Copahue Volcano: The Acid Waters of the Andes - Hiking & Hot Springs

Copahue (Mapudungun for “Place of Sulfur”) is a unique and restless stratovolcano located directly on the international border between the Biobío Region of Chile and the Neuquén Province of Argentina. Rising to 2,997 meters (9,833 feet), it is famous for hosting one of the most acidic crater lakes in the world and for the extensive geothermal system that surrounds it.

Unlike the sharp, conical volcanoes typical of the Andes, Copahue has a broad, elongated profile with a summit truncated by nine aligned craters. Only the easternmost crater is active, containing a steaming, grey-green lake that serves as a window into the volcano’s magmatic soul.

Geological Features: The Acid Crater Lake

The defining feature of Copahue is its active crater lake.

• Acidity: The water in the crater is a hyper-acidic brine, often having a pH of less than 1 (similar to battery acid). It is rich in dissolved minerals like sulfur, chlorine, and magnesium.
• The Agrio River: The acidic waters from the volcano feed the Rio Agrio (Sour River), which flows down the Argentine side. The river is renowned for its vibrant colors, ranging from deep reds to bright oranges due to the high mineral content.

The Chemistry of Acid: The Agrio River

The most stunning visual feature of the Copahue system is the Rio Agrio.

• The Source: It is fed directly by the leakage from the crater lake.
• The Cascade: As the river flows down the volcano, it cascades over basalt cliffs in spectacular waterfalls (Salto del Agrio).
• The Color: The water is a vivid, unnatural electric blue-green, while the rocks it touches are stained deep orange and red by precipitated iron. It looks like a river on Mars.
• Life in Acid: Amazingly, extremophile bacteria and algae thrive in this river, adapting to a pH that would dissolve metal.

Pehuenche Cosmology

For the Pehuenche (“People of the Pehuén” or Araucaria pine), the volcano is a central spiritual pillar.

• The Legend: Legends speak of a powerful spirit trapped within the mountain (“Pillán”), whose anger turns the waters sour.
• The Pinon: The slopes are covered in ancient Araucaria forests. The Pehuenche harvest the pine nuts (piñones) each autumn. The health of the forest is believed to be linked to the mood of the volcano. Volcanic ash, in moderation, fertilizes the pines; in excess, it kills them.

The Science of Healing: Balneology

The Termas de Copahue aren’t just a spa; they are a medical center.

• Vichy, France vs. Copahue: The waters here are compared to Vichy in France for their mineral complexity.
• The Mud: The grey volcanic mud is “matured” in special ponds. It is rich in sulfur, calcium, and magnesium.
• Treatments: Doctors at the complex prescribe specific waters for specific ailments— “Green Lagoon” water for skin, “Ferruginous” water for anemia, and “Sulfurous” steam for asthma. It is one of the few places in the world where geology is directly prescribed as medicine.

Glaciology: Fire and Ice

Copahue is capped by a permanent ice cap, which leads to dangerous interactions.

• Lahars: When the volcano heats up, it melts the base of the glacier. This can send torrents of water and mud (lahars) down the valleys, threatening the tourist infrastructure.
• Glacial monitoring: Scientists monitor the “snout” of the glaciers. A sudden retreat or fracturing of the ice is often the first sign that the mountain is heating up from the inside.

Eruptive History: A Persistent Threat

Copahue is one of the most active volcanoes in the region, with a history of frequent, low-to-moderate intensity eruptions.

• The 2000 Eruption: A significant eruption occurred in July 2000, ejecting ash and larger ballistic rocks. The crater lake disappeared completely as the water boiled off, only to reform later.
• The 2012-2016 Cycle: The volcano entered a period of unrest in late 2012, prompting red alerts and evacuations on both sides of the border. It produced ash plumes that disrupted flights and dropped ash on nearby communities. The activity continued intermittently until 2016, keeping the region on high alert.

Termas de Copahue: Healing Waters

On the Argentine side, at the foot of the volcano, lies the resort town of Copahue, home to one of the world’s most famous thermal spas.

• Geothermal Medicine: The Termas de Copahue utilize the volcano’s geothermal activity for therapeutic treatments. Visitors come to soak in sulfurous mud baths, green algae ponds, and steaming thermal pools, which are believed to help with skin conditions, arthritis, and respiratory issues.
• Seasonal Town: Due to heavy snowfall in the winter (often burying buildings up to their roofs), the town of Copahue is seasonal, open only from November to April.

The Geothermal Controversy

Copahue sits on a massive heat source, and humans want to tape it.

