Poás

Poás Volcano (Volcán Poás) is one of the most visited and prominent volcanoes in Costa Rica. Located in the Central Valley, just a short drive from the capital city of San José, it is the centerpiece of Poás Volcano National Park (Parque Nacional Volcán Poás). Standing at 2,708 meters (8,885 feet), this active stratovolcano is famous for its massive summit crater, which stretches over 1.6 kilometers (1 mile) in diameter and hosts a stunning, turquoise-colored acid lake.

The volcano is a powerful symbol of Costa Rica’s geothermal forces. Its accessibility allows visitors to stare directly into the heart of an active volcano, observing the steaming fumaroles and geysers that constantly reshape the crater floor. However, its beauty masks a volatile nature; frequent phreatic (steam-driven) eruptions often lead to temporary park closures to ensure visitor safety.

Geological Origins and Structure

Poás is a basaltic-andesite stratovolcano that is part of the Central Volcanic Range (Cordillera Central). It has been built up over the last 11 million years, but the current cone is relatively young, formed within the last 600,000 years.

The Craters

The summit area consists of three distinct craters:

1. Main Crater (Crater Principal): The active center of the volcano. It is 300 meters deep and contains the Laguna Caliente (“Hot Lagoon”).
2. Von Frantzius Crater: An older, extinct crater located to the north.
3. Botos Crater (Laguna Botos): A dormant crater filled with a cold, clear freshwater lake (Laguna Botos) surrounded by lush cloud forest. This lake drains into the Sarapiquí River and eventually the Caribbean Sea.

Laguna Caliente: An Acid Bath

The lake in the main crater is one of the most acidic natural bodies of water on Earth, with a pH often approaching 0 (similar to battery acid). The color of the lake fluctuates between turquoise, emerald green, and grey, depending on volcanic activity and mineral concentration. The water is heated by magma just beneath the surface, maintaining temperatures between 20°C and 50°C, though it can reach near-boiling points during periods of unrest.

Eruptive History

Poás has a long history of activity, with 39 recorded eruptive episodes since 1828. Most of these are phreatic eruptions, caused when groundwater comes into contact with hot rock or magma, flashing to steam and exploding violently.

The 1910 Eruption

One of the most notable historical eruptions occurred in 1910, when a massive column of steam and ash rose over 8,000 meters (26,000 feet) into the air.

2017: A New Cycle of Activity

In April 2017, Poás entered a vigorous eruptive phase. Large phreatic explosions destroyed the webcams and monitoring equipment inside the crater and damaged visitor infrastructure. This activity forced the national park to close for nearly 16 months. A new lava dome began to grow, and the crater lake evaporated significantly.

Recent Activity (2019–2024)

Since reopening in August 2018, the volcano has remained restless. Frequent small-scale ash and gas emissions continue. The park has implemented a new safety system, including gas sensors and concrete shelters, to protect tourists from sudden bursts of activity.

Flora and Fauna: The Cloud Forest

The slopes of Poás are covered in dense cloud forest and montane rainforest, creating a stark contrast with the barren, scarred landscape of the active crater. The high acidity of the volcanic rain (acid rain) near the summit has stunted the vegetation in the immediate vicinity, creating a unique “elfin forest” of gnarled, bonsai-like shrubs.

Plant Life

• Poor Man’s Umbrella (Gunnera insignis): Massive, broad-leafed plants that look like prehistoric rhubarb are common along the trails.
• Epiphytes: The trees are draped in mosses, bromeliads, orchids, and ferns that thrive in the constant mist.
• Myrtle and Oak: At lower elevations, magnificent stands of oak and myrtle trees dominate the canopy.

Wildlife

Despite the volcanic activity, the park is a haven for wildlife.

• Birds: It is home to the Resplendent Quetzal, the fiery-throated hummingbird, the clay-colored thrush (Costa Rica’s national bird), and the toucanet.
• Mammals: Visitors might spot coyotes, rabbits, armadillos, and the elusive weasel.

Extremophile Life: The Acid Survivors

Laguna Caliente is a proxy for Mars.

• Bacteria: Despite the pH of nearly zero, scientists have found Acidiphilium bacteria thriving in the lake. They feed on the sulfur and iron in the water.
• Astrobiology: NASA researchers study Poás to understand how life might exist in the hydrothermal systems of Mars or the acidic oceans of ancient Earth.

