Beneath the Sombrero: Bolivia’s Uturuncu and the Science of ‘Zombie’ Volcanoes

Once considered long dormant, Bolivia’s towering Uturuncu volcano is defying expectations. Rising over 19,700 feet above sea level in the Andes, Uturuncu has not erupted in over 250,000 years. Yet recent studies reveal that beneath its calm exterior, a complex hydrothermal system is very much alive. Unlike typical eruption patterns involving rising magma, Uturuncu's recent activity—marked by over 1,700 small quakes and visible ground deformation—appears to stem from deep subsurface fluid movement. Researchers have classified it as a “zombie volcano,” quietly releasing steam without posing immediate eruptive danger.

At the core of Uturuncu's behavior lies the Altiplano-Puna Magma Body—the largest known active magma reservoir in Earth’s crust. Situated 10 to 20 kilometers below the surface and spanning 200 kilometers, this body of molten rock is interacting with briny fluids and gases to create a unique geothermal dynamic. Instead of a buildup of eruptible magma, what scientists are observing is a “letting off steam” effect. This is evidenced by the volcano’s distinctive surface deformation—a rising center and sinking perimeter, reminiscent of a sombrero.

Using a combination of satellite radar imagery, seismic tomography, and computer modeling, researchers from institutions such as the University of Oxford and Cornell University have mapped the underground plumbing of Uturuncu. Their high-resolution models indicate that the fluids and gases from the magma body are migrating upward and pooling in chambers beneath the summit. This movement causes minor earthquakes and slow surface uplift—about 1 cm per year.

While the study confirms that Uturuncu is not primed for an eruption, its behavior offers invaluable data for geological monitoring and hazard assessment. Understanding hydrothermal interactions in dormant volcanoes helps scientists differentiate between eruptive and non-eruptive signs of activity. The techniques applied to Uturuncu—such as seismic tomography and integrated geochemical analysis—can now be replicated globally, especially on other “zombie” volcanoes that may hold geothermal or mineral potential.

Moreover, these findings highlight the need for continuous observation of ancient volcanic systems, particularly those situated near communities. Although Uturuncu poses minimal risk today, it lies within 25 miles of several towns and sits between major South American cities. If misinterpreted, changes in surface activity could lead to unnecessary panic or overlooked danger.

By demystifying zombie volcanoes, scientists offer not only reassurance but a deeper understanding of Earth’s evolving geology—one steam release at a time.