• Energy Potential: Estimates suggest the Copahue field could generate enough electricity to power the entire Neuquén province.
• The Conflict: However, the best drilling sites are located within the Copahue Provincial Park, a protected area featuring ancient Araucaria forests. Environmentalists argue that industrial drilling would destroy the fragile ecosystem and ruin the pristine nature of the thermal spa. The debate between green energy (geothermal) and conservation continues.

A Volcanic Winter Wonderland

Skiing at Caviahue is unlike skiing anywhere else.

• The Backdrop: You ski down the slopes of the volcano (an extinct side cone) with the smoking active crater looming above you.
• The Trees: The ski runs wind through forests of Araucaria (Monkey Puzzle) trees. These prehistoric giants, covered in snow, look like something out of the Cretaceous period.
• The Hazard: Occasionally, ash from the volcano will fall on the snow, turning the white pistes grey. While gritty, this “volcanic skiing” is a badge of honor for locals.

The 2000 Eruption: A Close Call

The eruption of 2000 was a reminder of the delicate balance.

• The Buildup: The crater lake turned from green to grey, indicating a massive injection of ash.
• The Explosion: Phreatic blasts sent rocks the size of cars flying 7 kilometers from the vent.
• The Spa: The Termas de Copahue had to be evacuated in the middle of the winter season. The ashfall was so acidic it damaged the paint on cars in the village. It proved that the “healing” volcano can turn hostile in a heartbeat.

Tourism and Adventure

Copahue offers year-round adventure, though access depends heavily on the season.

Climbing the Volcano

Ascending Copahue is a popular activity in the summer months.

• From Argentina: The hike starts from the thermal resort. It is a steady climb through rocky terrain, passing the “Las Mellizas” (The Twins) lagoons. As you near the top, the smell of sulfur becomes intense.
• From Chile: The approach is often from the Ralco National Reserve.
• The Summit View: Standing on the crater rim is an intense experience. You look down into the steaming, churning acid lake, while the panoramic view extends across the Andes to the distinct cone of Volcán Antuco and the vast Patagonian steppe.

Winter Sports

In winter, the nearby Argentine town of Caviahue (located on the shores of Lake Caviahue) becomes a ski resort. The volcano provides a stunning backdrop for skiing and snowboarding, and snowmobiles are often used to access the hot springs higher up the mountain.

Indigenous Significance

For the indigenous Pehuenche people, who inhabit the forests of Araucaria (Monkey Puzzle trees) on the volcano’s slopes, Copahue is a place of spiritual power. The name itself reflects its nature—sulfur waters were traditionally avoided or used with great caution and respect.

Conclusion

Copahue is a volcano where the Earth’s chemistry is on full display. It is a place of stark contrasts: between the healing powers of its thermal waters and the destructive potential of its eruptions; between the frozen white of winter snow and the burning yellow of elemental sulfur. It remains a shared giant, linking Chile and Argentina in a bond of fire and water.

A Martian Analog

Copahue is of great interest to astrobiologists.

• Early Mars: The combination of acidic water, sulfur-rich rocks, and hydrothermal heat is widely considered to be a perfect analog for the environment of early Mars.
• The Studies: NASA and ESA researchers study the bacteria in the Rio Agrio to understand what kind of life might have once existed in the Martian hot springs of Gusev Crater.

The Science of Mud (Pelotherapy)

The mud at Copahue is more than just dirt.

• Maturation: The grey mud is harvested and then “matured” in special ponds for months.
• Microbiome: During this time, thermophilic bacteria colonize the mud, producing enzymes and sulfolipids that are anti-inflammatory. It is a living medicine, created by the volcano and refined by biology.

Volcano Diplomacy

Copahue is a binational challenge.

• Variable Alerts: Sometimes Chile raises the alert level while Argentina keeps it low, or vice versa, causing confusion for tourists.
• OVDAS vs. SEGEMAR: The two monitoring agencies (OVDAS in Chile, SEGEMAR in Argentina) have learned to share data intimately. Seismometers on the Chilean side listen to the earthquakes, while webcams on the Argentine side watch the plume. It is a model of international scientific cooperation born of necessity.

Quick Facts

• Location: Border of Biobío (Chile) and Neuquén (Argentina)
• Coordinates: 37.850° S, 71.170° W
• Summit Elevation: 2,997 m (9,833 ft)
• Volcano Type: Stratovolcano
• Key Hazard: Phreatic explosions, acid lahars, ashfall.
• Nearest Towns: Caviahue (Argentina), Ralco (Chile).