The volcano was asleep for hundreds of years and exhibited a single crater at its summit, with minor fumaroles activity, covered by dense vegetation. In 1968 it had an eruption and destroyed the small town of Tabacón. Due to the eruption three more craters were created on the western flanks but only one of them still exists today.

Arenal is Costa Rica's most active volcano, and one of the ten most active volcanoes in the world. It has been studied by seismologists for many years. Although the activity nowadays is less damaging, you can still see ash columns, underground rumbling, glowing orange lava flows and sudden explosions on a daily basis. With its perfect conical shape it allows breathtaking views of volcanic action in the summit. Almost every night the crater offers a spectacle of lights with its explosive eruptions.

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. The temperature varies from 16 °C (61 °F) in the high areas to 24 °C (75 °F) in the low areas and the annual rainfall is around 500 centimetres (200 in).[citation needed]

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.[citation needed] Chato is a dormant stratovolcanic cone. It is believed Chato first erupted 38,000 years ago during the Pleistocene period and last erupted about 3,500 years ago. Chatito 10°26′17″N 84°41′13″W / 10.438°N 84.687°W / 10.438; -84.687 (Chatito volcano) is a lava dome with an elevation of 1,100 metres (3,609 ft). Espina is another lava dome.

Arenal is the youngest and most active of all the mountains in Costa Rica. Scientists have been able to date its activity back to more than 7000 years ago. The area remained largely unexplored until 1937, when a documented expedition took place to reach the summit.[citation needed] It has been considered eruptive since 1968.

At about 7:30 a.m. a powerful eruption took place and destroyed the west flank of the volcano. Three additional craters were formed. The town of Tabacón was buried, killing seventy eight people.

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 miles).

After a period of high effusive activity of intermittent lava flows, a new explosive phase began with 3 to 20 explosions per day of low to moderate magnitude. These explosions of steam, water, gases and ash reached altitudes of up to 5 kilometers where winds again carried the material across the Arenal reservoir (Lake Arenal) and over the town of Tilarán.

A northwest wall of one of the craters collapsed and generated several pyroclastic flows. The collapsed crater wall was shaped as a V, about 60 meters deep and 100 meters wide. In this V-shaped crater the lava started to flow again.