Arenal Volcano is an imposing, conically shaped stratovolcano located in northwestern Costa Rica, towering 1,633 meters above sea level near the town of La Fortuna. This iconic peak is one of the country’s most geologically young volcanoes, estimated to be less than 7,500 years old. The period of continuous, significant activity that defined its modern history officially concluded in October 2010.
The End of the Eruptive Cycle
The active phase of the Arenal Volcano began with a massive explosion in 1968 and continued for 42 consecutive years, making it one of the longest volcanic eruptions recorded since 1750. This long-term activity involved frequent lava flows, ash plumes, and incandescent rock avalanches. These events gradually decreased in intensity, and the last recorded lava flow occurred in late 2010, signaling the end of the effusive eruption cycle.
The volcano did not become extinct, but rather entered a “resting phase,” a natural process for many active volcanoes after prolonged activity. This cessation of visible, large-scale eruptive events marked a major shift from the nightly glow and audible rumbling that had been a fixture for decades. By the end of 2010, all significant activity, including the extrusion of new lava and major ash emissions, had stopped.
Arenal’s Defining 1968 Eruption
While the continuous eruption cycle concluded in 2010, the most destructive event in Arenal’s recent history occurred suddenly on July 29, 1968. Before this date, the peak had been dormant for centuries, appearing as a forested mountain locally referred to as Cerro Arenal. The eruption was completely unexpected and violent, shattering the tranquility of the region.
The initial explosion killed 87 people and completely buried three small villages—Tabacón, Pueblo Nuevo, and San Luís—under layers of ash, rock, and lava. Pyroclastic flows swept across the western flank, and the force of the blast flung giant incandescent boulders, some weighing several tons, over a kilometer away. The sudden eruption permanently altered the volcano’s structure, blowing out a new crater on the western flank and creating two others. This event transformed the landscape and began the 42-year period of continuous activity that followed.
Current Status and Geologic Monitoring
Arenal is currently classified as dormant, or resting, but is not considered extinct due to its geological youth and history of activity. Though the dramatic lava flows have stopped, the volcano still exhibits geothermal activity beneath its surface, which contributes to the region’s famous hot springs. This geological potential requires continuous, close monitoring by scientific bodies, primarily the Volcanological and Seismological Observatory of Costa Rica (OVSICORI).
Current monitoring techniques include measuring seismic activity, which has remained minimal since the resting phase began. Scientists use Electronic Distance Measurement (EDM) to detect any ground deformation, which would indicate magma movement below the surface. Thermal imaging and gas emission analysis are also performed, with observations noting a gradual reduction in the acidity of rainwater, an indication of reduced magmatic gas output. While the risk of a major eruption is low, the monitoring ensures that any subtle change in internal pressure or activity would be detected quickly, providing a layer of safety for the surrounding communities.