The Coffee Connection

Poás is not just a destroyer; it is a creator.

• Fertile Soil: The Central Valley’s coffee plantations, some of the best in the world, owe their success to the nutrient-rich volcanic ash (andisols) deposited by Poás over millennia.
• High Altitude: The volcano’s elevation provides the perfect cool climate for growing “Strictly Hard Bean” Arabica coffee, prized for its complex flavor profiles.

Legend of the Rualdo Bird

A lesser-known indigenous legend tells of the Rualdo.

• The Sacrifice: The Poás god demanded a human sacrifice to stop an eruption. A young maiden offered herself, but a Rualdo bird (a golden bird) flew into the crater instead, singing a song so beautiful that the volcano wept, filling the crater with tears (the lake) and sparing the girl.
• The Bird: Today, the Rualdo is a symbol of hope, and birdwatchers still scan the cloud forest in search of it.

The 2009 Earthquake

The volcano is also a tectonic trigger.

• Cinchona Earthquake: In 2009, a magnitude 6.1 earthquake struck the eastern flank of Poás (near Cinchona). It was likely triggered by stress adjustments in the volcanic system. It caused massive landslides that reshaped the geography of the park’s entrance and tragically killed 40 people.

Tourism and Visiting

Poás is one of the most accessible volcanoes in the world, making it a top destination for tourists.

The Crater Viewpoint

A paved path leads from the visitor center to the rim of the main crater. On a clear day, the view is breathtaking, revealing the steaming acid lake and the striated walls of the crater. However, the crater is often shrouded in mist (cloud cover), so early morning visits are strongly recommended for the best chance of visibility.

Laguna Botos Trail

A hiking trail leads from the main crater through the cloud forest to Laguna Botos. This ancient crater lake provides a peaceful counterpoint to the active main crater. The hike offers excellent opportunities for birdwatching and experiencing the lush biodiversity of the cloud forest.

The Blue Waterfall (Catarata del Río Agrio)

The water that drains from the volcano tells a story.

• Acid River: The Rio Agrio (Sour River) is so acidic that no fish can live in it.
• The Waterfall: However, the high mineral content gives the water a stunning electric blue color. The waterfall, located just outside the park, is a hidden gem for photographers.

Agro-Tourism: Strawberries and Cheese

The drive up to Poás is an attraction in itself.

• The “Little Switzerland” of Costa Rica: The road from Alajuela winds through green pastures and strawberry fields.
• Local Delicacies: Visitors typically stop at roadside stands to buy fresh strawberries (often served with condensed milk), palmito cheese, and sour cream (natilla). It is a culinary tradition that supports the local economy when the volcano acts up and closes the park.

Conservation Challenges

Climate change is affecting the cloud forest.

• Lifting Cloud Base: As temperatures rise, the cloud formation level is moving higher up the mountain.
• Impact: This threatens the epiphytes and amphibians that rely on the constant mist. Park rangers are now monitoring species migration as the ecosystem tries to adapt to a drying climate.

Accessibility for All

Poás is a leader in inclusive tourism.

• Wheelchair Access: It is the first national park in Costa Rica to be fully wheelchair accessible. The paved trails allow visitors with limited mobility to reach the crater rim and witness the geological spectacle.
• Signage: The park features Braille signage and sensory stations, ensuring that the volcano can be experienced by everyone.

Safety Regulations

Due to the ongoing volcanic hazard:

• Time Limits: Visits to the crater observation deck are typically limited to 20 minutes to reduce exposure to volcanic gases (sulfur dioxide).
• Hard Hats: In times of elevated activity, visitors may be required to wear safety helmets.
• Online Booking: Tickets must often be purchased in advance online to manage visitor numbers.

Monitoring

Poás is closely monitored by OVSICORI (Observatorio Vulcanológico y Sismológico de Costa Rica). A network of seismographs, gas spectrometers, and GPS stations tracks the volcano’s “pulse.” The high concentrations of sulfur dioxide and carbon dioxide emitted by the lake are constantly measured to ensure visitor safety.

Technical Facts

• Elevation: 2,708 m (8,885 ft)
• Crater Diameter: ~1.3 km (Main Crater)
• Location: 10.20°N 84.23°W
• Volcano Type: Stratovolcano
• Rock Type: Basalt / Andesite
• Status: Active
• Key Feature: Ultra-acidic Crater Lake (Laguna Caliente)