Volcano experienced a series of large eruptions on Tuesday afternoon, May 5, 1998. The first ratchet eruption was recorded at 1:05 p.m. when part of the northwest wall of the crater fell apart. Large amounts of lava, rocks, and ash flew out of the volcano during this explosion. Another eruption took place at 2:20 p.m. with material emerging from the same part of the volcano.

A specialist from the Costa Rica Volcanic and Seismic Observatory explained that the eruptions are nothing unusual for the volcano. Nevertheless, during this occasion, the amount of lava within the crater was significantly greater than normal and therefore more material was poured out. This time a landslide took place, too, as a part of the crater wall falling apart on the northwest side. This phenomenon occurs sporadically, although this time the consequences were greater than usual.

As a normal precaution, authorities declared a red alert, closed the road between La Fortuna and Tilarán, which runs around the north side of the volcano, and evacuated approximately 450 people (mostly tourists) from the immediate area, including several hotels and tourism-oriented businesses. There were no reports of injuries caused by the volcanic activity.

At 5:20 p.m. on Tuesday the volcano was still discharging material, but activity had decreased significantly.

The eruptions of May 7, 1998, damaged 2 square kilometers (0.77 sq mi) and destroyed a 400-by-100-meter (1,310 by 330 ft) area of green forest in the vicinity of Arenal Volcano. A fissure, 500 meters (1,600 ft) long and 10 meters (33 ft) deep, was opened up in the wall of the crater and all the material slid down the side of the volcano.

During this day 23 eruptions were reported, between 1:05 p.m. and 7:00 p.m., and thereafter the volcano returned to its normal state. Authorities reported no unusual behavior and the national park was reopened the same week. Local seismologists investigate the activity of the volcano and park rangers continue to vigorously enforce the safety perimeter.

Last eruption reported, presence of fumeroles at the summit, under Green Alert (calm status) by local authorities.

• List of volcanoes in Costa Rica
• Lake Arenal
• Arenal Volcano National Park
• Arenal Prehistory Project

• MTU Volcanoes Page: Arenal
• Updated (and detailed) information and images about Arenal and its recent activity
• Most recent pictures of Arenal eruptions
• Jorque Barquero's Personal Account of the 1968 Eruption
• Fortuna (Costa Rica) travel guide from Wikivoyage