Volcán Arenal is the youngest and most active stratovolcano – the term for a steep, conical volcano created by the eruption of thick lava flows – in Costa Rica. Geologists have determined that Arenal is no more than 2900 years old; by comparison, Cerro Chato, which flanks Arenal to the south, last erupted in the late Holocene period, around 10,000 years ago. Geologists speculate that Arenal is so active because it directly taps a magma chamber located on a fault about 22km below the surface.
Arenal’s growth over the ages has been characterized by massive eruptions every few centuries: it is thought to have erupted around 1750, 1525 and 1080 AD, and 220 and 900 BC. At the time of its most recent eruption in the late 1960s, Arenal seemed to be nothing but an unthreatening mountain, and locals had built small farms up its forested sides (take a look at Cerro Chato and you get the idea). But on July 29, 1968, an earthquake shook the area, blasting the top off Arenal and creating the majestic, lethal volcano seen today. Arenal killed 78 people that day, with fatalities caused by a combination of shockwaves, hot rocks and poisonous gases. The explosion created three craters, and Arenal has been active ever since, with almost daily rumblings and shakings.
While history would suggest that it’s not due another major blowout for a few hundred years yet, Arenal is still very much an active volcano, so a few safety tips are worth bearing in mind: never veer from trails or guided tours, and do not attempt to hike anywhere near the crater, since lethal gases, ballistic boulders and molten rock, all of which occur regularly, can appear or change direction without warning. Indeed, technically everything between the volcano and the roads that run from La Fortuna to Laguna de Arenal and El Castillo lie in a high-risk area – a guide and a young girl were killed by a pyroclastic flow while walking “safe” trails (now closed) in the Los Lagos complex in August 2000 – so choose your trip with care.