The Poás Volcano, a prominent stratovolcano located in Costa Rica’s Central Volcanic Range, is definitively classified as active. Its continuous, monitored activity makes it one of the most closely watched volcanoes in the region. The current activity is primarily focused on its main crater, which hosts an extremely acidic lake.
What Does “Active” Mean for a Stratovolcano?
For scientists, a volcano is classified as active if it is currently erupting or shows signs of unrest, such as significant seismic activity or gas emissions. Poás fits this description because it exhibits continuous degassing and frequent low-level seismic tremors, indicating magma movement beneath the surface. This differs from a dormant volcano, which is expected to erupt again, or an extinct volcano, which scientists believe will never erupt again.
The activity at Poás is mostly characterized by phreatic eruptions, which are steam-driven explosions that occur when superheated water flashes to steam. These events are distinct from magmatic eruptions, where molten rock is directly ejected from the vent. While Poás has experienced both, its frequent, smaller explosions are a product of the interaction between the hot magmatic system and the surface water.
The Unique Hydrothermal System and Crater Lake Activity
The core of Poás’s activity lies in its volatile hydrothermal system, centered around its northern crater lake, Laguna Caliente. This lake is renowned as one of the world’s most acidic natural bodies of water, with a pH often measured near zero. The extreme chemistry is a direct result of magmatic gases, particularly sulfur dioxide and chlorine, dissolving into the lake water.
This superheated reservoir acts as a pressure cooker, sitting directly atop the shallow magma body. When the water level is high, it prevents gases from escaping freely, forcing them to accumulate until a critical point is reached. The pressure buildup leads to the sudden, explosive ejection of water, mud, and rock fragments. Even when not erupting, the system generates continuous, powerful plumes of gas that rise hundreds of meters above the crater.
Tracking Poás: Historical Eruptions and Current Monitoring Methods
Poás has a long history of documented unrest. The most intense recent episode occurred in 2017, when powerful eruptions forced the closure of the national park for nearly 17 months. Analysis of the ejected material showed fresh magmatic particles, suggesting the underlying magma was ascending and interacting directly with the hydrothermal system. Scientists continuously track the volcano’s internal state using a suite of sophisticated monitoring techniques to assess the risk of such changes.
Seismic Monitoring
Seismic monitoring involves a network of seismographs that record volcanic tremor, a continuous, low-level vibration caused by the movement of magma and fluids through underground conduits. High-frequency earthquakes also signal rock fracturing due to rising pressure.
Gas Analysis
Gas analysis focuses on measuring the output and ratios of gases like sulfur dioxide (SO2) and carbon dioxide (CO2). An increase in the SO2/CO2 ratio often suggests that magma is moving closer to the surface, as sulfur is typically released at higher temperatures than carbon dioxide.
Ground Deformation
Ground deformation is monitored using GPS and tiltmeters (GNSS stations) to detect subtle swelling or uplift of the volcano’s flanks. Vertical uplift, sometimes measured in just centimeters, signals an inflation of the underlying magma reservoir, providing tangible evidence of the active pressure building within the stratovolcano.